Carburettor Cleaning FAQ
by Kristian #562
Please read the Disclaimer before attempting any work in this FAQ.

Looking for the OEM Inline Filter ?  or the Aftermarket Fuel Filter?

Please Note that this FAQ only deals with the resetting of your Carb to the STOCK configuration. Re-Jetting, Needle Changes and Modifying Float Heights are covered to some extent in the Exhaust-Rejetting Spreadsheet. Have a look there if this is the kind of information you need. Nor is this a treatise on how a Carb Works. There are lots of references about that on the Web. I will try and add some links I think are worthwhile references, at a later date, as I come across them. Perhaps you might like to suggest some too. Dynojet has pretty good simple one here: Dynojet’s Carb Theory. I need to refine some parts of the pictures and circle some specific parts of the photos, and I need a few additional pictures. Write to me and tell me what you think would make it better.

Carb Nomenclature:

Note that Slide is also often called Slide Piston (or Piston) or Slide Barrel. The Slide Carrier is often just referred to as the Carrier.

Why should I clean out/re-condition my Carburettor?  


  1. If you've read the Hard Starting FAQ and the Poor Mileage FAQs and nothing helps.

  2. When the bike has been standing a long time and it sputters and stalls and pops and wheezes (but try some carb cleaner in a tankful of gas first! ) and you’re worried (or you know) the rubber O-Rings have deteriorated.

  3. When the bike falls victim to the Pukin’ Petrol Syndrome and you have to take it out anyway for that reason.

  4. When the bike “bogs down” at  anything above just past idle, a sign of a Split Diaphragm and you KNOW it’s not your Voltage Rectifier or Plugs or Plug-Caps.

  5. When you are getting Poor Gas Mileage.

  6. When you are Rejetting (Warning, Rejetting is a long slow process and you may NEVER be satisfied with the results).

  7. When your bike seems to Surge or Stumble (even if it is NOT an F650GS) and you KNOW it’s not your Chain/Sprockets or a dirty Air Filter.

General FAQ’s about the F650 Carbs

Q. At what mileage can I expect to get a reduction in mileage due to worn Carb parts?
A. This is an extremely difficult question to answer, it depends on whether you ride in dirt, road or at cruise revs or high revs. Some inmates have noted a decrease in Gas Mileage around the 20,000 miles mark.

Q. So what gas mileage should I expect to get?
Again this is an extremely difficult question to answer, it depends on whether you ride in dirt, road or at cruise revs or high revs. Some inmates get 45-50, some 50-55, some 55-60. If you get less than 45-50 with a stock bike, normal around town riding on the road, and you have a high mileage bike, it’s probably time to check the Carbs. Same goes if you get less than about 50-55 on the open road. Refer the Poor Mileage FAQ for more details/feedback.

Q.  Who makes the F650 Carb?
  Mikuni. They are 2 x 36mm Mikuni BST B316 CV (Constant Velocity) Carburettors. There are two on the F650, side by side.

Q.  Is there another bike with the same carb. from which I can scavenge cheap Bike Parts.
  Yes, the Aprilia Pegaso, a 5-Valve Aprilia version of the bike, made in the same factory in Italy. It’s not identical though. You can also get SOME parts from the Suzuki GS500E Carb, which is very similar. Refer Alternative Carbs & Carb Parts for details.

Q.  Is there another supplier that supplies parts?
A.  If it’s just Main and Pilot Jets you are after you can probably get them from your local Bike Shop. Ask for Mikuni Jets and show them your old ones, as there are several different head Styles. I know you can get them from to Sudco-Mikuni in the U.S. however Sudco do NOT supply the needles. They say they were a special run for BMW and I’ve found the only supplier to be BMW or a BMW Agent. However Dave #365 (Thank you Dave !) got this precious Gem:

Dear Sir,

We checked your question. Your F650's BST33-B316 original slide springs are the following compatible part No's. Please order Yamaha or Suzuki your local dealers.
It is faster than delivery when you order to BMW or Rotax (engine maker).

Yours faithfully,

Mikuni Corp.
International Dept

See also Alternative Carbs & Carb Parts for details.

Q.  Do I need to “Balance” (Synchronise) the Carbs.
A.  You have probably read somewhere about needing to “balance” the Carbs. This really applies to multi-cylinder engines where individual Carbs feed individual Cylinders. You do NOT need to “Balance” the carbs on the F650 if you set the “Settings” (Needle, Float/Jet Size/Mix Screw)  the same, for each Carb. It’s a single cylinder engine. For more information refer The Carb Misc FAQ.

