I am running a 250X with a DG slip on and a K&N filter. I just got it and it runs very strong and has loads of compression. The guy that had it before me jeted up the main but not the pilot. I also remember him saying something about playing with the needle height. When I am into the throttle to 3/4 it is about average power. Anything past 3/4 and you hold the hell on because it feels like another machine. The problem is when I let off the gas it will backfire and sometimes give me a little flame thrower action. It rides great but the noise is getting VERY irritating. How do I fix this?

I was thinking of going back to factory jetting and factory needle position ot get a new pilot jet to match up. It smells a little rich when you are on the gas...is it backfiring because it leans out too much on the pilot?

It is supposed to be a 91 250X so what should I do?

 

Your problem could be jetting and it could be an air leak in the exhaust system. I not real sure on the jetting part, but I beleive if it's lean it can/will backfire. The lean condition should also show up on a plug test. As for the Exhaust system, that I've had experience with and if there's a leak it can and will backfire when lettting off the throtle. Check (cold) all area's that bolt together for a leak. Head pipe to cylinder, head pipe to muffler Etc. You should be able to feel If there's an exhaust leak or not.

Rude Dog

 

I am guessing it is too lean.

 

My experience with a lean condition is that you will get back firing at higher RPM / throttle, but, you'll also experience power loss. If you're not getting power loss, I don't know what to tell you.

I'd proceed cautiously before going stock on the needle and carb adjustments. With the aftermarket pipe, muffler, filter, etc., going back stock on the carb adjustments will worsen a lean condition.

 

Bakfiring on a 4-stroke usually means a lean condition. Do a plug check at 1/2 throttle and full throttle to see where it is that you are lean.

 

under what conditions does it backfire? what throttle setting? it's probably a lean condition. Rudedog right about exhaust leaks too, especially if they are up near the head. A quick way to find out is to have a friend put his foot over the end of the muffler while you are listening. Even a pinhole up near the head is enough to cause it. Also check to make sure carb is fully pushed in the intake boot and clamp is tight, intake boot in good condition.

 

Just thinkin' about this one a little more. Since that model has a butterfly type choke, I think, you could use partial settings , manipulation of the choke to determine if it's lean. If it runs better at a particular setting with a bit of choke, you know what circuit of the carb to address. If any choke at all makes it much worse, then you may be a bit rich. If it needs a lot of choke to start, is very cold blooded, adjust the mix screw or a richer slow jet might help.

 

Well it backfires whenever I get out of the throttle and the harder I let off the more it backfires. It is pretty micky mouse where they have it bolted onto the head so I will check that out and see if I have a leak. I think it has more to do with the jetting though. He upped the main jet and left the factory pilot jet in. It seems to run rich under full throttle (bit of smell) and I am thinking that when I back out of the throttle it is not getting enough fuel on the pilot but I don't know a lot about this. It very well could be an exhaust leak so I will check it tonight...I don't see any carbon deposits which are normal indications of a leak.

 

Just about every piped 4 stroke will backfire even if jetted dead on. Mine's running perfect and it still backfires a little it's just because of all that lost backpressure the stock pipe gives you. You're running a DG pipe, they require minimum jetting.
Stock main is I believe a 122 or 120 high altitude, I'm running a 132 with my FMF I'd try a 130 with the DG
Your pilot is a 38 stock it needs to be about a 40-42 or it will sputter at the lower part of RPM when letting off.
Your air-screw can fix some of the sputter, it's 2 1/4 stock put it at 1 1/2 it that should fix the problem.
try the air-screw thing first before you change jets. Leave the needle clip on stock position.

 

Thanks mojave...that is the exact info I was in need of. I think I have to take the carb off and give it a good cleaning and set everything back to factory spec. I don't know what he has done to it so that is my first task. I will take your advice and it is much appreciated.