backfiring

[dirtdawg]

i have a 250 ex w/ a powercore 4 muffler and i notice when i downshift from second to first and sometimes reving it in neutral, i noticed that it makes a loud popping noise like a backfire. do i have a loose connection between my carb and the engine, or what? i'm kind of scared but otherwise it runs great. i just shot some carb cleaner in there the other day so i don't know if that could potentially create any danger. i just stopped running it w/ out an airbox, so maybe the abrupt change hurt it.

i also noticed that when i first start it and give it 75% or more of the throttle in neutral, it kind of sputter, and sometimes dies out.

[bwamos]

On the second part.. if the quad is still cold it's normal for it to not want to go over 75% throttle. Mine doesn't like over 25% until it warms up a bit when it's cold.

On the first part is it popping when you let off of the gas? Or when you have it consistently on the gas?

If letting off the gas, you jstu need to adjust the air/fuel screw on the bottom of the carb to richen up the pilot circuit a bit.

[dirtdawg]

i found out that i have to rejet my carb. i'm running stock jets w/ an fmf powercore 4 w/ no airbox lid, so i just put it back on. So my main jet is a size 95. i need to bump it up to a 105 or 110, which will cost me around 10-12 bucks. that should do the trick.

[panther99]

I have the same pipe on an 01 stock 300 w/ a uni air filter. Any one know what mejet settings should be??

[bwamos]

Originally posted by dirtdawg
i found out that i have to rejet my carb. i'm running stock jets w/ an fmf powercore 4 w/ no airbox lid, so i just put it back on. So my main jet is a size 95. i need to bump it up to a 105 or 110, which will cost me around 10-12 bucks. that should do the trick.

Keihin jets should only be $4.50+/- at the stealership.

[dirtdawg]

i put the 110 main jet in, and problem solved. my 250 rips again with no backfire!

[dirtdawg]

nevermind. its not right again. i have a sooty spark plug, and my engine gets way too hot.

[bwamos]

Probably a tad lean on the main, and a tad rich on the fuel screw.

Opening it up will make it run hotter. More fuel & more air = more energy converted in the same period of time = more heat.

[dirtdawg]

i figured out that my slow jet is too small. stock is a #38, and i need a #40. its backfiring when i let off the gas, and the jet that controls idle to 1/4 throttle is the slow jet.