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Let us begin by warning you, though: an engine that is out of synch may perform AS IF it has a lean, or a rich, or a hanging idle, or a no-idle, or a run-away idle, or any variety of different conditions------if your engine (carbs) have not been synched, then that at is the #1 issue that you should attend to first, before you even READ any of the guidelines below and go and try to adjust and fiddle with things and change settings THAT AREN'T THE CAUSE OF THE REAL PROBLEM!!

And here are a couple of very good visuals of how a Hitachi or Mikuni CV ("constant velocity") carb actually works. For those of us who are not intimately familiar with these carbs, these videos provide a great basic understanding of what is actually "going on" with your carbs:



Question #1: Why Isn't It Running Right?:

Below is a semi-useful "general rules-of-thumb" list to help you recognize and diagnose fuel-mixture problems.

All of these descriptions assume that the carburetors are cleaned and operating properly, are stock (no jet kit), the valves are adjusted properly, and the engine has been synched.

If any of the above procedures, tasks, actions, or activities have NOT been checked or performed, then do them first, or otherwise all your other efforts will barely even give you "casino odds" at striking it rich and determining what the real cause(s) of your symptoms are......

Typical Symptoms of a Lean Fuel-Mixture Condition:

- Poor acceleration; the engine feels flat.

- The engine won't respond when the throttle is snapped open, but it picks up speed as the throttle is closed. (A too-large main jet also mimics this symptom.)

- Idle speed falls after you blip the throttle, then creeps back up.

- The engine runs hot, knocks, pings and overheats.

- A lean pilot circuit condition can be responsible for a creeping or hanging high idle, where the rpms stay high then slowly drop down to the set point.

- The engine surges or hunts when cruising at part-throttle.

- Popping or spitting through the carb occurs when the throttle is opened. Or popping and spitting occurs through the pipe on deceleration with a closed throttle.

- The engine runs better in warm weather, worse in cool.

- Performance gets worse when the air filter is removed.

- Fuel levels that are too low will cause a lean condition.

- In cases where an overly lean fuel condition is suspected, the application of a small amount of "choke" may decrease or eliminate the symptoms.

- Here's the main reason you don't want to run lean over an extended period of time:


A holed piston. Notice how it's right where the plug fires (the hottest point anyway, and made ultra-hotter by a lean fuel condition).

Typical Symptoms of a Rich Fuel-Mixture Condition:

- Engine acceleration is flat and uneven and loses that "crisp" feel.

- The engine "eight-strokes" as it loads up and skips combustion cycles.

- The engine's idle is rough or lumpy, and the engine won't return to idle without "blipping" the throttle.

- An overly rich pilot mixture is usually the culprit when the idle drops low then slowly recovers.

- The throttle needs to be open continuously to maintain acceleration.

- Black, sooty plugs, a sooty exhaust pipe and black smoke from the tailpipe that stinks of unburned fuel.

- Poor fuel economy.

- The engine works better when cold. Performance falls off as it warms up or the ambient temperature rises.

- Engine performance improves when the air cleaner is removed.

- Fuel levels that are too high will cause a rich condition, as will a choke system that is slightly hung open or has leaky plunger valves.

Some common causes of a high idle:

- Engine has not been synched, as each cylinder "fights" the others for dominance, it can lead to a run-away idle situation.

- A lean air-fuel mixture condition, which can be caused by a variety of problems......vacuum leaks, plugged or too-small fuel jets, etc. Old or "stale" gas may also cause a lean fuel-air mixture to occur temporarily (until the fuel is used up!).

- A vacuum leak somewhere in the intake system.....intake boots, internal o-ring seals, etc.

- Mixture screws not properly set or adjusted.

- Vacuum piston sticking or stuck in a partially raised position.

- Idle speed screw set too high, or set to create a "proper" idle speed when the engine was cold (and thus results in a "high idle" once the engine reaches operating temps). The idle speed when the engine is cold should be modulated via the use of the choke (enrichment) control system.

- Butterfly valves opened too far; synch screws out of adjustment.

- Throttle cable wear, adjustment, or throttle lever brackets installed incorrectly or interfering with other nearby objects (cylinder head fins, etc.).

- Choke (enrichment) circuit is stuck "open"....this can occur even if the choke lever is rotated to the fully closed position, if for some reasons the choke plungers are not fully closing (cable wear, cable adjustment, bent finger brackets, or installation problems). In addition, even if the choke plungers are "closing" fully, if the choke plunger valve face or its seat are worn or scarred, this will allow fuel to leak part the plunger and richen the mixture even if the plungers are closed.

Why your engine seems to be schizoid:

A hanging idle---one that stays high and slowly comes down when decelerating---is a sign of lean mixture. The opposite case, where the idle drops dangerously low then rises, is a sign of rich mixture.

A bike that runs better when hot is probably lean, and a bike that runs better when cold is probably rich.

But sometimes a bike that can be adjusted "just right" when cool, but actually starts exhibiting signs of running lean (hanging idle) as it warms up. And if the idle speed is then re-adjusted while the engine is warm, but eventually falls and the bike dies (especially when sitting at a stop light). Or if the pilot screws are adjusted, then it's too rich when cold---all in direct contradiction of the above observations.

