Miscellaneous Carb Questions FAQ
compiled & edited by Kristian #562
Please read the Disclaimer before attempting any work in this FAQ.

If you are looking for performance-related carb information See the Rejetting Spreadsheet accessed via. this link Performance Mods Section

Straight to the Carb Clean FAQ
For Aftermarket Fuel Filter Q's and the OEM In-line Fuel Filter.
For Petcock Q's.

For Idle Q's
For Canister Q's

Carb Primer
by Flash #412
Please read the Disclaimer before attempting any work in this FAQ.
October '01

This is a carb primer for those with little or no understanding of this device. If you're carburettor literate, move on. If you're carburettor ignorant, you might like this... "Carburettor is NOT Latin for 'don't mess with it.'"

Fuel and air need to be mixed in the proper proportions in order for your engine to run. The device that atomises the fuel is your carburettor (unless your motor is fuel injected). Due to the relationship between the volume of a sphere and it's surface area (spherical surface area is about 12.6 r^2 while spherical volume is about 4.2 r^3), smaller drops ignite better by a factor of three times the difference in the radii. Producing these small drops at idle as well as wide open (WFO) requires more than one method of atomisation. Hence, your carb has three distinct fuel circuits. The simplified view has one single air circuit.

If you look at a carb, you'll see that it is a fancy, glorified pipe through which air passes. When you twist the throttle, a slide raises in the carb throat. This may or may not happen with the engine NOT running due to differences in design. But anyway, this slide controls the amount of air allowed to go into the intake manifold. The intake manifold is whatever is between the carb and the hole in the head containing the intake valve(s).

The float chamber is an area inside your carb filled with gasoline, whose level is more or less carefully controlled. There is a float in there, sorta like in the back of your toilet. You access the float by removing the float bowl. The float is either solid or filled with air. In any case, it floats in gasoline and is made of material impermeable to that solvent. When the fuel level rises, so does the float. At a certain level, the float pushes a conically shaped round thing (float needle) into a conically shaped hole (seat) and shuts off the flow of gas. This "needle and seat" is the valve that controls gas flow into the bowl. Float level is important and so is a good seal here. If the level is too low, the carb won't get as much gas as it should (lean). Too high and it can run rich or leak.

The idle circuit is made by drilling some holes from the float bowl to the area on the *motor* side of the throttle slide. There is ambient air pressure on the air-filter side of the carb. There is vacuum on the motor side. This vacuum is used to draw fuel through this little hole, the diameter of which is normally controlled by an orifice, or pilot jet, installed in that hole. More vacuum at wider throttle opening draws more fuel. This is from idle, through off-idle to about 1/8 throttle. Normally idle SPEED is set by a screw that opens the slide a little more or a little less. Idle MIX is set by a screw that sticks a pointy tip into the little fuel passage (or a similar air passage used to draw the fuel).

At about 1/8 throttle, the effect of the next circuit swamps out the effects of the idle circuit. There is a bigger hole that goes straight up from the float chamber into the bottom of the carb throat. This hole has two precision fittings, one at either end, both removable. The one at the bowl end is the main jet. More on that later. The one at the top is the needle jet. Hanging off the bottom of the slide is the jet needle. The jet needle is a tapered needle that moves in and out of the needle jet. Normally, both the needle jet and jet needle can be changed. Needle jets have different size holes in them. Jet needles have different lengths and tapers. Usually, they are suspended from the slide in a manner that allows one to change how far they hang down, by moving a clip to a different notch. You can imagine a tapered needle withdrawing from a hole... the further out it is, the more surface area of the hole is available to flow fuel. This circuit works up to about 3/4 throttle, when the jet needle is completely withdrawn from the needle jet. At that point, the limiting factor becomes the main jet.

The main jet is used at wide full open (WFO) throttle. How much fuel does the engine NEED when it is breathing as fast as it can? That is *THE* limiting factor for your motor for any given combination of exhaust system, Airbox/filter setup, temperature, altitude or barometric pressure.

