Need a little help here, I have a 99 Honda 400EX completely stock and it has always run strong. Lately though I can't get the thing to stay running, it will start up and run for 25 to 40 seconds and then it sounds like it's starving and then dies. Sometimes I can get it to start again, but it does the exact same thing, sometimes it will just crank over without starting until I kill the battery. I recently tore the carb apart to make sure it wasn clogged, and I replaced the plug several times just in case, but yet it still does the same thing. I consider myself fairly proficient when it comes to wrenching on these things, but this has just got be baffled.

Any help here would be greatly appreciated.

SAS

 

Just some ideas to check:
Is the vent hose clogged?
What about the fuel line.
Change the plug, you did not say anything about this. Try this one first.
Do you have gas in the float bowl
See if you have spark
Put a plug in fist and see if it runs.

 

Without more information, I can give only guesses. Look into the compression, maybe a problem there; look into spark timing, or bad wiring. Check the valve clearance. I dont know how cold it is where you are, maybe its something as simple as running too lean?

 

I agree with tprender, check your vent hoses coming from the carb, I bet one of them is clogged....that's been the most common problem I've seen with the description you're giving us.

 

You know after taking things apart and checking the float and fuel line and re-assembling the carb, the one thing I didn't check was the #$&* vent lines, boy do I feel like an idiot. Well disregard that comment I made about being proficient on wrenching on quads. Thanks for the insight/info guys, I do appreciate it, I'll let you know if that was the problem.

I don't think it is a major motor or spark problem, because it does start and run decent if only for a short time. It did seem like some sort of carb problem like it was choking out, and since you all brought up the vent hose problems, that sounds more like the problem. Once again thanks for help!

SAS

 

Another thing to check would be the fuel hose from the tank to the carb. Disconnect it and turn on the shut-off valve and make sure it is flowing freely. Could be dirt in the petcock or if you have had the tank off recently, the line could be kinked or pinched.

 

I finally got it running, there was a small piece of sand/dirt lodged in the pilot jet, I couldn't tell it before because air would flow through the small holes in the side of that particular jet. I wouldn't have looked a second time if I hadn't noticed small amounts of dirt/sand in one of the vent hoses, so thanks guys for getting me to take a second look. I appreciate the help and info.