Experience shows that this is a sign of worn throttle shaft seals.....the aluminum carb bodies expand with the heat, but the steel throttle shafts---resting comfortably in a nice cool airflow---don't heat up and therefore don't expand. Once the shaft seals get old and hard, they no longer are able to flex and fill the gap.....so the carb starts drawing air around the shafts, leaning it out.

Why does my engine sometimes backfire when I first turn on the key (without attempting to start the engine)?:

- When you kill the engine, the intakes will still have some remaining (un-burned) air-fuel mixture remaining in them. Upon powering up the bike (turning the key on), the ignition system will apply 12 volts to the coils, thus charging them. Of course, that constant current is not good for the coils, so few seconds later (if the engine is not started) the TCI shuts down the coils by grounding them (to protect the coils from overheating) which cuts the current to the coils and thus triggers a spark to the plugs. If there is enough un-burned fuel in the intake manifolds or the combustion chambers, and if the valves are held open by the camshafts, a backfire thru the exhaust header and/or thru the intake manifolds/carbs/airbox can result.

Why are my plugs carbon-fouled (dry, sooty black deposits)?:

- Carbon fouling is the result of incomplete combustion----for any reason. It is most often associated with an overly rich fuel mixture (whatever the cause), but can also be caused by an overly lean fuel mixture (or poor spark, etc.) In a lean-mixture condition, most of the un-burnt mixture gets pumped out the tailpipe, but some fuel droplets remain in the cylinder and add themselves to the next intake charge. That's not a very precise way of metering the mixture, so when it's finally rich enough for a spark to ignite, that particular charge may be too rich, resulting in incomplete combustion and plug fouling. So carbon-fouled plugs can be due to rich or lean conditions.......your Colortune spark plug will tell you for sure. If you have a light blue or white-ish flame, intermittent flame, and/or intermittent flashes of yellow within an otherwise white-ish flame, then your fuel mixture is too lean.

And by the way.........if the spark that happens to ignite this overly-rich mixture is the "wasted spark" (which occurs in each cylinder at the top of the exhaust stroke,, it will occur JUST AS THE INTAKE VALVE IS OPENING ---- so the backfire pressure wave may be directed back up the intake tract!

Why are my plugs oil-fouled (wet, oily black deposits)?:

- worn or broken piston rings, excessive wear or damage to cylinders, *leaking intake valve stem seals.

* while bluish smoke from the exhaust can be caused by worn exhaust valve seals, that situation will not foul your spark plugs. Consider what happens: oil pools above the valve guides, and coats the valve stems every time they rise. These stem "seals" really aren't seals, they are more like wipers, and their function is mainly to wipe off the excess oil from the valve stem. Now, when these stem seals get old and hard, they leave an excess amount of oil on the valve stem, which then drops down into the path of the exhaust gas flow as the valve opens and the oil is evaporated off, leaving blue smoke from the exhaust. But note that none of that oil ever enters the combustion chamber, so it can't foul the plugs.

Of course, any excessive oil coating an intake valve stem will end up going through the cylinder, but intake valve stem seals rarely fail on these engines. Unlike the exhaust valves, which are constantly baked by superheated exhaust gas, the intake valves are bathed in cool and moist (with gasoline) air.

Why is the inside of my carbs covered with a brownish-green goo?:

When fuel mixes with water and sits around for a while, this is the result. Nasty looking, nasty smelling, and you can bet that the tiny passages inside the carb body are plugged solid with this stuff! Definitely time for a full rebuild.......

Question #2: What Parts Will I Need?:

The parts you'll need depends on what is there, what is missing, what is worn out, what gets broken or damaged during the dis-assembly process, etc., but the "basic items" needed for the rebuild process (besides any special tools and tuning equipment) is as follows:

- throttle shaft seals (you'll have to split the carbs from the rack to replace these).

- fuel supply tube o-rings (you'll have to split the carbs from the rack to replace these) on the models that use them (some Mikuni carbs do not use these).

- float valve needle and seat rebuild kit (seat, needle, clip, seat washer. The float valve seat filter screens are also available separately). Mikuni owners will also want to replace the float valve seat o-rings.

- float bowl gaskets.

- idle mixture washers & o-rings for sure, and the idle mixture screw springs and mixture screws if they are damaged.

- jets, if damaged or incorrectly sized.

- carb bowl fuel drain screws, if damaged.

- new replacement carb hardware (screws, etc.)---now is the time to upgrade to stainless steel and/or allen-head fasteners if so desired!

From Our Experience:

Since the carburetors are THE most troublesome (and thus most expensive) aspect of these bikes, let me offer a few dozen words and insights about them, their service, and expense:

These carbs are quite simple, mechanically speaking.

The process of making DARNED SURE that you get them unbelievably, positively zestfully clean is a matter of stick-to-it-ness and resisting all attempts to take "shortcuts" or to brush off the tedious aspects of it. Many of the passages in these carbs are tiny, and it's that "tiny-ness" that bites most people, as they don't want to or don't realize the amount of effort that has to be undertaken to deal with such small passages.