Tuners tune first to get the bike to start and run (idle circuit), then make power on the mains, THEN be "streetable" between idle and WFO. *HOW* to go about tuning is a different subject, maybe for a different day. Many whole books have been written on the subject.

by Flash #412
October '01

  1. Remove the seat.
  2. Remove the side covers. 
  3. Remove all the screws that secure the gas tank to the fairing. 
  4. Remove the left and right engine covers. 
  5. Remove the big bolt at the back of the tank.  Pull the fuel line off the petcock. 
  6. Pull the tank upward and note that there are a couple of vent hoses. 
  7. If you have removed your canister, one hose is short and plugged.  If you haven't, you should. 
  8. The other hose has a connector inline where you can "break" it. 
  9. Remove the tank and set it aside on top of something so that it doesn't hurt the petcock/nipple. 
  10. Remove the tank support bracket from the frame. Loosen the hose clamps which clamp BOTH the rubber carb boots and the Air Intake Boots. 
  11. You might want to use a flat screwdriver to sort of pop the lip of the boot up and spray some Armour All (tm) or Son of A Gun (tm) or other rubber treatment/lubricant in there to get the rubber unstuck from the metal and lubricated all around the lip. 
  12. Repeat for all the rest. 
  13. Wiggle the rubber, pull on the carbs and cuss a lot until they are free. 
  14. Pull them out.
  15. "Install in reverse order."

For more details about removing the Gas Tank see the Gas Tank Removal FAQ.


by Flash #412
October '01

Last night I stopped and filled up 0.8 miles from home. I was careful not to overfill. NO gas escaped into the "well" below the metal cap. I parked the bike in the garage. At about 4AM my wife woke me up because she smelled gas. There was a large puddle below my bike. I put a coffee can under the hose and mopped up the puddle. Thinking to improve the situation, I put the bike on the centerstand. When I did, another table spoon or more poured out. In the morning, there was almost an INCH of gas in the coffee can. I pulled the bike outside and parked it on the centerstand while I drank my coffee. When I went to leave, a half hour later, there was about a two foot diameter wet spot on the concrete under the bike. It was hard as hell to start. I stalled it at the bottom of the driveway. I happened to look back and see a trail of drips from the big wet to where I was. I rode it about eight miles and stopped just to check things out. No more drip.

I've smelled gas in the garage every night this week, even when parked with more than 100 miles on the tank. The temps lately have not been extreme. I think there was not such a great differential between the temp of gas in an underground tank and the temp last night. If anything, the air was cooler.

So what causes it.....Pukin' Petrol (The Answer!)

  1. When you remove the float bowl, there is a plastic assembly which consists of the floats and a holder.
  2. The float needle clips on to the floats. There is an o-ring on holder.
  3. This o-ring had a positive feel to it when I removed and reinstalled the holder.
  4. With the assembly removed, there is a bronze piece which is (or contains) the SEAT.
  5. This bronze piece also has an o-ring. However, BOTH of the seats just plain fell out when I turned the carbs upside down.
  6. I figured that if the o-ring did not have enough interference to hold the thing in, then it would not do much of a job keeping gas from getting by.
  7. I replaced the o-rings with some slightly larger (non-metric) standard ones I had in an o-ring assortment box which fit better than the ones I took out.

After putting it all back together, it started right up like it's supposed to, full choke & zero throttle. I rode around the block. When I got back, I put it on the sidestand and a few drops of gas came out one of the hoses on the opposite side from where it HAD been leaking. Being an optimist, I put a coffee can under there, turned the petcock off and left it overnight. All was well this morning.

BTW, I also installed the fuel filter while I was at it. There are four carb screws, which I replaced with Allen's. They are 5M0.8 x 12. But if you try to use ones that are too long, they won't go in the front holes, only the rear ones. See the Carb Cleaning FAQ for details and pictures of the Carbs.

Q. What are the Symptoms:

Q. Can (not should) you still drive with "Puking Petrol" syndrome.

All about Floats

by A Watson

Floats serve 2 purposes: First, they keep the carb bowl from overflowing by floating up against the float valve and shutting off the (gravity feed) flow of fuel. Second, properly adjusted, they maintain the fuel level in the bowl at the right height. This is important because the height of the fuel in the bowl determines how much fuel gets sucked up through the main jet for a given RPM, which in turn has a direct affect (along with needle size and adjustment) on the air-fuel mixture that reaches the cylinder.

