I bought a used 2000 Honda 400ex a couple months ago and I'm having a few problems with it. The quad starts up and idles fine, but when I accelerate, the quad sputters and pops up to the higher RPM range. Then when I shift, the quad cuts out for a second, and then opens up again in the next gear. I cleaned the carb and put it back on, but that didn't help. I also noticed that there was some gas running down toward the air filter in the air duct. I also checked the spark plug and it is almost carbon fouled with black soot. I'm running an aftermarket pipe with a K&N Highflow air filter. The carb has a 150 main and a 42 pilot jet. Any idea of what is causing the quad to act up like this? Thanks for the help in advance!

 

hi, are you sure you cleaned out the carb throughly, have you checked the timing? you should also get a multimeter and measure current going in and out of alternator and rectifier, you may also have a bad ground connx somewhere, you should also get a service manual because it has all the specs for your quad.. have anymore q's lemme kno..

 

I've cleaned the carb twice since it started happening. I also ordered the new jets because I thought the old ones looked scored from the previous owner pulling them out before. I'll check the currents on the regulator and rectifier next. I'm not too sure, but how do you check the timing?

 

The 150 is to small of a jet most likely. what's your elevation and do you have the lid off?

You don't have adjustable timing. You line a couple marks up when putting on the timing chain.

 

I'm about 1100 feet above sea level and I have the lid on. I threw the 148 jet back in and I'm still getting the carbon black spark plug like I'm running rich. I would think myself that I would need at least a 155 main with my air filter and aftermarket exhaust.

 

Don't go by much on the plug. You actually have to do a "Plug Chop" to get a true reading. Those 400ex's are infamous for burning a little oil. That will mess up your view as well.

You can also try to pull the choke and see if it helps the pop. (more fuel less air) If it does help you need a bigger jet. If your close the the right mixture it may still run bad.

I'd try a 155 before I did much of anything else. It's was what I had to run with the lid on.

 

I think I'm getting too much fuel though if it's running into the air box from the carb.

 

The only think i know of that would cause that would be the backfiring maybe.

Popping and backfiring is a symptom of running lean. When your running rich you usually will bog terribly, be powerless, and even cut out. Backfiring and popping won't usually occur in rich conditions.

 

I had close to the same issues with my carb. Stock pipe, welds removed from header, uni filter @ 1500'. When I removed the welds and put the filter on she was super lean and would idle but when you got past 1/4 throttle it would bogg and stall. I sprayed it down with carb cleaner 2 times with no luck. My friend took the carb and striped it down, soaked it then blew it out. That took care of most of the bogging. Now I have a 155 jet in it and she runs perfect. I think with a pipe and filter you are looking at 155-160 on the jets.

 

Maybe your float is off, or your needle and seat is worn, sounds like too much gas is filling up the carb bowl, then running down to the air box, that would also make you run way rich, check your float level and make sure needle and seat seals tight, and is clean.