Thanks in advance--I have been poring over this site for the past two weeks to try to rule out everything and can't figure it out. 2001 400ex, was rebuilt with .050 oversized pistons and only other mod is a slip on Yoshimura pipe.

Everything I've done:
Pulled valve cover to verify cam is aligned (the two marks are flush w/head when the "T" is on the mark)
Valves adjusted to .004 intake, .005 exhaust per spec
Carb needle in center (3rd) position
Stock 38 pilot jet, 170 main
Mixture has been tried at 2, 2-1/2, 2-5/8, and 3 turns out
Carb has been COMPLETELY disassembled and cleaned (twice, actually)
Gas tank was emptied, sprayed carb cleaner through petcock backwards, and it all dried for two days before re-assembling.

With all of that said, for the first 2-3 minutes it will only run on full choke. After a few seconds it starts sputtering and spitting gas out of the carb inlet. To reduce the choke only makes it worse and it literally has to run for 5-6 minutes before it will come close to idling without a choke (still popping backwards through the carb). If I take the lid off of the airbox it only gets worse.

This sounds lean from what I've read, but I would think that a 170 is plenty for nothing more than a slip-on. Any other ideas? Motor is very tight--no smoke at all. I also have a prestine, bone-stock 2006 400ex (the wife's ride) that I swapped carbs with and the 2001 idles great with that carb--the 2006 idles like crap with the 2001's carb, so I think I've isolated the problem to something within the carb/setup.

Thanks!!

 

I would switch jets between the 2 carbs. and see what happens then you can narrow it down further. To me a 170 sounds kind of fat for a slip on and .050 over however it won't really effect the idle, it will make it bog from 5/8 ish throttle to w.o. What happens below 2 turns out? Does it spit while it's idling? Have you cleaned the passages in the throat of the carb? A fine piece of wire works good.

 

Thank you for the reply--for some reason this post didn't appear within the first two days of posting it so I gave up thinking I had hit a wrong button or something....

Turns out that after disassembling the carb twice and cleaning it with spray carb cleaner, it still wasn't adequately clean. I disassembled it a third time, soaked it overnight in Chem-Dip, cleaned the pilot jet with speaker wire, and then blew everything out with about 50psi of air pressure and it purrs like a brand new one. Idles equally well with the mixture screw anywhere from 2-1/2 to 3 turns out, so I just need to tweak it for possible easier starting.

The 170 main may be a bit much--it sometimes smells a little rich if the wife is riding and I'm following--but it is significantly faster than the stock '06. I'm gonna keep on eye on the condition of the plug.

Thanks!!

 

Good to hear you got it running. Thing to note is the 170 main will have no effect on idle. It won't take over until half throttle or so. The 170 maybe about right for you. Running rich is OK. Running lean is where it can hurt ya.