All the wisdom I've heard is that floats, which are made of plastic (they appear to resemble extruded - closed cell - styrofoam), can over time absorb fuel and become less buoyant. (The way older ones and some newer ones may be made of copper, but that's another story.) Floats that are less buoyant will allow the fuel level in the bowl to become too high before they shut off the flow, even if they are properly adjusted. This in turn will permit more fuel to be sucked up, which will enrich the air/fuel mixture, which will affect fuel economy.

Floats are cheap and relatively easy to replace. As someone said, it makes sense to replace them now and then. 13 years seems a long time to me.

The post about weighing the floats was very interesting and this should permit a very precise determination of their condition/buoyancy. However, a visual examination can also help. If they're heavily stained, porous-looking or just plain cruddy, change 'em.

See also:  

Proper Choke Use

Q. Ok here is my question. Is the choke supposed to move by itself? I have found that when the bike is cold and I have the choke fully on and I ride aggressively (read rev to 7000 then shift then wide open again) the choke closes and often time the bike will stall going down the off ramp when I declutch. Is it supposed to move like this? If I pull the choke back it runs fine again. It can be a pain staling like that. Is this indicative of another problem? I do know you are supposed to let it warm up a bit before opening up but it seems to be warming up slower. Is that a symptom of some other problem and if so what? Thanks for all your help.

A. No, this is not normal. When the choke is off it is supposed to stay off. It should have a spring near the choke mechanism on the carbs that must be overcome to activate the choke. It this spring missing? The reverse problem is more typical, with the choke refusing to stay on because the tension is too loose at the choke lever on the handlebar. Usually it is not too hard to increase the friction of the plastic choke lever by placing some heavy grease or rubber cement between the choke lever and the adjacent housing. If that doesn't work, try using a small plastic shim between the choke lever ring and the mirror/clutch handle mount to provide some additional drag on the ring. Looking at my bike, I don't believe that there is a screw to tighten. I would just loosen the mirror/lever mount and move it over closer to the choke ring and see if you can put a little more friction on the ring. I don't have an answer about your idle, just turn it up to 1400 rpm and see what happens. Richard #230, Pacifica, CA.

Q. Which way is the Choke on?
The Choke is on when the Lever is BACK toward you. It often goes off by itself, rotating AWAY from you (when you are sitting one the bike).

Q. How long should I leave the Choke On?
Do not leave it on TOO LONG. You will foul your plugs, carbon your cylinder and head and and stuff up your fuel economy. Most folks can switch it off after half a mile to a mile, depending on the weather.

Choke Stuck?

If it is idling high, just Check the Cables. BOTH Choke Cable and Throttle cables.


Do I need a Carburettor Adjustment for High Altitude ?
by Kristian #562,
Dick #420, Mark #403, HsN, Richard #230 & Unknown

Going from Low to High Altitude:
Possible Symptoms:

You may experience rich running, stalling, hard starting and irregular idling at slow engine rpms at high altitude. The bike will also run rich at full throttle, because too much fuel will flow through the main jet. So do I need to adjust it:?

Going from High to Low Altitude:
Possible Symptoms:

You may experience popping on throttle slowdowns. i.e. popping/minor backfiring during compression braking/deceleration. BUT check your carbs are adjusted correctly, not worn and also check the Backfiring FAQ, Exhaust Gasket Replacement FAQ and Strange Noises FAQ. So do I need to adjust it?