Q.  What are the STOCK Carb Settings?
The Stock Carb Settings are:

1993-1995 Models:  Thanks to Spakur & Fede

Q. What are the Carb Parts Numbers?
Steve#417 supplied these: (Thanks Steve)

        Here is the OEM Parts List:

Refer also the Carb Alternatives

Q. Are there any Websites to tell me how to tune my Carb?
Well the best thing you can do if you have a Stock Setup or even Stock Carb with an aftermarket exhaust is to Check for and Replace all the worn parts discussed above and set the Carburettor Floats/Idle Screw/Clip Location to the specified Stock Settings. Otherwise:

Q. What Parts of the Carb get worn?
Pretty much all the moving parts or parts against which moving parts work, like any mechanical device. However there are some parts which don’t move that deteriorate with age, some to a greater extent than others. The parts that most often wear or deteriorate are the Carb-O-RingsNeedles (shown are the Stock (L) vs. Dynojet (R) needles), Diaphragm Rubbers (shown on top of the Slide), Slide Carrier (Grey Plastic Block) and the Slide (Black barrel with two Side Fins). The jets (Main Jet, Pilot Jet) do not really wear, except for the so-called Carb Venturis or “Needle Jets”, due to the action of the needle vibrating within in it. Here's a link to an Oblong Venturi Jet, on Factory Pro's Website, to give you some idea of the wear that can occur in the needle jet. It is this wear, along with the corresponding rubbing/scratching of the needle. See Worn Needles, also on Factory’s Site. However if the gas is dirty the Main or Pilot Jets can also get blocked (fairly common) and the hole CAN wear bigger, although this is not that common. The Float Valve Tips and or Springs can also wear, but take a bit longer than other parts.

AND.... Don't forget to check the rubber intake manifold for cracks as well as the rubber Air-intake boot.!

One thing you might also want to watch out for. If you've been using Coolant with Silicates in it, (which you should NOT as it will destroy your water pump), you might accidentally spill some into the intake snorkel when you fill your coolant and the fine Silicates that may pass through the Air Filter will quickly wear your Carb Venturis and Needles.

The sizes of the Four Replaceable O-Rings, (4 in EACH Carb i.e. Total 8) available from any bearing shop are, Smallest to Largest:

Q. What about alternative Sources for the O-Rings?

Q. How do I adjust the Idle/Adjust the Idle MIX Screws ON the bike. (Note they ARE different).
A. Refer the Idle Mix Screw FAQ.


So how do I get the Carb Out ?

1.     Complete Carb Removal



  2. Remove the Seat.

  3. Remove the Tank See the Gas Tank Removal-Replacement FAQ.

  4. Remove the Seating Plate at the Rear of the Tank, held in Place by two 10mm Bolts.

  5. Undo the 2 Hose-Pipe Clips on the Air Intake Side (Rear of Carb) and the 2 on the Cylinder Head Side (Front of Carb) and Remove the Carbs.

  6. For Carb Removal, see Flash’s FAQ. This is a bit hard on the rubbers and you might think you’re doing some serious damage, but if you do it right and don’t use a sharp screwdriver, it’s fine. The alternative is to remove the exhaust pipe and take out the Airbox, which is VERY time consuming and a Royal PITA. When you do it, in addition to Flash’s great comments I’d recommend pushing each of the Clamps back away from the Carb as you can and also to wear thick cotton gloves for your hands as the Metal edges, specially the Airbox Side are a bit Sharp. Lift the Air-Intake Side up first and get them past the thinner and more pliable Air-intake rubbers, by pushing the rubbers down past the Carb, while simultaneously pulling the Carbs up and out.

  7. Unlatch the Throttle Cable. To do this first twist the throttle (at the Carb) against the spring, then with the cable now slack pull the little steel bend out of the socket in the side of the Carb. With the wire now slack, but still attached to the throttle at the Carb, just rotate the cable in its holder at the throttle-mounting until the cylindrical head and the wire come free.

  8. Unscrew the Plastic 12mm Nut to the Choke Cable and pull it out gently. You can only use an open ended spanner. Do NOT turn it the wrong way and screw it up tight now (or later) it’s just a PLASTIC Nut and will Break.! Give the Choke Cable Barrel a bit of clean with some “Jif” or some other polishing fluid. If you want to lubricate the choke cable, this is a good time to do it. Recommendations for lubricating cables run from (a). It’s not required because it’s ion a plastic Sheath (I subscribe to this notion, especially for Choke Cable) to (b) Tape a SMALL STRONG plastic bag of cable lubricant around the cable then hang it up higher than your handlebars, leaving it overnight.

