rough runing bike

a friend of mine just bought a 2001 wr426, and it seems to backfire an awful lot. it runs really nice, other than the backfiring - with the choke on. after letting it warm up for a while, turn the choke off, and the bike dies. there is an fmf powercore iv2 on the bike - could this be the reason the bike isn't running well? there was also gas in the bike that was 2 years old. we drained as much as we could, put in new gas, and it still dies once the choke is off, unless we give it some throttle. also, there is a small amount of oil coming out of the black overflow tube. does this bike need some jetting work b/c of the new fmf pipe? should we try putting the original pipe on the bike?


If the bike had gas in it for 2 years then you are due for a good carb cleaning. Pull the carb, and remove all of the jets to make sure they are clean, you should be able to see through all of the holes in the jets. Clean it all up with carb cleaner and you should be good to go. You may need to do some jetting for the new pipe but it should not keep it from running.


what explains that minor oil leak then........

If your talking about the tube that comes out of the valve cover and runs down the left side of the bike, that's the breather tube and a little oily drip now and then is normal.

If I were your buddy, I'd pull the carb and do everything short of a complete rebuild. Have him remove all the jets and soak them overnight in carb cleaner (make a note of the jet sizes and compare them to stock.) Check the needle valve and replace it if necessary. Verify the float condition and level. Check the hotstart and choke to make sure they're not sticking. Replace the fuel lines with some Tygon. With 2 year old gas in the thing, it could be anything or a combination. Also, put in a brand new spark plug and air filter. Then do a filter and oil change.

Once all that's done, retorque the head pipe to the cylinder head, remove and reseal the midpipe and header connection. Check for leaks at all the junctions in the exhaust. After that, provided the bike starts and runs, rejet according to your symptoms. BTW, if your buddy doesn't have a factory manual, that is step number one...SC

Ditto on what Clark said. I guess I was a little short in my answer, Sorry bout that. Good luck, and let us know how you come out.


Check for leaks at all the junctions in the exhaust. SC

Hey guys,

I know how to check for leaks on the intake side of things, but how do you check for leaks at the junctions of the exhaust system?


is there any way that this problem could have anything to do with the throttle positioning? if you hold the throttle barely open, the bike runs very well. as soon as you close it off, it shuts off.

any suggestions?

Its possible the idle is set too low. But it should idle at a nice slow thump.

Based on what you said sounds like a plugged pilot jet. Take a flash light and look into the tank, if you see brown sludge in it, that stuff is also in your carb.

After cleaning the carb, make sure you clean the tank too or that sludge will right back in a plug the carb again.

sounds like the pilot circuit is plugged up.. .mine did the same thing after sitting for a year..

sounds like the pilot circuit is plugged up.. .mine did the same thing after sitting for a year..

ISBB (or anyone else),

I'm still learning the basics about these carbs and jetting in general so have questions about this.

By the pilot circuit are you are referring to more than the pilot jet?

Should a guy pull the carb right off the bike in order to do a proper job cleaning it up?


If the gas is 2 years old the oil is probably that old or more so there will undoubtedly be some condensation in the crankcase, especially due to the weather cycles around here, this could be the oily sludge being dripped out of the breather tube. How is the compression? As for the carb, yes I would remove it from the bike for a total cleaning. The exhaust connection between the head pipe and the mid/muffler pipe has to be sealed well by the gasket that is probably still in the stock pipe you removed, buy new as I doubt that the old gasket will come out in one peice. Good Luck. WR Dave.

well we tried a few things tonite - took off the air filter and sprayed a whole bottle of carb cleaner into it - no change. we played with the air/fuel screw at the bottom of the carb, opened it all the way, closed it all the way, set it to what the manual said - the bike ran exactly the same with all the settings. we even tried putting on the stock pipe with no success.

does the carb have to be stripped right down? is there any other suggestions?


Where in Alberta are you? I can't beleive you used carb cleaner with the carb still on the engine!!!!!!This may cost some beer !! WR Dave

I have read at least 3 different times on this thread where myself or someone else said "pull the carb" and you still are trying everything but removing the carb and cleaning it. By pull the carb we mean remove it from the bike, dis-assemble it, if need be soak the jets and hard parts in carb cleaner, re-assemble and then try adjusting the jetting. If you are only spraying carb cleaner at it then you are not cleaning the internals of the carb, only the surfaces that are exposed to the pressurized spray from your can. Trust me, it is not very difficult to remove the carb and get it cleaned. You have probably spent enough time trying to avoid it to have done it 3 times already. Hope this helps.


Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now