If you read through these forums, you'll see many tales of woe of people who are now "cleaning" their carbs for the 2nd, 3rd, or 4th time BEFORE they get it right. The problem is, they never really "cleaned" them correctly the first, 2nd, and 3rd time. After all of the frustration and hassle involved, by the time they get around to the 4th go-around, then they understand that when people who have SUCCESSFULLY rebuilt carbs say things like "you'd better make SURE that you get clear flow through each and every passage, and you'd better polish that piston diaphram bore", etc. aren't just saying that to be over-the-top retentive about their own habits and style of working on these carbs, what they're really doing is issuing an experienced WARNING: do it right, or keep doing it again, and again, and again. And although they're kind of fun little critters to play with, taking them all apart and putting Humpty Carby all back together again, well, it's time better spent on other things.

Think we're kidding? The following thread is a "must read", because the title says it all:


One last note: "cleaning" the carbs (internally, not just making them look pretty on the outside) is just one of the small subsets of tasks necessary to the proper REBUILDING of the carbs. I cannot stress this point enough. The use of the term "cleaning" is used as if it were the end goal of the process; in reality, the proper way to think about it would be this:

"A complete and exceptional level of carb CLEANING is a vital and necessary part of the process of properly REBUILDING the carbs."

And what are the other components of the rebuilding puzzle?

a) getting the darned things off the bike! This might help:




b) replacing the worn, missing, or necessary "wear parts" on the carb.

c) Proper "service" work to the carb bodies, especially:

- polishing the piston bore for silky smoothness.
- repair or clean up of any stripped threads in the carb bodies.
- replacement of any damaged or worn parts. In fact, the ability the recognize what is damaged and worn (besides the parts above which are designed to wear out) is the most difficult part of the entire task, since it requires a level of experience: are my needle tips worn? What, exactly, does a worn needle tip look like? How "smooth" is smooth enough? Etc. This is where the advice and experience of the members of this forum are invaluable.

d) Proper "settings" of the various components during re-assembly:
- the fuel levels (float heights).
- the "basic or bench synch" of the throttle valves:

If your carbs have been removed from the engine for a rebuild, it is suggested that a “bench synch” procedure be performed, which does not require any tools to perform….the only purpose of the bench synch procedure is merely to get the carbs set up “close enough” to allow the engine to start and run, at which time a running (vacuum) synch of the engine should be immediately performed, using the HCP96Q synch gauges, A great visual guide to the bench synch process can be seen at:


e) On-bike "settings":

- first and foremost, the measurement and setting to specifications of the valve train (shim) clearances. Failure to do this "wastes" 90% of your efforts involved with the carbs.
- final synch (using some type of manometer and the YICS tool if your engine is YICS-equipped)
- idle mixture screw setting, preferably using a Colortune plug.

One last thought about these carbs, since they are THE most troublesome aspects of these bikes, and something to consider:

- the four carbs concept is really neat looking, and is certainly performance-oriented to the extreme.

- for many of us who had lots of experience with automotive carbs, just beware that while these Hitachi and Mikuni carbs are a bit different in operation (and thus parts), a carb is a carb, and they're pretty basic little devices. The main difference is that the fuel circuit passages on these carbs can get bizarrely tiny, especially if you're used to working on automotive carbs (which have canyon-sized fuel passages compared to these carbs).

- like I've said, the cleanliness part is really just tedious work, with a couple of "tricks" thrown into the mix, given the small sizes you're having to deal with. The "rebuild" part is basic mechanical knowledge and skills, knowing which way to turn a screwdriver to loosen vs. tighten, when and how much force to use or not, being organized, stuff like that.

- but the "tuning" part will require some special tools, but no type of any rocket-science knowledge or skills----once you master the rebuild and tuning process with these bikes, you are pretty much a Carb Tuning God, as these carbs are about as "complicated" as it gets with carburetors. I mean, VERY few vehicles have multi-carbs, not until you get to the really high-performance machines level in the automotive world.

- well, as "complicated as it gets" until you start adding pods, etc.----which if you do, I sure hope you've got every last one of the basics listed above down to a science, and you can do it all, properly, and blindfolded, too!

- finally, if you do not have a service manual(s) for your particular bike(s), and you're going to wrench on it and want to do things right, then the only thing I can surmise from such a course of action is:

a) you're already an expert, or.....

b) you aren't really serious about doing things right, and that's okay, too---just don't expect good, quick, or cheap results!

The following guide may save untold hours of frustration and grief:











http://www.xj4ever.com/inside your carbs.pdf




http://www.xj4ever.com/hitachi throttle shaft seals.pdf






http://www.xj4ever.com/mikuni carb cleaning.pdf



Note that the above link shows the rebuild for the BS28 series of Mikuni carbs (same as on XJ550 models) that are off a Suzuki, and while some minor differences exist ( i.e the choke plunger system and the locknuts on the synch screws), 99% of the information (and pictures) will crossover to the XJ550 model carbs.



Note that the above link shows the exploded view of the BS28/BS30 series of Mikuni carbs (used on XJ550 and XJ650 Turbo models);, the BS33-and-larger series move the pilot air jet from under the vac piston diaphram to the carb throat.........and have the choke plunger coming out of the side of the carb body rather than the top.

