400ex breaking up

[Jns400ex]

Alright at times at full throttle it will run awesome and now all of a sudden it will just cut out like quads do in reverse. Its a 99 400ex with alot of motor work, i just cleaned the carb and it still does it. Im thinking its electrical maybe in the kill switch. Any help would be appreciated

[JOHNDOE83]

Depending on what "alot of motor work" means your issue could be anything.

But if you have "alot of motor work" we would need to know what type of jetting you have and what "alot" actually is.

Id be willing to bet with it being summer time, your jetting went slightly to rich or to lean because of temperature and climate changes.

If you still use your air box lid, try to remove it "or" put it back on and see if the problem gets worse or better.

That break up like a rev limiter is usually from being to rich but it can have the same effect if your overly lean also.

[Jns400ex]

Jettings good, plugs good

[pdpdrummer123]

Check and see how strong of spark your getting. your solenoid could only be producing weak spark and sometimes strong so that could cause it. Just check that i can't imagine it being something with the killswitch because it should just stop not miss. so try testing the spark and let me know.

[JOHNDOE83]

Originally posted by Jns400ex
Jettings good, plugs good

Well, doing that test or atleast checking the jetting other then pulling the plug will eliminate that as a issue.

You cant just pull a plug, look at it and get a jetting reading "especially for a main jet" I dont care what anyone says.

Without buying a replacement cdi or coil, you really cant test it to see if thats the issue. Without completely taking the wire harness apart, you cant "really" test it.

It's not the kill switch, atleast try the air box thing or atleast tell us what alot of motor work means so we can try to help better.

[pdpdrummer123]

and if its been sitting for a while it could be bad fuel and make sure your using the correct octane.

[Jns400ex]

Floats sticking maybe?

[JOHNDOE83]

Nope, a sticking float would most likely leak gas out of the carb over flow tube.

Have you tried any of the above mentioned?

[Jns400ex]

I had it out today, it ran real good for about 20 minutes then it started acting up. Didnt want to go once you started giving it gas. I dont know if its electrical, carb, or what.

It has a wiseco piston, bigger valves, cam, ported, exhaust, kn filter without lid.

Keep in mind ive had it out for a few weekends since the motors done and it ran great

[JOHNDOE83]

Running good for 20mins then acting up is a sign to me that its extremely lean.

I have a 440, with bigger valves and a 450r carb with no porting and I have to run a 220main jet for it to run correctly.

A new piston and new "bigger" valves with a new cam deffintely needs re-jetted or youll have alot of issues later on. The exhaust and lid mod would need atleast a 155 main jet, probly more like 160ish.

What jets are in there now? With your mods I would think a 180main and 42pilot would be the smallest you could run.

[Jns400ex]

It will run good then at times. It seems like theres an electrical problem or something