I have a 2000 Honda 400ex. It is all stock except for a K&N High Flow air filter. I was originally running a 148 main, 42 pilot, and needle in the 3rd position when the quad had the stock air filter. With the K&N air filter, the jetting I had was too lean. Would bumping up the main to a 152 and turning out the air/fuel screw help to solve my lean problem? Thanks

 

Is the exhaust totally stock? I would be surprised that just a K&N would run you that lean, since the restriction of a stock exhaust would limit how much more the intake could flow,you can only pull air in as fast as the exhaust lets it out. Are you running the stock air lid as well? In that case, I would think going up just one size on the main should be fine, unless you uncork the exhaust by pulling the baffle and run a hi flow end cap.Then the 152 minght be needed. Best thing is to run a plug chop test and see where you are for mid and top end jetting, see how the plug looks at mid and then run it up to wide open and see how bad its leaning out. Is it missing on the top end? One thing I have run into on Hondas is the restrictive rev limit, I added a pipe and filter to my son's 300ex, and it ran great with a bump in the jetting, but would sputter and miss revving through the gears, turned out it revved so much better with the restrictive intake and exhaust opened up, that it was hitting the rev limit. I bought a +1000 rpm cdi and it made a world of difference.

 

Yep, the exhaust is completely stock and the airbox lid is still on the bike. I actually tried the 150 main and I'm still slightly lean which surprised me. The end of the plug was pure white and the motor was really heating up so I know that jetting is too lean. I'll have to try out the 152 and see what happens.

 

Well, Honda did jet lean from the factory for emissions, so adding the K&N must have really leaned it out, sounds like you are right on top of the plug reads, so I would do as you think and go up to the 152. If a 150 is white, you may need one more up from the 152 !! Keep us posted.

 

I have the 152 main in the quad now. I took it for a ride and it definitely fixed the overheating, I'll just have to wait until it cools down and see what kind of color I'm reading on the plug. I'll keep you updated.

 

So I've been running the 152 main in the quad for 2 days now and it's spot on. I'm getting the darker brown color on the plug and no more overheating issues. Thanks for the help!