Always remember to take copious notes as you are disassembling your carbs, or otherwise you will forget some important details when it’s time to put those Humpty-Dumpty’s back together again. For example, would you remember how to re-install the throttle closure springs correctly if you didn’t have this?:



Having the carb piston bore fully polished is an incredibly important task when rebuilding a set of carbs, as any restriction to the free and almost friction-less movement of the vacuum piston will create a variety of performance-related problems. You can review this issue (the vacuum piston "CLUNK TEST") at:


and see a nice video of a clunking piston at:

And the reason you want your vacuum pistons to operate as closely as possible to a "zero-resistance" state is because they must be able to perform, constantly, in the following manner; and if not, tuning and performance becomes troublesome or downright impossible:

Note closely the difference in vac piston operation between a no-load condition:

and how they operate under load:



NOTE: these throttle shaft retaining screws are an absolute BEAST to remove, and it is very easy to destroy the screw heads as well as the throttle shaft threads while trying to remove them---if proper precautions are not taken. You will want to review the information in this forum thread before attempting removal:





http://www.xj4ever.com/hitachi throttle shaft seals.pdf



P.S. how do you know when your starter jet is really, truly, zestfully clean? One of two ways:

a) shine a strong penlight or mini flashlight into the bottom of the bowl, where this jet passage "intake" is located. Look through the top of the bowl down into the jet passage "outflow" passage (this is the passage that the brass suction tube in the bottom of the carb body actually fits down into). Focus your eye carefully on the jet opening and make sure it's clean. P.S. it helps to do all this while in a darkened area.........see the picture "starter-jet.jpg" on page 2 of this forum thread topic for a great image of what you want to see!:


b) put the spray tip (you may have to gently shape the end of it to a fine point, see the video below) of a can of carb or brake cleaner into the intake opening of the starter jet and let rip a spray. A STEADY, FINE, POWERFUL STREAM OF FLUID WILL COME OUT OF THE OUTFLOW PASSAGE ON THE TOP OF THE CARB BOWL IF THE JET IS PERFECTLY CLEAN AND OPEN. I mean this stream will absolutely spit out a good 5-10 feet. If the stream isn't powerful and laser-like precise coming out of the jet, then the jet isn't zestfully clean.....

And here's how to re-size an aerosol spray-can tube to clean out those carb jets and passages:


By the way, when using the spray-stream method of checking the jet, don't even THINK of putting your eye or face anywhere even NEAR the jet outflow path, unless you like a painful and potentially serious trip to the emergency room.




You're not alone.........



Q: and how do you know when your pilot fuel circuit is really, truly, zestfully clean?

Ensuring that your pilot fuel circuit is internally clear and free is one of the most difficult tasks to perform on these carbs due to its incredibly small size (so fuel varnish will accumulate and solidify in there over time):

http://www.xj4ever.com/inside your carbs.pdf

To make sure that your internal pilot fuel circuit is truly “zestfully clean” after your clearing efforts:

1. Drill out an old pilot fuel jet so that the straw from the carb cleaner fits snuggly in the hole (or some other suitable method of “sealing off” a straw into the pilot fuel jet passage where the jet screws into the carb body).
2. Leave the pilot air jet in, but block it off with a wooden q-tip that is covered with a bit of heat shrink tubing (or use your finger to firmly block it off).
3. Install the pilot screw without its o-ring, and set it to about 2.5 turns out from soft bottoming.
4. Blast the carb cleaner through the modified pilot jet and it should look something like this:


By the way, when using the spray-stream method of checking the jet, don't even THINK of putting your eye or face anywhere even NEAR the jet outflow path, unless you like a painful and potentially serious trip to the emergency room


You're not alone, either.........



Here's the proper fuel-level settings by model when using the "clear-tube" method of measurement:

Hitachi all HSC32 series models:

NOTE: all of the following models used HSC32 carbs:
XJ650 Maxim, Midnight Maxim, XJ650RJ Seca (non-turbo), XJ650 Euro all use HSC32 carbs (various versions)
XJ750 all USA 1981-83 models use HSC32 version 5G200 (Seca) and 15R00 (Maxim and Midnight Maxim)
XJ750 all Canadian 1981-83 models use HSC32 version 5H200 (Seca) and 15T00 (Maxim and Midnight Maxim)
XJ750 UK/Europe 11M models, XJ750 Police models 24L and 37H use HSC32 version 5N100

-Fuel level measured via the clear-tube gauge: 3mm +/- 1mm (.12 +/- .04 inches)

-Float height: 17.5 +/- 0.5mm

Hitachi HSC33 series (XJ700 air-cooled models):

-Fuel level measured via the clear-tube gauge: 1.0mm +/- 1mm (.039" +/- .039")

-Float height: 16.0 +/- 1.0mm

Hitachi HSC33 series (XJ750E-II model):

-Fuel level measured via the clear-tube gauge: 5.0mm +/- 1mm (.197" +/- .039")

-Float height: unknown

Hitachi HSC33 series (XJ750RL models):

-Fuel level measured via the clear-tube gauge: 1.0mm +/- 1mm (.039" +/- .039")

