Here's my story..
Bought a 1999 400ex in parts 2 months before Christmas for my son figuring could rebuild and have done by Christmas. Took to a friend/does atv work on the side. We basically built it together. It ate the backside of the first piston in less then 5 miles(was a new piston with new bore). Took back to machine shop checked bore and claimed good so rehoned and sold me new piston and rings. Put that in and guess what ate back side of that piston. Supposedly my friend took it to another place and bore was checked and was all good. Rebuilt again with new piston and rings and after a whopping 20 miles it started to high rev with no throttle being pushed. Shut it down right away checked carbon all looked good bought rebuild kit put in and fired up and was still reving high went down road about half a mile and revs came down when i turned around but when i got back to the yard was reving again. Shut it off and now it will not restart. Checked timing and was close but got it on and it still just turns over for about 20 revolutions and then give a very weak pop. It sounds like compression is down. I have checked for spark and its all good. Later this week i will get my compression tester and know forsure just thought i would post here for help and suggestions.. I have well over 2 grand into this with new tires and stuff and Im not the happiest. I will be doing the next build myself ao i can't blame or complain..

Any help would be great. My ideas are new jug with new piston and rings if compression is low but with the soft pop Im sure it is.

Thanks look forward to the ideas

Jeff

 

Posted with droid and made a few mistakes sorry about that hopefully can understand my gibberish : )

 

Sounds like a air/fuel problem. Check for leaks around the carb boots. That would certainly cause pops. Make sure gas is getting through. Did you do a good carb cleaning after putting it all together?

I'm not sure about the timing thing. There isn't adjustable time on these. Line up the dots and thats it. They are known however to stretch the cam chains. I"d put a heavy duty chain on there if i was rebuilding anyway.

 

Iam with koopa air/fuel issue. What does your plug look like, white or black? You say it ate the back of the piston. Was it melted? If the piston was melted in a few miles that ex must of been running so hot it had to be uncomfortable to ride.
Any mods, what is the bore? Bigger bore means bigger jets.

 

Thats a good point. What exactly happened to the piston? Like ate up as chopped up? Something else entirely must be wrong if that's the case. Something like clearance issues with the head or something. Are the valves OK? Nothing bent or chipped? That could mess up a piston quickly. You did line up all the marks when installing the cam? That's basically your timing. If it's off you could be banging a valve off a piston. You'll find out quick when you do a compression test.

 

The back side of the piston rubs in the bore of the jug on both of the builds and probably this one. It was bored twenty over on the first build and honed for the 2nd and 3rd. Did seem to have quite a bit of metal in the two oil changes i gave but just hoped break in. Spark plug has been nice and brownish and put new one in just now with no help. It has a decent header but a pretty quiet stockish pipe. Was all perfect until all of a sudden just like the last two.

 

And i can only get the pop if i give it some gas or choke it and it gets the spark plug wet. So Im assuming fuel isn't a problem i could spray some starting fluid in it while turning it over?

 

Hmm.. Some wear is normal as the rings seat in the newly honed cylinder but, this sounds bad. If the rings are proper and if the actual piston is damaged, I'd find me someone else to mic the cylinder next time. The guy I used not only goes by spec's but, has me bring him the new piston as well to check it against the cylinder. With the proper bore and piston there shouldn't be enough room for it to do that kind of damage.

 

Again Iam with Koopa. With the first post I was under the impression that the piston was melting but if its rubbing the cylinder the bore is to big or the piston is to small imho. As a rule of thumb your suppost to have the cylinder bored to the piston that will be used. So dont bore until the machine shop has the piston that will be used. I dont know if it is possible to have a issue like this from the wrist pin, connecting rod or crank bearings. But when you tear it down I would check them out.

 

I agree that the bore is probably too big for the piston, causing it to slap back and forth at the bottom. But the other issue I am hearing is that since the carb side of the piston took the hit everytime, I think there may be a carb issue too. Just guessing here.