  9. The Carb is now free for you to work with in a nice warm comfortable and CLEAN environment, all of which are highly recommended.

  10. Bear in mind the Carb Bowls are still full of GAS, so undo the Float Bowl Screws (Circled Red) a few turns and rotate the Carbs around until all the GAS you can get out of it comes out. DON’T SMOKE.


A2.     Removal of the Diaphragm Cap only (on the Bike): 


  2. Remove the Seat.

  3. Remove the Tank See the Gas Tank Removal-Replacement FAQ.

  4. Remove the Seating Plate at the Rear of the Tank, held in Place by two 10mm Bolts.

  5. You now have access to the Diaphragm Caps, which after removal allow you to check and replace: The Diaphragm, Slides (But NOT the Slide Carriers), The Needles, Springs, Needle E-Clips, Needle Spacer and Washer. This also allows you to change the E-Clip position or add a small thin washer, for an equivalent ½ E-clip position change, all without removing the carb from the Bike. Great for fine-tuning Rejetting. Not so good if it’s snowing outside and your bike lives outside.

Q. Should I run the carbs dry by turning off the petcock and running the engine until it dies, before I remove the tank? Or, when you remove the tank and drain the fuel hose going from the petcock to the carbs, do the carbs drain too? If not, is there a reason this is not mentioned in the FAQs? (Perhaps work on the carburettors is easier/better with their "bellies full")?
A. No. You don't need to run the carbs dry. After you turn off the Petcock, if you are REALLY paranoid, you CAN empty the bowls using the drain screws if you really want to. (The Drain Screws are here
Float Bowl Drain Screws (Circled Red). Wind them out a few turns, either on the bike beforehand (or after you get the Carb's out and rotate the Carbs around until all the GAS you can get out of it comes out). DON’T SMOKE.


Enough of the Preamble, How do I actually Clean & Replace Carb Parts.



Q. How do I undo those Float Bowl Screws!
A. This is a good place to discuss the 2 Screws holding on each Float Bowl.

On with the Cleaning.



Diaphragm Trouble


Diaphragms don't go back into place after lifting off the Black Covers?

I lifted the diaphragm covers today in order to check the diaphragms and needles out, as I've been getting rather few mpg. I have no idea when the insides of the carbs were changed out the last time, but I guess it's probably been done since the bike's got more than 62,000 kms on it. The needles looked brand new to my (untrained) eyes though. So did the diaphragms, and thorough checking revealed no rips or punctures of any size or kind. HOWEVER, I did get a bit puzzled over the fact that my diaphragms are way too large to fit in the circular indentations they're supposed to settle in before you screw the covers back on. (They were indeed pinched under the covers by whoever screwed them back on previously). I've checked the FAQs, but I only found this (from the carb cleaning faq):

"Press the Diaphragm into the Groove on the Carb Body and make sure it (more or less) stays in. It might try to pop out, but don’t worry when you put the Cap on it will slip into the groove, so to speak. So put the Cap on and slide it around a little bit on the rubber, just to make sure the rubber is down in that groove and you haven’t pinched it somewhere. "

My problem is that it is positively impossible to get the entire edge of the diaphragm into said groove. My diaphragms are at least a couple of millimeters too large (across) to fit, and after 45 minutes of trying to get the edge to seat in the groove all the way around, I gave up. I had to screw the covers back on, pinching both diaphragms in the process. Looking from above, either diaphragm would protrude from under its cover for maybe a fifth of the total circumference.. this can't be good. Has anyone else experienced this problem? Could it be that whoever replaced the diaphragms before me got the wrong size? In your experience, is the diaphragm exactly, or at least almost exactly, the same diameter as the groove it's supposed to sit in? I mean, this was NOT a case of the rubber "trying to pop out".. I can't see how anyone could have managed to get the entire edge down there, let alone keeping it down while reaching for the cover. Last question: could this be (part of) the reason why my mpg is low? I'm imagining that pinched diaphragm edges could be fairly equivalent to rips in the rubber. No, there was nothing sprayed on them. Also, they were already pinched before I even started unscrewing the covers, so the previous attempt to get it right apparently failed, too.

I called the dealer today. It appears to be a well-known issue with older diaphragms, they simply grow larger with age, due to contact with gas fumes. The mechanic I spoke to thought my diaphragms are the original ones, which would make them almost nine years old. (They are approximately three millimeters too large, across.) He also said getting older diaphragms back in the slot is a major PITA. They wanted $18 per diaphragm (must have remembered the price of some other part), so I called the local Suzuki shop for a quote of the GS500 diaphragms... which were priced at almost $40. So I ordered them from a BMW car dealership instead, saving $2 compared to the bike shop.. guess I'll use my savings and buy a snow cone or something :) Emil '94 F650 -- Malmö, Sweden.

