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Xr400 carb issue/running rough

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Hey guys. Long time lurker and made a few posts.

I started work at a motorcycle shop and there is a xr400 that has come back a few times to get the carby adjusted right. It is an 04 model with only 3500kms on it. It has an aftermarket mukini carb in it. Not sure what type. It may be a tm 36? It is a pumper carb btw.

The problem we are getting is at 1/4 - 1/2 throttle and onwards it starts popping and backfiring and running rough in the mid to top RPMS. Cleaned the plug and ran a high-speed test. Pulled the plug out and showed a lean condition. Cleaned the carb multiple times over. Also cleaned tank out, removed the fuel filter inside the tank and added a inline fuel filter.

The jetting is

Main: currently 160. Started at 130 and getting same issues as 160.

Needle: is on the second from the top. Have moved it to the middle and back with no luck.

Pilot: is 15 and haven't changed it.

Mixture screw has been played with at all settings we have had it.

We are at about 500m above sea level and temperatures are at around 30℃ and cooling off.

Any help is appreciated.

Thanks.

Edited by ejboaklyn_crf250r

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Check the o ring between the intake manifold (front carb boot) and head. Check the for good seal at both ends of the carb, no air leaks.

Make sure there's an air filter installed.

Make sure the spark plug is good. Check to make sure the spark plug boot is tight on the end of the coil wire and no corrosion.

Pull the resistor inside the plug boot and check it or replace it temporarily with a piece of stainless or copper of the same size to rule out a bad resistor.

Be sure electrical grounds for the ignition system are clean and tight. Connectors are clean and tight.

Make sure the gas tank cap is venting properly. Carb vent lines are clear.

Last resort is to be sure the coil or CDI isn't trying to go bad .

Sometimes a problem that seems carb related, isn't .

Edited by Trailryder42
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Get a can of starting fluid and spray around the intake from the carb to the motor to check for intake leaks. The bike will rev up a little bit if there is a leak. I bet that is what it is.

Long live the thumper!.... BRRRAAAAP

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Good advice, get a new o-ring for the manifold too. If the manifold is dried out/cracking at all, replace it...does sound like an air leak.

 

cheers

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Found out a problem. Not an airleak at all. Replaced the o ring twice. Only because I pinched it the first time.haha. found the fuel screw tip to be broken off. Removed that and order a new one. Hopefully it will run better.

We did put a fuel screw in from an 02 crf450 carby we had laying around just to see if it was the problem. We did find that it ran better but still ran rough on the high rpm. After we get.the fuel screw in it will be jetted right.

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