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Your motorcycle’s carburetors can be a source for many no-start or poor running conditions.
This is especially true if you’ve got a project motorcycle that has been sitting for a long time.
Carbs that have been sitting are prone to having their small passages gummed up. Dirty, clogged carbs can cause:
- No fuel or too little fuel getting to the engine.
- Poor idle.
- Poor acceleration at different throttle positions.
How to Clean your Motorcycle Carbs
- Remove the carbs from the engine.
- Refer to your manufacturer’s shop manual for specific disassembly procedures.
- Disconnect the carbs from the mounting bracket.
- Remove the carb tops and float bowls, keeping all the parts to each specific carb separate.
- Remove the throttle slides, jets, float pins, and float.
- Inspect all parts for excessive varnish build up, clean with small wire or guitar string.
- Remove all rubber parts – o-rings, floats, vacuum diaphragms, etc.
- One carb at a time, lower the metal parts into your carburetor cleaning dip and let it sit for a few hours.
- Remove the carb and set on a towel.
- Carefully wipe and clean each individual carb body and jet with paper towels and cotton swabs. Use wire to get in between small jet holes.
- Spray each airway, fuel passage, and jet with carb cleaner spray to ensure that each hole is clear. Dry them out with compressed air.
- Repeat for the other carbs.
- Replace any o-rings, rubber hoses, and gaskets with a carburetor rebuild kit.
- Replace any jets that you couldn’t unclog with the same size. Look on the head of the jet to find the correct size, it’s usually stamped in very small numbers. (If you’re modifying your exhaust or air filters, you’ll likely need to rejet your carburetors.)
- Reassemble your carbs according to your service manual.
Clean carbs will help your motorcycle engine run better.
If you’ve just bought a motorcycle that has been sitting for a long time, then the chances are pretty good that the carburetors will need a thorough cleaning.
How to Remove the Carburetors
Get your motorcycle shop manual out to read about the exact sequence for removing the carbs from your motorcycle and taking them apart.
The typical procedure for removing your carbs is:
- Disconnect the fuel line.
- Loosen the screws that tighten the manifold clamps.
- Remove the airbox.
- Pop off the carb rack.
Be sure to go slow and keep things organized. Wait until you have a dedicated workspace before you start disconnecting the carbs from their rack and getting into each individual one.
How to Take Apart Motorcycle Carbs
With your rack of carbs off the bike, you can disconnect them by unscrewing them from the mounting plate.
There are typically 2 to 4 screws holding each carb in place. Before you can get into each individual carb you may also need to remove the choke rod and the throttle shaft.
Set your workspace up with separate containers for each of your motorcycle’s carburetors. You’ll want to keep all the internal components with the carb body they came from.
Remove and set aside any rubber parts before you use any carb cleaner. This is especially important if your throttle slides work with a rubber diaphragm. The rubber diaphragms are easily damaged with carb cleaner and are often expensive to replace. If the bike has been sitting for a long time, be gentle when you remove the throttle slide because the rubber diaphragm may be stuck.
If the metal throttle slide is stuck inside the carb body, you can try soaking it with penetrating oil or apply heat with a heat gun.
Unscrew the throttle slide tops and the float bowls. You can access and remove most of the jets from the bottom of the carb. Be sure to remove any o-rings and rubber/plastic carb floats before using carb cleaner.
Before removing the air screw or pilot screw, turn it in until it stops and count the turns. When you reassemble the carbs, count the same number of turns when installing each air screw.
Remove the jets and be careful not to strip them or break any legs/supports within the body of the carb. Be gentle and use carb cleaner to loosen up varnish that may be fusing things in place. Certain jets are pressed in. Certain jets are not designed to be removed.
How to Remove a Frozen Float Pin
The float pin is what hold the carb floats in place. On older carbs that have been sitting for a long time, these pins can freeze in place and become difficult to remove.
The best method for removing a stuck pin is to:
- Soak it in some carb dip or lubricant.
- Use a press pin or spring loaded punch to carefully knock it free.
If you have a stuck float pin, be sure to work slowly and carefully. If you rush, you run the risk of breaking a post off and rendering the carb useless.
How to Remove a Carburetor Welch Plug
Many carbs feature a welch plug, which is a factory installed cover that goes over the fuel adjustment screw.
As part of your carb cleaning and rebuild, you’ll want to remove the welch plug so you can get at the fuel/air screw.
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You’ll want to do this for two reasons:
- To ensure the passage is clean – this is a common area that gets clogged and gummed up.
- So you’re able to better tune your carbs once they’re back on the bike.
To remove a carburetor welch plug or welch cap, use a narrow set of picks.
How to Test a Float Needle
The carb’s float needle is a common point of failure. If you’ve got an old carb, your best bet is probably to replace it. However, even replacement float needles can fail.
A bad float needle will not be able to stop fuel from flowing into the float bowl, and can lead to:
- Fuel overflow.
- Fuel leaking into the air filter.
- Fuel leaking into the crankcase.
If you’re experiencing these symptoms and are sure that the float height is correct and all passages are clean and clear, you can test your float needle with an air gun.
To air pressure test a float needle:
- Make sure the float needle and floats are in their seats.
- Use a rubber-tipped air blog gun and set the pressure on your air tank to low (~3 psi).
- Insert or mate the rubber tip of the air gun to the fuel inlet.
- Apply air pressure.
- Move the float up and down to test the needle.
- If you hear air pressure when the needle is supposed to be closed, you’ve got a bad float needle.
Best Ways to Clean your Motorcycle Carbs
Once you remove all the rubber and the jets, you can dip the carbs in carb cleaner if they are badly clogged up with varnish.
Berryman Chem-dip and B-12 Chemtool Carb Spray are the best products for cleaning carbs. Just be careful about using them on rubber parts.
After soaking them for a while, remove the carb, wipe off the excess carb cleaner, and spray each passage with spray carburetor cleaner. Watch to make sure the the carb cleaner spray come out of a different hole that it is sprayed in.
You can also use a small wire guitar string to clean the small passageways. You should wear gloves, goggles, and a respirator when cleaning with carb chemicals.
After going through the clogged carb parts with carb cleaner and guitar string if necessary, a pass with compressed air can be useful.
Soda Blasting Carbs
Soda blasting is a great option for really dirty, varnish covered carbs.
Soda blasting is a good option because it:
- Is relatively cheap.
- Will dissolve in water.
- Safe to use on all carb materials and surfaces.
- Will get through the small passages.
Ultrasonic Cleaner for Carbs
An ultrasonic cleaner can help as a final step before your rebuild the carbs to get all residue and debris cleaned.
Hard or Cracked Rubber Carburetor Boots/Manifolds
If the rubber manifolds that connect the carburetors to the engine and the airbox are old and hard, you should either replace them or try to restore them. Replacements are easier to find for some models than others.
If you absolutely can’t find a replacement, you can try to restore the rubber by soaking them in Armor All or Wintergreen oil.
If the rubber carb boots are too hard, they can crack and cause vacuum leaks. Spray some carb cleaner around the boots when the engine is running and watch/listen to the RPMs. If there’s a fluctuation when you spray, the old carb boots are likely the culprit.
Motorcycle Carburetor Rebuilds
After you’ve thoroughly cleaned the carbs, you’ll need to rebuild them. You can usually find aftermarket motorcycle carburetor rebuild kits, some of which are better than others.
If you’re able to clean and reuse the jets that were already in your carbs, try to do so. Aftermarket jets tend to be slightly different than the OEM parts and can cause tuning issues.
O-rings, gaskets, and washers from aftermarket carb rebuild kits are usually okay to use.
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