Do I need to Synchronise the Carbs

Synchronization is possible with anything more than one carb. Two, as in F650's case can be synched and may not be noticeable to rider, other than less HP. I am assuming a bit here, because the 650 carbs may have no adjustment ability (I've not looked), individually, but my experience with other bikes has always been that the individual carbs can be adjusted separately. Usually a screw on the actuating bar, for each carb. Factory set, these are rarely off, but possible. Most Japanese bikes with multiple carbs have single cable, bar actuating all four as unit and they are synch'd as a regular service item. Single cylinder has no input here, except acting as a single, the input to rider will be less obvious in rough running vs. less HP. The domes on F650 carbs do have access ports. Two 33mm throats feed the 650 and less than two equally opened butterflies, will mean less HP. I imagine the settings are very close to perfect from factory. Comparison; My R100 only has two 32mm carbs! The 650 has 4mm more feeding power than the 1000cc bike! A lot going through a 650 and thus the 48HP it delivers and the somewhat equal mpg to the R100. The R100/7 delivers 58-60HP, as I recall, but with one more cylinder and very close to same carb size (total). The 650 breathes better with 4 valves, which creates more power, where the old boxer has two valves per hole. Anyhow, the 650 rams a lot through one cylinder, with two 33's behind the intake and I imagine without water cooling, high tech metallurgy, etc. the engine would burn up. One fun bike!

Fast Idle Problems ?

Bob #550
October '01

This is to share a possible fix for your pre-fuel injection F if the idle suddenly or unexpectedly refuses to drop down to normal.

Tonight I took the tank off my '99 F650 to see what was causing the throttle to hang up. What I found was that one of the electrical wire connectors located near the left carb had worked its way down to where it was rubbing against the throttle cable and had snagged the spring-loaded linkage at the carb body. I moved the connector back and away from the carb, and all was well...until I decided to check the diaphragms that control the needles in the carbs.

I took the top off the right carb first, and checked things out. The rubber diaphragm was just fine, and everything looked in order. Then I tried to refit the diaphragm and its top cover back in place. No go. After an hour of failed attempts, I finally smeared a little engine oil on the edge of the diaphragm, thinking that may help it slip into place. It did, and I was finally able to close everything up.

What is the Effect of Dropping the Needles.
Flash #412
December ‘01

Effect of Dropping the Needles: Flash vs. Matthew


CV carbs (sometimes called Constant Velocity or sometimes called Constant Vacuum) use the pressure differential between the two sides of a diaphragm to raise the slide. The needle is attached to the slide. By removing the snorkle, you lower the "effort" required by the motor side to raise the slide. You have effectively "raised" the outside air pressure by removing the snorkle. In other words, for a given manifold vacuum, the slide now rises higher than it did before you removed the snorkle. This means that the needle is allowing more fuel in for the same throttle-plate opening than it was before. To counteract this effect, you need to drop the needle, physically, one notch. You do this by putting the clip in the next higher notch, physically.


CV carbs operate of the velocity of the air passing through them. Removing the snorkel or opening up an airbox will slow the speed the slide raises. It will have no effect on the distance the slide rises.

Can anyone enlighten us on Practical Observation vs. the Theory.? (ed)

What are the Holes in the Bottom of the Slides for ?
Curt Martin
October '01

While I'm not even close to being a guru, I'll take a crack at answering your question. The vent holes bleed off vacuum that operates the needle slides.  Ok, you probably figured that out already. So why change the hole diameter?   To change the ratio of vacuum to slide position. You've had your carbs apart, so you have seen the slide return springs and know that the slides carry the needles as they move.

With that in mind, the chamber & diaphragm at the top of the slides gets a supply of intake vacuum, fed through the little hole at the lip of the cover.  The vacuum pulls the slides up, exposing more of the needle.  That vacuum is pulling against the return spring, and also being bled off by the vent hole at the other end of the chamber.

If that makes sense so far, then you'll realize that there are three ways to alter the needle position vs. engine load (vacuum):

1.)     Change the diameter of the vent hole. A larger will bleed the vacuum off quicker, smaller will bleed the vac off slower.

2.)     Change the rate the return spring. A heavier spring will have the same effect as a larger vent hole.

3.)     Change the position and shape of the needle.


You can use any combination you wish, provided it produces the results that you (or should I say, the engine) wants.

I've seen it done all three ways by different shops and kits.  I know one successful race shop that prefers to use the stock needles (straight taper) and slides.  He only changes spring rates and jet sizes.  Others,  Dyno-Jet kits included, use custom needles (stepped taper/adjustable position) and larger vent holes in combination with stock spring rates.  And some just alter the needle shape while leaving the vent and springs alone.

