Backfireing after a month

I haven`t started my bike for a month. So today I started it up and let it idle with choke on until it was warmed up. When I gave some gas and let of the throttle it was backfireing like hell, flames coming out of the exhaust.

What could the reason be? It has had a minimum backfire before, but rarely. Also it is a lot cooler around here then a month ago. Could in be a jetting issue or something else?

PS. It makes "buff" second after shut down. Sometimes even a little flame out of the exhaust.

Two things spring to mind:

>The bike sat and varnish from the gas partly blocked the pilot jet or the passage around the end of the fuel screw. In that case, it may clear up, you may be able to run the fuel screw in, and then back out to its current position to clear the passage, or you may have to clean the carb out.

>It got colder in the meantime, and the bike simply wants to be richer for the weather change.

So i should maybe open up the pilot screw a bit?

I know it sounds silly but shouldn`t the fuel screw be on a side of the carburator. I can`t find mine :S

Not as silly as where they put it. Yes, it really should be on the side of the carb--but it's not :crazy:

It is on the bottom, going straight up inside a small well just in front of the main jet access plug. There is a picture on page 3-17 of the manual. Try backing it out to adjust for the weather, but don't back it out more than 2 1/2 to 2 3/4 turns, as you will get diminishing results after that, and there is some risk of the screw falling out from too little spring tension. If it needs more than that, go one size larger on your pilot jet.

I must thank you again. And probably it won`t be the last time :crazy:

I haven`t started my bike for a month. So today I started it up and let it idle with choke on until it was warmed up. When I gave some gas and let of the throttle it was backfireing like hell, flames coming out of the exhaust.

What could the reason be? It has had a minimum backfire before, but rarely. Also it is a lot cooler around here then a month ago. Could in be a jetting issue or something else?

PS. It makes "buff" second after shut down. Sometimes even a little flame out of the exhaust.

Change the sparkplug while you're working on it......

Ok, my fuel screw was about 1.5 turns out. There seemed to be minimum backfire when I turned it 3 turns out. Probably need a larger pilot jet, but won`t the screw fall out? Maybe somebody knows how many turns I can turn the fuel screw out?

Btw I made my own little fuel screw tool. Just take a nice big nail and make it`s one end flat and shorten it up to about 40mm. Then tack a shorter nail to its other end by welding. So you don`t have to even loosen up your carburator.

3 turns is too far out. When you put in a go one size larger on the pilot jet, then you should'nt need the fuel screw that far out.

I know, but is there a potential risk for screw falling out?

If you keep it at 3 turns out, the risk is greater than at 2 turns out, but once you install a larger pilot, you will no longer need to have it backed out that far, so the risk of losing the screw will be far less.

I find there should be a rubber cap on the hole of the fuel screw. Anyways thank you for help.

Another quick question since my bike is doing the same thing. I'm going to get a larger pilot jet, but should I have to adjust the clip position?

Please don't use big words when dealing with carbs, for I'm slightly retarded when dealing w/this stuff.

Another quick question since my bike is doing the same thing. I'm going to get a larger pilot jet, but should I have to adjust the clip position?

Please don't use big words when dealing with carbs, for I'm slightly retarded when dealing w/this stuff.

What, like "multisyllabacy"? :crazy:

No, the needle clip position is not related to the pilot jet. The needle is a part of the main circuit, not the pilot circuit. The needle has an influence on jetting between 1/4 and 3/4 throttle.

There are two circuits in the carb through which air and fuel flow into the intake air stream: the Idle, or Pilot circuit, and the Main circuit. The pilot circuit works at idle and when the throttle is closed. The main circuit does not, since there is not enough air flowing through the carb to activate it. As the throttle opens past about 1/8-1/4, the main circuit is started up by the increased air flow The more the throttle opens, the more effective the main circuit becomes, and the less effective the pilot circuit becomes.

What, like "multisyllabacy"? :D

No, the needle clip position is not related to the pilot jet. The needle is a part of the main circuit, not the pilot circuit. The needle has an influence on jetting between 1/4 and 3/4 throttle.

There are two circuits in the carb through which air and fuel flow into the intake air stream: the Idle, or Pilot circuit, and the Main circuit. The pilot circuit works at idle and when the throttle is closed. The main circuit does not, since there is not enough air flowing through the carb to activate it. As the throttle opens past about 1/8-1/4, the main circuit is started up by the increased air flow The more the throttle opens, the more effective the main circuit becomes, and the less effective the pilot circuit becomes.

So we go up one size on the pilot jet because the pilot isn't allowing enough fuel through once the throttle is closed shut after acceleration. Thus causing the backfiring/popping?:crazy:

Yes, but no.

The degree to which a YZ450 pops on decel MAY be a symptom of lean idle jetting. They'll pop some when they're jetted right and there's no air leaks. It's just how they are. People who try to eliminate all of the popping by richening the pilot end up with a bike that idles too rich, stalls on a throttle chops, etc.

The reason kasjok, for instance, is going to increase his pilot jet size is so that he can get the bike to RUN right. It has not that much to do with whether it pops or not. In his case, he cannot get it idle in a normal fashion without backing the fuel screw too far out, so he needs a larger jet. When he gets done, he'll be able to find the sweet spot in the idle adjustment without going beyond 2 turns out.

So what do you suggest I do? Last season it seemed like my jetting was spot on. This season, my bike is backfiring and making abnormally loud popping sounds when I get off the throttle. Even when I rev the bike, blue flames come out. I have adjusted the fuel mixture screw, but it isn't doing much.

If the weather is the same (within 15-20 degrees) and the the bike runs alright (idle quality, throttle response) then you should look for an air leak at the mid pipe/header joint, or the seal at the exhaust port.

If the weather is the same (within 15-20 degrees) and the the bike runs alright (idle quality, throttle response) then you should look for an air leak at the mid pipe/header joint, or the seal at the exhaust port.

Holy Crap! I just checked my header pipe and there is a bolt missing at the exhaust port! :D Gray, have you been snooping around my garage? :crazy:

Gray, have you been snooping around my garage? :crazy:
Why? Is something missing?
Why? Is something missing?

:crazy: No, I was just impressed that you could diagnose the problem w/o even physically looking at my bike. Especially since I have been emailing various people like mad trying to figure this problem out. Thanks for your help Gray!

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