I'm enquiring on behalf of a young man (15yrs) who bought a used 400 ex as a first quad. The quad is stock except for a performance cam. Initially the quad ran well but on the last couple outings it developed a problem we can't seem to resolve. When a new spark plug of the correct specs is put in it seems to run well for a couple hours then starts to run rough and miss. When the plug is checked it seems to be burning OK (color). The spark seems strong. We've confirmed fuel flow to the carb should be adequate. There doesn't seem to be a short in the spark plug wire.

I'm beginning to think it's the electronic ignition. I'm reluctant to take it to a shop as the owner doesn't have a lot of money to spend on it. I also suspect the shop would change the plug, quad works fine, send it out the door and 3 hours of riding later we have the same problem.

If anyone has experience the same problem and was able to resolve it I'd like to hear how. If anyone has any suggestions of how we cold better diagnose the problem or suggest fixes I'd like to hear that too.

Cheers

 

Check for water in the steering shaft where the fuel cap vent hose is normally located, if the hose is into water the tank won't vent right and they run rough. Pull the hose out and see if it runs any better. The hollow shaft needs a drain hole at the bottom to let any water accumulation out if that's the problem......

 

Id rule out the electronic ignition. Its either gonna work or not, it wont be intermitent. You could try removing the connectors at the spark box, and looking for water or corosion it there. Other than that, it should be good.
Hows the valve lash on the cam? Is it set correctly?
Compression good?
It concerns me that only the cam was changed. Usually the air box /filter /exhaust would be changed also. To put a hotter cam in, and use stock everything else, isnt gonna do much performance wise.
When it starts to run crappy, try setting the choke in different positions, and see if it improves or gets worse. If you go to the first detent or even the second detent on the choke, it should run either worse or better. If the problem is jetting or fuel flow, this will help direct which way to go.
Its posible the coil could be bad, or even the pick up coil, but those are kind of long shots. Id stick with making sure the jetting is correct, and the compression is good first, before throwing any parts at it.