The results are all that matter.  (btw, this is usually a mid-range adjustment. Low range is still governed primarily by the pilot jets & float height, and top end by the jet size.)

Flat spot in carburetion?

Refer the Canisterectomy FAQ, Idle Mix Screws FAQ, Poor Mileage FAQ, Carb Clean FAQ.

The following responses are for PRIMARILY from people with Stock setup. If you know or suspect you have a Jet Kit, please also refer the Performance Modifications FAQ.

Slide Barrel Wear

Morgan Seim has a solution for the G's carb crappiness. Not for the F650 but interesting all the same.

“This letter is the culmination of many hours of trouble and tinkering with Ellen's 1100G's carbs. Riding my new 1200 Bandit back to back with her 1100G really highlighted the gross carb troubles her bike had with the original carbs.  You may have read my earlier "fixes" on the Worship page, little did I know how wrong I  actually was.  My "fixes" were more of a band-aid that made the bike run better, but didn't cure the root cause.  A letter sent in to the November 2000 issue of Cycle World magazine's Service section by Sasha Shapiro of New Paltz, New York, describing the exact symptoms I had experienced prompted me to finally get hot on the problem.  I must note that Cycle World's response was way off target!  ( I will be sending them a letter regarding the true cause of her troubles.  I contacted her, and she eventually replaced her carbs, curing the bike.) I finally got fed up enough to do some serious research, and I found a letter on the Web from a guy in England that said he had noted emulsion tube wear on his 1200 Bandit, and after replacing them, it cured his low speed mixture problem.  I reasoned that with 56K on our bike, we must have this problem as well.  I rushed out and bought new needles, emulsion tubes, and slides, confident that I had solved the problem at last.  Wrong!  The bike ran no differently, which prompted me to completely tear down the carbs for some direct inspection of my own.  After getting the carbs apart and taking some careful measurements, I finally found the true problem, which is the plastic pieces that the slide mates with.  This piece eventually develops an uneven mating surface with the slide, which leads to needle and emulsion tube wear, and inaccurate air and fuel metering at idle and low speeds.  I also noted that the problem can cause sticking of the slide if the wear is severe enough, since the wear results in such a misalignment of the needle and the emulsion tube that the side load on the slide from intake vacuum binds the needle in the emulsion tube.  The original needles I removed from Ellen's bike had two steps worn into them, causing the slides to not fully lower as well. I had always thought the Bandit carbs would work on the  1100G, so I called American Suzuki, and one of their reps said he had both a Bandit and an  1100G, and had put Bandit carbs on the 1100G.  He said I needed the Bandit intake boots, and I had to use pod type air filters.  It turns out he was incorrect, it's a direct bolt-on.  The used carbs I put on Ellen's bike supposedly had about 4K on them, and they looked like that was accurate mileage.  There is always the unconscious expectation when you work on your  bike that it will run better since you have just expended so much effort, and I think this can sometimes create false expectations.  Before putting the used carbs on her bike, I opened up the mixture screws a bit, and raised the needles about .040".  After getting her bike all buttoned up, I

anxiously started it, and it idled perfectly.  I was so relieved I was nearly beside myself with disbelief.  Of course, when buying anything used, you are at the mercy of the guy at the junkyard, who usually has a different perspective.  To him, if the donor bike was in good shape, it must be low mileage, but fortunately, these carbs actually were.  I suspect the wear problem with the plastic pieces starts to affect carburetion around the 15-20K point.  I think not everybody notices how bad their bike has begun to run, since it is a very gradual degradation, and some people are not as particular about their vehicles as I am. Here's the arrangement with the carbs and why I haven't had any luck getting

the parts I need.  Suzuki buys the carbs from Mikuni Japan, and in order to get the part that wears out, you have to buy the entire carb body for approximately $350, and of course you need four of them.  Dealer reps told me to contact Mikuni, so I did, and it turns out that Mikuni Japan is separate from Mikuni America, and Mikuni America said they couldn't help me. So far, Suzuki of North America, Mikuni America, and all the dealers I have talked to have said they can't help me.  Their stance is that the part is available, you just have to spend $1500 for four carb bodies, that's all.