-Float height: unknown, but possibly the same as XJ700 air-cooled models (16.0 +/- 1.0mm)

Mikuni BS28 (all XJ550 except 1984 XJ550L):

-Fuel level measured via the clear-tube gauge: 2mm +/- 1mm (.08 +/- .039 inches)

-Float height: 21.5 +/- 1.0mm

Mikuni BDS26 (USA all 1992-98 XJ600 Seca II):

-Fuel level measured via the clear-tube gauge:
USA: 4 - 6mm (.016 - 0.24”) below float chamber line

-Float height: 6.2 - 8.2mm (0.24 - 0.32”)

Mikuni BDST28 (non-USA all 1992-98 XJ600 Seca II):

-Fuel level measured via the clear-tube gauge:
1992-96 Canada and Australia: 3 - 5mm (0.12 - .0.20”) above float chamber line
1992-95 UK: 3 - 5mm (0.12 - .0.20”) above float chamber line
1996-98 UK: 8.5 - 9.5mm (0.34 - .0.37”) above float chamber line

-Float height:
1992-96 Canada and Australia: 11 - 13mm (0.43 - 0.512”)
1992-95 UK: 11 - 13mm (0.43 - 0.51”)
1996-98 UK: 8.8 - 10.8mm (0.35 - 0.42”)

Mikuni BS32 (all XJ550L, 1984-85 FJ600, and 1984-87 / 1989-91 XJ600):

-Fuel level measured via the clear-tube gauge: 3mm +/- 1mm (.12 +/- .04 inches)

-Float height: unknown

Mikuni BS30 (all XJ650 Turbo):

-Fuel level measured via the clear-tube gauge: 2mm +/- 1mm (.08 +/- .04 inches)

-Float height: 17.5 +/- 0.5mm

Mikuni BS33 (all XJ700-X and XJ750-X):

-Fuel level measured via the clear-tube gauge: 3mm +/- 1mm (.12 +/- .04 inches)

-Float height: 17.5 +/- 1.0mm

Mikuni BS35 (all XJ900RK/RL):

-Fuel level measured via the clear-tube gauge: 5mm +/- 1mm (.20 +/- .04 inches)

-Float height: 22.3 +/- 0.5mm

Mikuni BS36 (all XJ900F, FN, N, etc):

-Fuel level measured via the clear-tube gauge: 5mm +/- 1mm (.20 +/- .04 inches)

-Float height: 22.3 +/- 0.5mm

Mikuni BS34 (all XJ1100 and XS1100):

-Fuel level measured via the clear-tube gauge: 3mm +/- 1mm (.12 +/- .04 inches)

-Float height:
25.7 +/- 1.0mm (1978-79 all models)
23.0 +/- 0.5mm (1980 all models)
not specified (1981-82 all models)

And here's the Holy Grail on this subject:

http://www.xj4ever.com/setting fuel levels.pdf

And here’s a good visual guide to some problems that you might run into:


and here’s a wonderful video from TurpentyneTV that shows the whole process in excruciating detail:












http://www.xj4ever.com/HCP5460 YICS tool.pdf










And here's a super-sized colortune plug, don't try this at home kids!:








A great visual journal of the original automatic petcock rebuilding process is documented at:





Seems like if one fuel line is good, then seven must be even better........... although the pictured carbs are from XJ900 models, the XJ700 model carbs suffer a similar fate.





Bolt-on tank interchanges:

All XJ650 (except Turbo) and 1981-83 XJ750 (except XJ750RL) tanks will swap between those models and fit, but may experience some minor interference issues with the front of the seat/seat pans. Many XJ650 Maxim and Midnight Maxim owners like to upgrade to the larger XJ750 Maxim tanks, and that is basically a bolt-on swap.

XJ750RL (1984) and XJ900 tanks are basically identical and will swap easily.

All XJ550 tanks (Maxim and Seca models) will interchange with each other.

[n/Tanks that will not interchange at all or without a LOT of work[/b]:

All XJ550 tanks (Maxim and Seca models) will interchange with each other, but will NOT swap with XJ650 or above models (the tank mounting dimensions are different). XJ700 and XJ750-X tanks will NOT interchange with any other models. XJ650 Turbo tanks will NOT interchange with any other models. XJ750RL and XJ900 tanks will NOT interchange with any other models.

Further insights into tank-swaps can be reviewed at:




And if you want to increase the rather small fuel capacity of your XJ700 or XJ750-X tank, here's the correct way to do it:



For most owners, replacement fuel tanks are not available, and so the issue becomes "how to de-rust" an original tank. Although there are a variety of methods which can be used, there are plus-and-minus considerations to each method or technique.

Here are a few other good ways to de-rust metal products:

- soak iron (steel) parts in heated vinegar (doesn’t matter what kind) overnight, then rinse with clean water. and finally spray with fogging oil (or similar) to prevent rust from re-forming.











