I just got this bike on trade 1999 YZ400F and when I rode it on the test ride it backfired on deceleration and while reving it was popping. First let me say I know I'm a NOOB here but I know forum edicate and I have been reading for about 5 hours about this issue. I'm posting this thread in hopes that I might get some good advice and also help out anyone else who has the same or similar issues. I will post my findings once I figure this out :-) So I cleaned the carb completely today, 175 main but I didn't check the pilot. Stock pipe so I seriously doubt they changed that but I'll go back in if I need to. The carb was pretty clean no real dirt or build up. I completely drained the fuel tank and put in fresh 92 as well. Symptoms: - Kick start takes a LOT of kicks when cold. fires up with choke out and runs pertty high RPM's. - Once warm(er) after maybe 30-seconds to 1 minute I turn off the choke and it sputters and dies immediatly. - Will start again without choke but will not idle at all, adjusting the big black knob on the side is very hard to turn and seems to have no real effect on the idle. - If I rev the bike it pops pops pops, and loud. once I let off the gas it dies. It pops through all of the above, even with the choke on. I put a brand new plug in it today and verified gap, the old one was black and looked like it was rather rich with lots of carbon buildup. Tomorrow morning I'll check the pilot screw on the bottom, I'll start with 2 turns out and work my way from there. I'm assuming idle screw should start at around 1 1/2 turns out and then adjust to right RPM range. Thanks!