Jetting problem?

[Antifaith]

Hi,

I have a 2001 400ex that runs great in summer. I wanted to try some winter riding this weekend but my quad is acting up.
Whenever i start it, i have to keep the choke on full, otherwise it backfires like crazy. If i let it idle, it dies right away, so i always need to give it some gas to keep it running. Even when it's running, it's not smooth at all.

I have a 42 pilot jet, a 155 main jet, a white brothers E2 pipe, K&N filter, everything else is stock in the engine/intake etc.

Could the jetting be causing this, since it seems to run fine in summer? The temperature is usually between -10 and 0 down here in montreal.

[huffdaddy_400EX]

Check the airfilter, check the plug, check the gas.

When was the last time you rode?

[Antifaith]

I put in a brand new plug yesterday. The old one still worked, but it was dark black. What does that mean?

The airfilter is a brand new K&N. Is a performance filter like that be bad for winter riding?

And there is gas going into the carb. There's no pinched hose or anything either.

Last time i rode the bike was around 4 weeks ago. Had no problems back then.

[huffdaddy_400EX]

How long do you give it to get warm? Since it below 0 where you are at it will take some time to get the thing hot.

You may need to rejet for the winter. The back firing could be coming from too much fuel and not enough air mixture.

[Antifaith]

I let it run for almost 20 minutes. I did not even bother trying to drive it, i wanted to get it running smoothly first, but i never managed to.

Should i get a bigger main jet, pilot jet, or both?

[huffdaddy_400EX]

http://www.atvriders.com/articles/jetting.html

If you are too rich you will need to go down on the jets.

When you said the plug was black...was it dark black...even wet? If so, you are too rich.

[400EXtreme]

Sounds like the jetting

[Antifaith]

The plug was fairly black, but not very dark either. It wasn't wet when i removed it, even though i spent a good 5 minutes trying to start the 400ex.

[04TRX400EX]

It sounds like your jetting is too rich, like others have said. A dark black spark plug indicates too much fuel and therefore not enough air. Colder temperatures can result in not as much fuel being burned, relating to poor performance and/or backfiring. Considering that you are having mostly start-up and idle problems, your pilot jet is probably the problem. Go down to a #40 pilot and see what that does for you. Play around with lower fuel jets until you get the quad running well, then start looking at the main jet. If your quad runs good at the 1/2 to WOT, then the main is probably ok. If it feels like it is starting to crap out on you, go to a smaller main jet as well, like a 152 or possibly 150. Hope this helps.

[Antifaith]

Thanks for the elaborate answer. I'm going to go buy a #40 thuesday and see how that works. I Might pick up a stock #38 as wekk just in case, those jets are cheap anyway.

[atvracer980]

I'd go to a 50 pilot jet and not use your choke- remove it, hit your throttle a few times and away you should go