A great visual tour of the internal bits and pieces (and the rebuild process) for the original pop-open gas caps can be seen at:

http://www.xj4ever.com/gas cap.pdf

And now with video:




A great visual tour of the internal bits and pieces (and the rebuild process) for the original twist-open gas caps (FJ600, XJ650 Turbo, and XJ900) can be seen at:

http://www.xj4ever.com/turbo gas cap rebuild.pdf




Filter, filter in that hole,
Who's the fairest of them all?




Yamaha specifies a service life for stock pleated-paper air filter elements under "normal" conditions as being approximately 15,000 miles.


It's a question we get asked often and unfortunately, one that we cannot answer honestly about your specific bike besides with "it depends".

Which is a nice way of saying "you're about to enter the seventh circle of hell......."!

Carb jet tuning required by aftermarket modifications is somewhat of a black art, part science, part skill, part luck. It depends on the current state of tune of your engine, your altitude, the mix of aftermarket parts on your bike, etc........a lot of variables.

The best advice we can offer is: Just Say No!. Don't do it! Leave everything stock!

But, since most people---with good reason, I might add---don't always listen to our well-intentioned advice, then the next best recommendation we can offer is: "if you want more power get a bigger bike!".

And since that doesn't cut it with many owners, either, for the remaining stalwarts out there who insist on "experimenting" with aftermarket intake and exhaust systems, here's the best information that we've come across to give you some GUIDANCE, which you should take as just that, and not as ANSWERS, because it isn't!

Definitely not plug-and-play:


And some pod filters make a difficult situation even worse.........read all about it:


Yoji himself cautions against it:



Typical Exhaust Changes:

+2 main fuel jet size for custom 4-into-2 exhaust


+4 main fuel jet sizes for 4-into-1 exhaust


+4 main jet sizes for no muffler (open headers)

Typical Intake Changes:

+2 main fuel jet sizes for single K&N filter (inside a stock airbox)


+2 main fuel jet size for drilling holes in the airbox with stock filter


+4 main fuel jet sizes for individual pod filters (no airbox)

Additional changes:

- Add up all the main fuel jet size increases and subtract 2 sizes.

- Decrease main fuel jet size by 2 sizes per every 2000' above sea level.

- Under a mis-match condition, such as when using pod filters with a 100% stock exhaust, or 4-into-1 header with stock filter and air box, then subtract 2 main fuel jet sizes.


Pilot fuel jet size changes are related only to the change in main fuel jet sizes according to the main fuel jet size formula described above. Note that this pilot fuel jet rule is for the main fuel jet size change BEFORE any main fuel jet altitude compensation is factored in:

Increase the pilot fuel jet size +1 for every +3 main fuel jet size increases.

Additional changes:

- Decrease pilot fuel jet size by 1 for every 6000' above sea level.


- Make sure your carbs are in perfect working order before making ANY jet changes....meaning fully cleaned internally and rebuilt, operating properly in their stock configuration, proper sized air jets and needles, etc. Otherwise, you'll like find that all of your efforts are going to be a HUGE waste of time.

- Check plug color often and adjust as needed, 2 main fuel jet sizes at a time and 1 pilot fuel jet size at a time. Bright white plug insulators are a sign of an overly lean fuel mixture condition and WILL cause damage to your engine over time, up to and including engine seizure!

- Synch the carbs after each jet change.

- Make sure the floats are set correctly

- Seriously consider purchasing a Colortune Plug Tuning kit.

- You may find it necessary to make changes to the size or shimming of the main jet needle. There are no guidelines on what or how to do these changes, this is true trial-and-error tuning!


A 1982 XJ750RJ Seca using an aftermarket Supertrapp 4-into-1 exhaust and a single K&N air filter in the stock, unmodified airbox. Bike is primarily operated at an altitude of 2600 feet above sea level.

XJ750 Seca Stock Hitachi HSC32 Carb Jetting:

#120 Main Fuel Jet
#40 Pilot Fuel Jet
#50 Main Air Jet
#225 Pilot Air Jet
Y-13 Needle


Changes made:

4 into 1 with Supertrapp = +4 Sizes Main Fuel Jet

K&N Pod Filters = +4 sizes Main Fuel Jet
Equals: +8 main fuel jet sizes above baseline
Subtract: -2 main fuel jet size per formula above
Equals: +6 main fuel jet sizes due to modifications, thus:

Stock main fuel jet size is: #120
+ 6 additional sizes
= a #126 main fuel jet size
Subtract: -2 main fuel jet sizes for Altitude of 2500' Average

= #126 calculated from above
-2 jet sizes for altitude adjustment

= a #124 main fuel jet size.


The formula is: +1 pilot jet size increase for every +3 main jet sizes increased.

Stock pilot fuel jet size is: #40
+ 2 additional jet sizes (since we went up +6 main fuel jet sizes before the altitude compensation was factored in):

= a #42 pilot fuel jet size.

Note that no altitude compensation is needed on the pilot fuel jet since our elevation is less than 6000' a-s-l.



A #124 Main and #42 Pilot is A GOOD STARTING POINT.


Note that the above calculations do NOT take into account any possible changes in the sizes of the main or pilot air jets, the main needle or main needle jet size, and thus are additional variables and opportunities for tuning excellence. These are areas which are largely unexplored by most tuners, but logically should allow for additional fine tuning or additional rage and frustration.........

