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How To: Carburetor Rebuild

Old 01-15-2010, 12:15 PM
alpiniste's Avatar
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Posts: 24
Default How To: Carburetor Rebuild

I have a lot of people asking me why their bike does not run right. If my answer is carbs, 9 times out of 10 I hear “I just cleaned them” in return.

Cleaning your carburetors is not a difficult task, but you do have to do a thorough job to make sure you don’t have to pull everything apart again. I have seen several topics with incomplete or wrong information on the matter, so I decided to share my knowledge with everyone here.

Before you start, you need to go to a store, such as a car quest, and get yourself some Berryman’s Chemdip. It is expensive, 30 bucks for a gallon, but it is the only way to be sure that the crud inside the carb body is dissolved and removed. If you get a gallon then you might have to take the individual carburetors off the rack because you can only fit two carburetors in the can at most. If you can afford it, get the 5 gallon jug of it. It does not go bad, and you can use it in the future.

I have heard of and seen people using other chemicals like brake cleaner, carb cleaner and even WD40. Don’t waste your time with those. These simply were not designed for this job.

1. Remove the carburetor assembly from the bike. This may involve removing your plastics, the gas tank, the air box and sometime require some elbow grease if you are working on a V4 engine or something of a similar configuration.

Shut off fuel and remove the gas line as well as the vacuum line which activates the petcock (if equipped). Remove throttle cables and choke cable.

Loosen the clamps (engine manifolds and air box)

Pull the carburetor assembly out.

2. Take the cabs to the work bench or a suitable work area. I have an oil pan handy and open the float bowl drain screws to empty the carbs of any gasoline remaining the float bowls.

3. start at the top. Remove the vacuum slide cap (usually held on by 4, sometimes 3 screws). Work in sequence, first remove the left most carburetor’s cap, spring, vacuum slide, remove the needle, remove the air jets (if any found on the carburetor body), them move to the next carburetor.

***Check the rubber diaphragm for tears. If any found – replace!***

4. Once you have finished disassembling the top of the carbs, move onto the bottom. Remove the float bowl, the float pin and float, the float needle and the seat (some have a screen attached to them), the pilot jet, the main jet, the main jet emulsion tube and collar. Put all parts from this carburetor into the float bowl. As you go se aside anything that is made of rubber: the float bowl needle, remove the o-ring and the screen from the float needle seat, remove the float bowl gasket/o-ring. These rubber parts will not get dipped as the solution dissolves rubber.

5. Remove the enrichment circuit bracket (which connects all enrichment circuit pistons between the individual carburetors) and unscrew the enrichment (choke) pistons. Clean those by hand if required. Most times these do not require attention.

6. Each constant velocity carburetor has Idle air/fuel mixture screws. The screw is located underneath each carburetor outlet, the side closest to the engine. These are set at the factory and then capped with aluminum caps as an EPA standard. You have to drill these out. Use a reverse bit that is close to the diameter of the cap. Drill slowly, once you are far enough into the cap, it will come out with the bit exposing the screw. With a flathead screw driver, turn each screw in until lightly seated. Count the number of turns – you will need to set the screw the same at reassembly. Do not tighten the mixture screw, it will break off and get stuck in the carb – then it’s time to get a new carb. Take out the screw, the spring, the washer and the o-ring. Do not loose the o-ring, those are hard to find. In fact, do not loose any parts because they might not be available for purchase depending on your model.

7. Omit this step if you have the 5 gallon just of chemdip.

Mark every carburetor 1 through 4 according to its position the rack. Mark each float bowl and cap accordingly. Note the size of each jet as they may be different from one carb to the next.

Find which bolts hold your carburetors together. Loosen these bolts and separate the carb assembly carefully: each carburetor is connected to another by a spring and adjustment screw. Remove any connecting vent and fuel tubes, which may have rubber o-rings. Remove the o-rings and put them in your rubber pile/container.

8. Put all jets, screws, carbs and anything else metal into the tray. Don’t worry about mixing up parts since they are all marked and you know what size jets go where. Submerge the basket in the chemdip for at least 2 hours. Note that if you leave your parts in for longer than 24 hours, the crud will start to solidify again.

9. Take the basket out and let it drip dry for a few minutes. Take the basket to a sink and wash every part under scolding hot water, as hot as you can stand. Be sure not to loose any parts to the drain.

10. Blow off every part with compressed air with a blow gun. Pay particular attention to the carburetor body, jets and emulsion tubes. Blow through every passage in the carburetor, ensuring good flow through the carb. Look through the jets and compare them one to another. Make sure they are clean.

11. Assemble in reverse order, take your time. Take pictures during disassembly to help you with assembling the whole thing back together. Once you have the carburetor rack back together, adjust the synch screws. Eyeball the throttle plate opening to be about the same all across. This will also give you an idea which screw controls which throttle plate.

***Be sure to check, and set if necessary, the float height. The procedure and specification can be found in the service manual.***

12. Reinstall the carb rack back on the motor. Connect throttle and choke cables, fuel and vacuum lines and so on.

13. Open the petcock and start the bike. It might take considerably longer since the bowls are completely empty of gasoline.

14. Warm up the engine, set the idle as per service manual.

15. Synchronize the throttle plates. This step is required to ensure crisp throttle response and an even, rhythmic idle. This procedure is done to all carburetors, fixed venture, constant velocity and mechanical slide. Even if you were able to get the throttle plates set exactly the same, synchronization is required because no two cylinders draw the same amount of vacuum due to slight mechanical differences – valve clearance, blow-by, exhaust length and so on.

16. Reassemble the rest of the bike and take it out for a spin!!! And don’t forget to wear your gear!!!
Attached Thumbnails How To: Carburetor Rebuild-cvcarbflow.jpg  

Last edited by alpiniste; 01-15-2010 at 03:52 PM.
Old 01-31-2010, 08:42 PM
Junior Member
Join Date: Jan 2010
Posts: 2

I'm about to clean my carb using your guide, 1996 CBR 600F3, do I need to replace the O rings and other rubber?? whats is involved in rebuilding the carb???
Old 04-20-2010, 05:42 AM
Junior Member
Join Date: Apr 2010
Posts: 3

im trying to clean mine as well i have a 97 f3 and im having trouble getting them off the bike?
Old 04-20-2010, 07:35 PM
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Join Date: Jun 2008
Location: Plymouth, MA
Posts: 19

You did loosen all the clamps? They can be a pain to get loose. I had to pry mine off. I couldn't get them loose by hand.
Old 04-26-2010, 06:01 AM
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Join Date: Apr 2010
Posts: 3

yeah i tried to lossen the clamps but couldnt get to them all didnt know if there was any out there that had any good ideas as to how and as far as changing the rubbers and o rings do it its a best if you do
Old 04-26-2010, 01:27 PM
johnnyx's Avatar
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Join Date: Apr 2009
Location: Chicago, IL
Posts: 1,641

Yeah, they're a BITCH to get off. Loosen all the clamps, and just lift while rocking the assembly back and forth. Helps to have a friend assisting.

Here's a PHENOMENAL HowTo on the subject, with tons of pictures, and pages and pages of troubleshooting:
Old 04-26-2010, 10:00 PM
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Join Date: Apr 2010
Posts: 3

thanks man looks great will let you know how it goes!
Old 02-22-2011, 02:36 AM
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Join Date: Feb 2011
Location: Aurora Colorado
Posts: 32

I know its an old thread but just throwing this out there. If you run the bike for a bit first, the rubber boots will relax with the heat and come off with ZERO fuss. Just FYI to future readers.
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