Q. How Do I Replace the Carbs.

A. With Difficulty, Patience, Cotton Gloves and Vaseline.  


Putting them back can almost feel more gruesome than taking them out. You think your ruining something, but the rubbers are pretty hardy and with a bit of patience you’ll get there. I suggest you use some of the lubricants Flash mentions on his removal procedure or coat the inside of the rubbers (Both Airbox and Engine Side) with Vaseline. Reconnect the Choke Cable BEFORE you start pushing the Carbs in (You can do it afterward but it’s very hard to get to the plastic nut.) Again, please, do NOT over-tighten the Nut. It’s just Plastic! The Throttle cable can be reconnected after you get the Carbs back in, but you can do it now to save yourself the trouble. Affix the Wire-Head to the Throttle-Plate then twist the throttle Plate (or pull the wire) until there is enough slack to get the bend back into it’s mounting. Work the Carbs down into the cavity, again pulling back the softer Airbox-side rubbers, to fit the Engine Side first. After you get the Carb ports mounted into the Engine Intake Manifold Rubbers (They are stiffer and relatively easy), the Airbox side rubbers will probably still be all squished up. Try and free the bottom first and get them fitted to the Carb, then with a finger or BLUNT instrument (e.g. Flash notes a Tyre Iron works well), working from the sides towards the top, stretch the rubbers over the Airbox Side Intake Ports. These Ports are not that wide, only about the width of the Steel Hose-Clamps, so after you get them in place pull the whole assembly Horizontally BACK toward the Airbox so you know they are as far on as they can go. THEN do up both sets of clamps, lining the clamp heads up so they are easy to access with a screwdriver.


Replace the Tank Seating-Plate, making sure the choke and throttle cables drape OVER the top of it and they are free and not caught anywhere between the Carb and the Throttle on the Handlebars. Replace the Tank as indicated in the Gas Tank Removal-Replacement FAQ.


That’s it.! Sounds long and arduous, but in reality it’s not, I just wrote a lot of detail. I can get the tank off, whip the Carbs out, change the Jets and Needles, put it back and get the Tank back on in about 45 minutes, during my lunch hour and still have time for a sandwich. But I’ve done it way too many times. I’m not suggesting you should hurry through it though, take your time. A job worth doing is worth doing right. It’s a job you can do in a few hours over the course of a leisurely day.


Any comments on the FAQ, improvements or suggestions, drop us a line. For all you poor people with F.I., aren’t Carbs FUN.!


Thanks to Flash and Todd, who always manage to be both entertaining and informative, to Mark for getting out there and doing it and to Craig, Tom and the other guys who went and are still going crazy trying to get the jetting right, having pulled their Carbs for the umpteenth time. Big thanks to Mal for his great close-ups of the Float Valve.

Note.: I have no affiliation with Factory Pro who are mentioned often in this FAQ, though they do have some very good tips. They didn’t like it when I started asking questions about their jet kits without buying it, but hey if you can’t ask about something they want to sell ! Others have had a good experience with them and find them helpful.

Cheers & Rgds




Is there any way of Setting/Checking the Float Height with the Carbs ON the bike?

by Richard #230

The Downtime Files of the April issue of Motorcycle Consumer News has a really great tip for checking float level height without removing the carbs: Connect a piece of clear tubing to the float bowl drain tube, bend the tubing up against the outside of the float bowl and turn on the float bowl drain screws. The gas will rise in the tubing and you can take your measurement by observing the top of the level of the fuel in the tubing, as it will rise to the same level that is in the inside of the float bowls.  The Classic has bowl drain taps on the bottom of the float bowls that you can hook up a plastic tube to. The only problem would be finding clear plastic tubing that will fit tightly over the drain nozzles. I think you could find this tubing in a pet fish store. If the tubing was a bit large, you could use wire wrapped around the tubing and twisted with pliers to compress it over the nozzles so that the fuel wouldn't leak while you were checking the float level.


The best way to use the tip is to check your float level when the bike is new, or at least running well. Place the tube on the float bowl drain and then turn on the bowl drain tap. Wait until the level stabilizes, then scribe a mark on the bowl for future reference. Then in the future, if you have a carb problem, you can perform the test again and check it against the mark. If it is higher than the mark, then you might have a leaking float or bad float needle valve.

Carburettor LINKS                                                                                                                              


Some good  resources for Jetting and Plug Reading Information - Not F650 Specific.