Normally, changing to an aftermarket exhaust does NOT require re-jetting, (or minimal re-jetting) as almost ALL of the airflow restriction in the airflow path thru the engine (meaning: ATMOSPHERE > AIRBOX > FILTER > AIRBOX BOOTS > CARBS > INTAKE MANIFOLDS > CYLINDER HEAD PASSAGES > EXHAUST SYSTEM > BACK OUT INTO THE ATMOSPHERE) is within the intake side of this air flow path, and primarily within the stock airbox/air filter. Your stock EXHAUST system can already flow more air volume than the stock INTAKE system allows.

Thus changing only the stock EXHAUST system, with no changes to the intake side of the heads, normally makes NO DIFFERENCE IN TOTAL AIRFLOW, and "no difference in total airflow" means "no difference in TOTAL FUEL FLOW" either, and thus bigger jets are not needed.

But once you start freeing up the INTAKE side of the entire system, you will produce more system airflow, even with a stock exhaust system (because, the stock exhaust system has the capability to flow more air through it than what the stock intake system allows).

This is why re-jetting is usually needed even if you keep the stock airbox and the stock exhaust, but use a K&N low-restriction filter, or even if you drill holes in the stock airbox, or leave the filter lid off.

All such actions free up the intake side airflow restrictions; the stock exhaust will move this additional airflow, and without providing addition FUEL flow to match the increased airflow (within limits, an engine will gobble up the maximum amount of airflow that it can; an internal combustion engine is actually just a self-powered AIR PUMP) then the engine will run "lean"---meaning not enough fuel to match the amount of airflow that the engine can (and now will) gulp.

Most pod type filters allow for vastly increased airflow, and thus require fuel re-jetting, and although no one really talks much about it, probably also require AIR JET changes to match the additional fuel flow, but since no one likes to deal with two parameters at once, it becomes a "tuning nightmare".

BUT, when you read all of the common symptoms of people who use pods, you quickly come to the conclusion that it's not possible to reproduce the stock "smooth in all rpm ranges" engine response. The reality is that you SHOULD be able to match it pretty darn closely, even with the increased airflow through the system, but ONLY changing the fuel jets isn't going to accomplish that. There are also air jets in the system, and they are there for a reason, as well as needle tapers and vacuum piston responsiveness issues.

For further insights and understanding, the Holy Grail (meaning: the whole miserable, un-varnished truth of what a real chore carb tuning is going to be, written by people who actually know what they're talking about, rather than by people who are trying to sell you something) can be found at:


and then click on the "Product Support/Technical Support" link at the top of the page, then on the "Motorcycle Tuning Tech" link, and then the "CV Carb Tuning" link........and then read, weep, study, and do....if you still dare to! HINT: if reading through it makes you think to yourself "sheesh, this sounds like an incredible amount of effort!", well, you're right! That's just some of the joys (and pitfalls) of getting to play "tuning engineer", which is what you're going to be doing. Yamaha probably has 10 of those types of guys on staff, and millions of dollars of test equipment, both physical and computer-aided, that allowed them to get the mixture settings just right---from an overall drivability AND power output standpoint----and now, since you're changing the airflow parameters thru the engine, you'll have to figure it all out "from scratch", but WITHOUT the benefit of 10 trained engineers and all that test equipment and experience.

That's why we warn you that setting up a bike for pods can be quite a bit of trial-and-error procedure. You can make the calculations according to what is shown in that guideline and then order the jets that the "formula" recommends, and that should serve as a good STARTING POINT............you may (or may not!) have to do more tuning and trial-and-erroring substitution of different jet sizes, etc. to get it performing to you satisfaction, with the recognition that you may ALWAYS end up with a situation that has some kinds of trade-offs.....lazy at the lower end but runs well at mid/upper-ranges, or runs well at the lower end but a "flat-spot" at some other rpm range, etc. Unfortunately there is no magic formula........you might want to read through the factorypro.com article that I list at the end of that section, and you will get a better understanding of what is involved to get the carbs set-up properly in a non-stock configuration.

As one of our favorite experts says about pod filters: "Get a Rubik's Cube instead.....it's less trouble and actually has an eventual solution!"


The first step on your road to hell:


Hint: it's the funky background music that makes it all possible!


Purchasing a replacement airbox to replace the original cut-up version (https://www.youtube.com/watch?v=48PJGVf4xqk&list=RD48PJGVf4xqk) when someone thought it was a great idea to install pod filters:



Sometimes, it's not as easy as you'd think!



EFI to solve all of your carb woes............








As with many other systems on these bikes, the Yamaha XJ-series of exhaust systems are a mini marvel-to-behold, well thought out and engineered......so please do keep this in mind when considering aftermarket "performance" exhaust systems.

Let's start with the headpipes: many of them are actually of a double-wall design......and inner tube (that carries that gas) and an outer tube that can stay relatively cooler and thus maintains its appearance much better and longer. Additionally, they are tuned in their internal (gas-carrying) pipe diameter to match the flow characteristics needed for their intended use.....thus, for example, even though the headpipes for an XJ650RJ Seca can physically be installed onto an XJ700 engine, the inner pipe diameters do differ, and thus the swap may cause more harm than good, as it messes up with the balance of airflow thru the engine.

