Backfiring??

[jiggaman2401]

The quad i am rebuilding is backfiring kinda bad its the first couple times we started it up..I know its got an FMF2 pipe on it the guy that had it before me was running it with no air box i have one on there now..I dont know if its jetted or not? What are some things that would make the bike backfire?

[250EXking]

First thing would probably be jetting. If it is piped and it is not rejetted, that would be causing the backfiring. I would get a kit, and raise the jet a few sizes, that should help it out.

[jiggaman2401]

Like i said im not sure if its jetted or not kinda sounding like its not.. Do i need to take apart the carb to check?

[Aceman]

Yeah, see what it has in it for jet sizes and then go up a few sizes until it's too rich then work your way down to the right one.

[cady#1]

When is it backfiring? If its coming off off the gas then turn you air screw out a half a turn each time(you should be able to get 2.5 to 3 full turns out of it before it falls out. ) If that doesnt do it then you need to go up on the pilot jet.

[jiggaman2401]

I mainly notiice it on idle.. Tomorrow im gonna thoroughly diagnois it see if it does it when i let off the throttle..

[MyFlying450r]

Is it really cold where you live? If it is that could be part of the issue.

[jiggaman2401]

I live in New York and it was about 55 yesterday..

[397ccEX]

Dumb question on my part but.....
How can you tell the jet size?

[cady#1]

Originally posted by 397ccEX
Dumb question on my part but.....
How can you tell the jet size?

It will be marked..

[jiggaman2401]

I dont know what the stock jet sizes are but the ones on the quad are main 125 and pilot 38. Also the spark plug is getting carbon deposit on it? Theres also some screws missing from the pipe is that helps any of you maybe no backpressure?

[jiggaman2401]

bump

[MyFlying450r]

The missing screws are an issue and the black on the spark plug is a sign that you are running to rich. It should be a light brown if your jetting is correct.

[panteramatt]

If you put an airbox back on and it was jetted for without one than you need to lean your jetting a tad.

[jiggaman2401]

I think i got everything figured out now i went with a 128 main and a 40 pilot jet.. Spark plug looks good and the backfiring has stopped the screws were def a big issue..

[jiggaman2401]

Alright lets try this again.. The backfiring didnt stop like i thought.. Its not bad at all just backfires a little when i let off the throttle.. My main jet is 128 and my pilot is 42.. I got a pipe with an aftermarket header.. Does anyone elses backfire a little? any other suggestions? Should i not worry about it?

[EPDP99]

a 128!!! why would you go lower than stock with a full exhaust. mine came stock with a 148. you should be around 155-160 i would think. whats your elevation and do you run with air box lid on or off and what kind of filter?

[jiggaman2401]

a 300ex comes stock with a 122 i believe.. I run with an air box and stock filter right now..

[EPDP99]

ok i was like wtf???. anyways I never saw that it was for a 300 ex i thought he was asking for a 400 ex.

[jiggaman2401]

Its cool just wanted to clear everything up.

[MyFlyin450R]

You can get a little back fire in cold weather its normal. As long as the plug is a brown or light black you are OK.

[cals400ex]

if that has a fuel screw, try turning that out a 1/4 or 1/2 turn.