Hey all, this is my first post, although I have read countless questions and answers trying to figure out this particular problem. I am working on a 2002 400EX for a neighbor. Before he asked me to look at it, he let "some guys" (that is all he ever told me about them) try to fix his broken tail light. At some point, the quad stopped running for them, and they assumed it was a carb problem, so they bought a brand new carb for it. When that didn't fix the problem, and they still hadn't fixed the tail light, my neighbor took it back and then passed it to me.
Currently, it has the new carb with stock jets (main: 148, pilot: 38) with a White Brothers full exhaust upgrade, but stock airbox and air cleaner. It also has a dead battery, so I jump it with a car battery when working on it (I don't think this has anything to do with the problem, but want to give all the information I have).
I have done the following work on it: new sparkplug, new ignition coil, cleaned and recleaned and recleaned and recleaned the brand new carb, checked the cam timing, adjusted the valve clearances, new ICM, new tail light, new gas, checked for air leaks, bought a new needle jet because the stock one was not adjustable (it is now on the third detent), and drilled through the cap over the air adjustment screw to adjust it (it is now 2-1/2 turns out, but I have tried it from all in to all out at 1/4 turn increments).
The symptoms: Will not start and run with anything less or more than about half throttle and no choke. When it runs, it pops terribly, creates clouds of dark smoke, and drinks gas like crazy. If I let off the throttle, it dies immediately. I can get it to idle like this if I adjust the throttle cable to some extreme configuration that matches about half throttle, but it can't be good for the quad and leaves the spark plug wet fouled in no time. The oddest thing about it is, if I have it running like this and I turn the fuel off, as the gas empties from the float bowl, the engine will rev really high for about ten seconds. If I am careful within these ten seconds, I can let off the throttle gently and I can get it to idle normally, but only until the gas in the float bowl is totally gone. One time I was able to get it to idle like normal, but only because I was throttling the fuel by alternately turning on and off the fuel shutoff valve, allowing just the right amount of fuel through.
Please help! This is driving me crazy. It seems like it is running way too rich, but with a high-flow exhaust system and stock jets, I don't know how this can be.