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FUEL SYSTEM ITEMS: Including Airbox, Carbs, Intakes, Fuel Tanks, Sending Units Petcocks, Gas Caps, Tank Mounts, Etc.



Let us begin by warning you: 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:

Now here is another interesting video: a fully see-thru operational carburetor. Although a lot of it is somewhat tedious to listen to, the video is great demonstration of the basic concepts of how a carb operates, and especially how the fuel becomes “atomized or aerated” (vapourized) by the airflow thru the carb throat (the venturi).


However, note that the following items are missing from this basic set-up, and hopefully you’ll be able to visualize where and how the placement of the following items will modify (improve) the behavior of this simplified carb:

- no pilot fuel circuit.
- no replaceable jets to regulate the amount of fuel coming thru the main fuel circuit (or, thru the missing pilot fuel circuit).
- no vacuum piston or attached needle to (also) further regulate (control) fuel flow thru the main fuel circuit (in response to varying vacuum conditions present in the engine)…….this is a lawn tool engine, so basically after start up, it runs at a more-or-less constant speed (rpm’s), as opposed to our engines.
- no emulsion tube with its little holes in the shaft to pre-aerate the fuel mixture coming up thru the main fuel circuit.
- the system shown has a true “choke” system for cold-starting (it “chokes” or closes off the air supply thru the carb), complete with a separate, secondary butterfly valve system. On our engines, the carbs use a “starter” system which operates the same as the main or pilot fuel circuit does, but via a different passage and control mechanism…..no independent butterfly valve, no “choking” off of the air supply, no need to open the throttle valve to increase the airflow thru that closed (“choked”) passageway in the carb……rather, the “starter circuit” on Hitachi and Mikuni carbs simply open and close (via the choke plunger valves) a fully separate source/passagway of fuel from the bowl, and thus let none, a little, or a lot of extra fuel into the carb throat. And please never mind that the label on the left control switch says “CHOKE”…………

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......

Here are the 3 rules you should always follow whenever trying to troubleshoot anything:

Typical Symptoms and Causes 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 fuel circuit condition can be responsible for a creeping or "hanging (high) idle", where rpm's are very slow to return to a normal idle speed, is often a sign of being set lean. Conversely, if the idle drops way low and then (maybe) slowly recovers is often a sign of an overly rich condition.

- 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.

- Pilot fuel circuit that are partially or completely clogged.

- Fuel jets that are too small for your application.

- Pilot mixture screws being set too far "in".

- 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 below normal 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.

- A choke system that is slightly hung open or has leaky plunger valves. Leaky valves can be determined via a measurement of vacuum using your synch sticks (or vac gauge) --- while watching the gauge, apply a small amount of downward force on the top of the plunger valve. If it vac draw changes, the plunger isn't sealing very well. Over time a small ridge may build up on the tapered brass plunger valve surface (Hitachi carbs) that may be possible to polish out, or a deformity forms on the rubber seals (Mikuni carbs) that may be possible to reform back into shape.

- Fuel jets that are too large for your application.

- Pilot and main FUEL jets being reversed.

- Pilot and main AIR jets being reversed.

- Pilot mixture screws being set too far "out".

- Main jet needle set at too "high" of a position (on some Mikuni models), or that has "risen up" within its head cap (on Hitachi models).

- A simple test for an overly-rich condition is to remove the air filter element, and see if performance improves. A very dirty air filter will cause an overly rich condition, AND, even if the filter was in good condition, the great increase in airflow accomplished by removing the air filter will tend to dilute an overly rich fuel mixture, and thus allow you to focus on determining the cause of that situation.

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!).

- 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, rather than cranking the idle speed screw up.

- 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.

Some common causes of a “run-away” idle speed:

If the idle speed starts building and building and eventually “runs-away” to a level that will grenade the engine without you shutting it off:


This is typically caused by an out-of-synch condition:


After reading all of the above, you should by now know the solution……….. ?

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 a 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 is my engine hard to start, and using the choke makes no difference?:

Most probably, the starter jets --- which are tiny, and are down inside the carb bowl ---- are plugged up with fuel varnish or the like.

Why does my engine run poorly (or is hard to start) after it is fully warmed up?:

This could be due to leaking throttle shaft seals (where extra seal-to-carb body clearance occurs after the carbs warm up and expand), or, it could point to valve shim clearances that are dangerously tight (the clearances close up after the engine gets fully warmed up).

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.......

What are my fuel levels supposed to be?:

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:

Fuel Level Cheat Sheet:

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: 21.0 +/- 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)

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

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.

f) A lot of useful "how-to" threads can be found at:


and scroll down to the CARBS and FUEL SYSTEM: section........

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:

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