No, I haven't contacted Mikuni of Japan, and at this point I don't expect a different answer from them either. Since the supply of good used carbs is finite, I have developed an inexpensive way to correct the worn plastic pieces by re-facing it with brass, and at this writing, I will have the first prototypes installed in a bike in about two weeks.  It is a rather labour intensive undertaking, but it sure beats $1500 for some plastic pieces that should cost $10 each.  So far, I have found about seven different types of bikes that use carbs with this same configuration, mainly big Suzuki’s and the fix is applicable to all of them as well .(Pretty much any bike with 36-38mm Mikuni's.) Please post this ASAP, I welcome correspondence from other Worshippers. Happy Trails, Morgan."

Carb Alternatives (Complete)
by Mtbiero (Cugino Pegaso)

I found a pair of 39mm Keihins here: http://www.rotax.net/BMW_F650.htm for $760:

and more cheaply here: http://www.pro-flo.com/proflo_keihin_fcr_carb_kits.htm#BMW for $625

However I think dual 39's is best for all out racers, and feel the dual 36mm Mikunis offered here:
http://www.team-pami.de/ENGLISCH/index.htm for 448 euros ($392) would work best.

Since the F650 and Pegaso share dual Mikuni BST33's, I think the F650 carbs would work on my Pegaso.

Info I got from Team Pami:

PAMI Performance Kits
by Mtbiero (Cugino Pegaso)

I don't have the entire kit (won't work with my Pegaso) but I do have a set of team Pami 36mm Mikuni radial slide race carbs for a F650 on my Pegaso. Just the carbs alone gave me a very noticeable power boost, more top end, and allows me to cruise the bike at the 2000 to 3000 rpm range without any lurchiness...

When I first put them in, I used my stock airbox, when I finally removed it and went to Uni filters, the bike really woke up. Right now I'm still trying to get the idle right, I messed up, I assumed the idle mix screws should have been at 2 turns out nominally. finally got a tuning manual from mikuni.com, that says unlike every other carb Mikuni makes, the RS idle mix screws range is 1/8 to 1/2 turns MAX! Been wrongly going to leaner idle jets to compensate, and causing other issues. I'm back on track and expect to have them idling perfect after about one or two more iterations. The nice thing about removing the airbox is I can have the carb out and on the bench in 6 minutes as opposed to 1 hour before!

Alternative Carb Parts:

OEM Parts List:
OEM Carb is a Dual Mikuni BST33-B316 CV carb

For the diaphragm try one from a Suzuki GS500, part number 13507-17c01. I don't know if the other parts are the same or not.  Charlie #070 from Pennsylvania.

Some parts are exactly the same parts as a Suzuki GS500E. Note the GS500E uses Mikuni BST33 x 2 Carbs.

These are from a Suzuki GS500E

For GS500E


The Aprilia Pegaso (Sister of the Funduro), also uses the same Mikuni BST33 Carb as the Funduro.


How do I get Water in the Float Bowls?

Q. How did I get water into my float bowls?

I emptied the fuel tank, I replaced the green gasket under the filler cap but somehow I still get water into my float bowls. Also, I don't have a cannister. Are there any other ''suspicious" passages for water? The problem arises only in wet weather (under rain) and cold. Then, the bike drops its idle and stalls or surges as if it runs out of gas.

Q. How can I drain water from my float bowls?

Fuel Line Replacement
by Chris in Santa Cruz, CA

It is possible to replace the fuel line without removing the carbs. You must have long fingers. You remove the rear shock preload adjuster (this ones an Ohlins, but the location is the same), grease the hose a little, and do without the clamp. I will take the carbs out at some later date to reinstall the clamp. Meanwhile I will inspect the connection for a while. I used 6mm Gates PVC fuel line.

This MAY allow you to remove the OEM In-line Fuel Filter. Maybe. If anyone can confirm this, great. The inline fuel filter is in the brown T-Piece connector between the Carbs. Refer the Carb Cleaning FAQ for more Details.

Fuel Lines: