what causes backfire/afterfire?

[Miami_Vice454]

when i got my new exhaust i re-jetted and it ran great but now when i am full speed and decel it pops once or twice, it does not happen often. just sometimes when i do that and that only. is it normal?

[rob_990]

put a bigger pilot in it

[Miami_Vice454]

why does a bigger pilot jet fix that. i thought the pilot jet only has a big role in your idle

[rocky_mtn_honda]

Pilots or slow jets usually help if the engine backfires when you let off the throttle. Even if no backfiring is present the pilot (slow) jet may be too lean. The pilot (slow) jet is one of the most commonly overlooked jet in the carburetor. If the engine needs the choke for a long time in cold weather a larger pilot or slow jet will help. If unable to obtain a smooth idle with the mixture screw going up a size or two in pilot (slow) jet will help.
courtesy once again to http://www.jetsrus.com/main_page.htm

[Miami_Vice454]

thanks for clearing that up, i will be ordering a new pilot jet soon

[TRXRacer1]

Originally posted by Miami_Vice454
why does a bigger pilot jet fix that. i thought the pilot jet only has a big role in your idle The pilot is important up to 1/4 throttle. Sometimes for bad backfiring you may also need to up the needle a clip.

You have a mixture screw that is currently choking back your pilot. If you want to save a couple bucks turn the screw out more. That's probably all you need to do.

[tri5ron]

Originally posted by TRXRacer1
The pilot is important up to 1/4 throttle. Sometimes for bad backfiring you may also need to up the needle a clip.

You have a mixture screw that is currently choking back your pilot. If you want to save a couple bucks turn the screw out more. That's probably all you need to do. ditto :)

[Miami_Vice454]

ill try doin that, it really doesnt do it that often but i kno people who have it dead on so i kno it can be done i should get it runnin perfect. thanks guys

[400man]

not trying to butt in on this thread, but my 400 has a bad case of popping and back firing when its cold and has set for a day or more, I mean real bad to where it wont even try to rev out anywhere from 1/8 throttle to wide open, it just pops and farts and has no power and wants to just fall on its face the more gas you give it, but once the motor gets up to operating temps then it all clears up nice and runs great and starts and idles great. my mods are just a uni foam filter and full big gun exhaust, and the jetting is:

needle clip --3rd from the top
fuel mix screw --3 turns out from lightly seated
pilot -- 38
main --152

[Miami_Vice454]

ur mods and jetting is pretty close to mine yet, mine only rarely does it

[dariusld]

A backfire is also a sign of an exhaust leak.

[Miami_Vice454]

interesting because i just noticed yesterday that where the muffler clamps to the header there was some carbon so i tightened it even more, i may put some gasket maker on it