I have a new to me atv that has a new aftermarket carb. I swapped the jets in the carb with Mikuni jets to make sure it was accurately sized. I am experiencing a lean condition when I push the throttle right off idle. I am getting a backfire (poping) in the intake boot/carb area on acceleration typically between 1/8 to 1/4 throttle. After careful examination I can't find any vacuum leaks. If I block off a portion of airflow into the airbox I can push the accelerator to full throttle without backfire which leads me to believe it's lean.

I have adjusted the pilot up one size from stock and still get the same results wit the mixture screw bcaked out 4x. In theory if I were to turn the fuel mixture screw counter clockwise and the pilot was the correct size wouldn't it typically start to idle rough after 4+ turns out if the pilot was the correct size?

I guess I'm not sure If it's the pilot or needle. I have put the needle on the richest setting and still get the same results. I have a few richer pilot sizes but I wanted some input before I tried richer pilot jets or shimmed the needle. Thanks

 

Warm up to operating temp. Now adjust idle adjusting screw until it's about 500 rpms higher than normal idle. Turn PMS/AIR screw all the way clockwise until it bottoms out. Now once it settle down , back it out 1/4 and wait 15 seconds for it to settle down. Keep repeating until 2 turns out. If max rpm is achieved from 0-1 turn= pilot jet is Too Lean. If it's between 1 and 2 turns= Perfect. And if it's 2-3 turns out, pilot jet is= Too Rich. OK now set the idle down to normal after finding the max rpm adjustment. Don't turn out more than 3 turns, btw.

 

Go to a bigger pilot, if that mix screw has to be much more than 2.5 turns out. 1/8 to 1/4 throttle is needle territory, so it may be beneficial to change clip location. Usually lower the clip so the needle doesn't go as deep.