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2002 400EX Will Not Run

1381 Views 9 Replies 4 Participants Last post by  JohnnySolo
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.
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Hey jzleister, welcome to the forums! Sounds like you got some project, I'm sure we can help you out.

First thing Id check is your throttle, be sure the cable is all the way through the spring and not coming out the side. This should help the throttle seat as low as it can in carb. If you can, take a picture of inside carb so we can see how far down the throttle is seating.

Also did the guys that replace the carb even try to fix the old one? For someone that isn't bright enough to try and fix the carb and just replace it, maybe they bought wrong carb. Just something to check in to.

How does the oil look in this thing? If it's burning through gas like crazy lets be sure there's enough oil in it and be sure it's not gassy.
Thank you JohnnySolo, I will check the throttle, take a picture through the carb, and recheck the oil.
No problem, let us know what you find.
I have attached a picture down the barrel of the carb. I checked the oil and it seemed pretty thin and might smell like gas, but I'm not totally sure. If it is mixed with gas, does that mean the piston rings are bad or did gas maybe just sit too long in the cylinder? Thanks.
Throttle is fine. Oil isn't good. It can cause damage to the cylinder but we don't know if it did. I had a carb run gas into my oil and it didn't do damage but that's bc we caught it soon enough. So drain the oil and get you some new oil. Be sure it's for atvs!
Possibly a vacuum leak at the intake manifold. Your throttle slide is definitely open too wide for a stock idle but you probably already know that.

With the bike running as best you can, spray some flammable carb cleaner at the intake manifold areas and see if the rpm changes. If yes, fix the vacuum leak there.


Sent from my SAMSUNG-SM-N900A using Tapatalk
My throttle on my Recon 250 2004 looks identical and idles just fine.


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I'm assuming your slide is a touch high because you said you adjusted it up to get it to run.
So do this .... get it to run and take a flash light and look into the carb while it's running and see if you see excessive gas flowing in there because maybe it's me but I think you're missing your idle jet.
It's the brass fitting that the needle slides into. I circled where it should be in the pic.
Check it and report back.
If this is the case it would explain gas in your oil.
Never thought of that. Look at my pic of my throttle, you can see the fitting that Carolyn is talking about. It comes in a carb rebuild kit. Should've replaced it when cleaning, probably is your problem if you're missing it.
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