Tuning CV Carbs: Mikuni-Sudco recommends tuning the circuit from the Pilot upwards as they say  each circuit is additive. Factory recommend starting with the main and going DOWN. I cannot put  copies of the information on this Spreadsheet as Sudco has a copyright on this information. It's  about a 13 US$ Manual from Sudco.        Note: Actually for Harley V-Twin with CV Carbs, but general information is useful.  On Carb Circuit Ranges        On adjusting the Carbs  On Harley CV Carbs Mods      

 Also on Harley CV Carbs Mods      On Spark Plugs and Spark Plug Reading    On Reading Spark Plugs, with colours of Plugs 
Dynojet        On Carb Circuits in general       
Detonation  More Detailed information on Fuel Detonation 
Carb Circuits    Diagram of Overlapping Carb Circuits - See Diagram under needles & jets 


Opinions (On Carb Cleaning/Particular Parts Replacement).


Some of the comments below might seem to indicate the 14.6mm measurement is no good. But if it is measured properly, I believe this measurement is fine. At present there is just not enough feedback to say either way. So set it stock, measuring it right and if it doesn't work for you, (runs outta gas at HIGH rpm/flat out speeds), LET US KNOW!

Detailed Feedback: on Carb Reconditioning


Rejet of the 97 F650 Carb with Tech Tips/Problems/Tools/General Whining

by Langlois

First: Get your airbox out...see the F650 Faq, remember to take the muffler off, much easier with it out of the way. I struggled for a while and could not get the angle right to pop it out without the pipe removed (the headers stay, the muffler is removed with the cross-over/connector pipe). NOTE: You do NOT need to remove the Airbox. Just pull the Carbs (ed).

Second: Have a cup of coffee, I am sure you just spent half an hour dicking with this and you will need your wits about you for the rest of the job!

Now, see photos:

Carbs still hanging from Throttle Cable still attached, there is a little room to mess with the carbs.


Plug the intakes with paper towel, keep the nuts/bolts/washers/birds from getting in!



Here is what you see to work on the jets, the float bowls.


Ideally you will be able to use a Phillips Screwdriver drop the float bowls...but......sometimes they are frozen...



Float Bowl Removal for difficult screws, vice grip on screwdriver......


When that does not work, soak with your favorite penetrating oil.


Then latch onto them with a mini-vice grip and ruin them completely. (see bottom of page for contact info on nice new stainless ones!)




Floats off, note bubble wrap and genuine factory-tool plywood plank.

Skinny screwdriver for the jets

Stubby one for the others

Diaphragm with slide spring

Note needle with lock ring and 2 plastic bushings (4th from the top for those who asked!)

Eurosport Jet Kit as sold by Factory Pro Tuning, these guys are AWESOME, they really helped me out when I bunged up my float screws and went the extra mile for me to get some replacements (photos of the jets and screws below).  go there and spend money.


Nicely packed Jet Kit w/decent data sheet.

Stainless Carb Screws from Factory, these guys overnighted me the parts in the afternoon of one day and they actually got here before 10AM to finish the project. While this might seem common to you, I have had SOOOOOOO many supposedly "overnighted" parts show up a week later...

Tasty new SS fasteners, being the "coal to Newcastle" kinda guy, I ordered 2 sets. 5*16 and 5*12 for those who need to know!

Installing with gobs and gobs and gobs of Antiseize.



Looks purty now! See the gold anodized bar that holds the carbs together? Be sure to remove those screws and hit them with antiseize too.


This is the flexi-gas spigot that is a pain in the you-know-what to get a line on when your old, stiff, factory gas line decides that it wants a new home. Unless you are a hampster, you cannot get under the carbs easily to put it back on. My fix was NEW line with a REAL screw type hose clamp (no photos sorry, but get a close fitting clamp to fit in this spot)


Sporty Stainless Diaphragm screws with liberal smearings of Anti-Seize.



Why can't I get the Airbox rubber intakes to fit over the carbs after jiggling it for 20 minutes???


Do this (you did remove the snorkel right? you did loosen the clamps right? you also lubed the rubbers right?)

1: Take hand, remove watch, grab screwdriver/hooktool/whatever.

2: Insert hand, arm and tool into airbox.

My hand and tool is well hidden but I am pushing up on the left side carb rubber that is being problematic with my screwdriver tip...

Below HERE  |  




Finally, I cannot recommend the guys at enough, these guys saved my butt. (the SS screws and jets are nice too!) But, saying that, jets are can buy them everywhere (almost) it's the service you need during and after the sale, they most certainly provided that!


Identifying a Float Valve Problem