Pipe diameter helps determines (among other things) both the volume of exhaust gas that can be carried away, and, equally as important, the speed at which those gases will flow thru the pipe. All other things being equal, a bigger diameter pipe --- while surely capable of flowing more gas volume, will also slow down the progress of gas thru that pipe......which, in the world of exhaust gas flow --- and its important effects on intake gas flow (how well each cylinder can be scavenged of its burnt exhaust gases and allow for a full and complete incoming fuel/air charge) --- is a mighty important consideration.


By the way......although we've touched on some of the reasons above.....in regards to "swapping" headpipes between models, our best advice is: don't. Besides the whole "gas capacity and velocity" issues discussed above, there also the issue of fit: although the headpipes between a 550 and a 750 (for example) may look the same, and may actually bolt onto the cylinders heads, both their length and their unique bend angles ---- needed to follow the frame and mate up with the exhaust collector ---- will likely be different.

Next: the collector. Although these weren't used on the XJ650 Maxim and XJ1100 models --- those bikes use a crossover pipe to perform the same function --- the collector box performs a few not-so-obvious but very important functions, and does it in a very admirable manner. First, of course, it provides a transition between the headpipes and the mufflers. Second, it keeps the decibel levels down to a reasonable volume. And finally....and very important for overall engine performance.....it provides a method of assisting that all-important "gas velocity" by providing a sufficient volume of space for "warehousing" (temporarily) the exhaust gas volume, while also providing (via its unique design) the ability to assist in providing maximal exhaust gas flow.

If you've ever looked carefully at your heavy (and probably-rusty) collector, you'll notice that it is designed ("split") in such a way so that cylinders 1 & 4 (the outer two) "share" a certain internal cavity space, while cylinders 2 & 3 "share" the rest of it. This is important because of the firing order of your engine: 1-2-4-3

If cylinders #1 and 2 --- which fire sequentially (and, thus need to exhaust their spent gases sequentially) ---- were to share the same exhaust cavity, then their (sequential) exhaust gas flows would be "spaced" too close to each other (in time) and would see vastly different back-pressures (cylinder 2, which fires after cylinder 1, would see more back-pressure than cylinder 1, which emptied itself into a, well, into an "empty" exhaust collector).

To put it another way, cylinder #2 sees a lot more back-pressure than cylinder #1, because the gas pulse from #1 doesn't have much time to exit the system before the pulse from #2 is trying to squeeze itself in the same pipe. Notice that cylinders #3 and 4 have a similar situation.

The same thought goes for the other two cylinders (# 2 and 3). Notice how even the aftermarket "dual" exhaust systems suffer from the above problem, since they are really two separate system, one system being for cylinders #1 and 2 (left side) and the other being for #3 and 4. Notice how it's the "sequential" problem.....since cylinders 1 and 2 fire (and exhaust themselves) sequentially, and since those gases vent into one common chamber, that cylinder #2 is going to see more backpressure than cylinder #1 will......and same for #3 and 4.

This un-balanced situation....with different cylinders seeing different exhaust system back-pressures......can (and does) result in different amounts of burnt gas scavenging from the differing cylinders, which is not the way to get an engine to make the most amount of power.

Aftermarket 4-into-1 systems overcome this "differential scavenging" situation by re-introducing, somewhat, a "collector".......where the fours pipes come together into a common chamber, just before the muffler. This collector allows the system to maintain some amount of equalized back-pressure (and thus burnt gas scavenging), while the particular placement of the four headpipes, as they come into the collector, allows a little bit of "pull-thru" between each cylinder, helping to reduce overall system back-pressure levels, while still maintaining a somewhat equal amount of back-pressure to each cylinder.

Or, maybe, aftermarket systems do no such thing:


Of course, the final piece of the puzzle is the mufflers, which are primarily decorative pieces....they look nice, stylish to the lines of the bike, contribute almost no back pressure to the system, and provide a nice throaty (but not too loud or droning or buzzing) sound output.

So just remember that the real heavy-lifting in the exhaust system is actually done by the headpipes and the unique, well-designed (but often overlooked) collector box.


straight pipes: just say no:


straight pipes tuning tricks ---- in case you didn't listen to the above advice:


stock and custom exhaust sounds, for your comparison. Note that bikes parked in a garage (or their exhaust facing a wall) will sound a bit louder than they actually are "in the wild":


stock exhaust:


XJ650 (except Turbo):

stock exhaust:

Emgo replacement mufflers on stock system:

Mac 4-into-2:

Open mufflers:

Mac 4-into-1:

Unknown brand 4-into-1:

Supertrapp 4-into-1:

Marhsall 4-into-1:

Something different:


air-cooled models stock exhaust:

water-cooled models stock exhaust:

water-cooled models 4-into-1:


stock exhaust:

4-into-1 Supertrapp:

XJ1100 and XS1100:

stock exhaust:

4-into-1 Kerker:


exhaust port gasket hide-and-seek.....and replacement:




changing the coolant on your water-head engines:

http://www.xj4ever.com/maxim-x coolant flush.pdf

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