Need help identifying a problem with single cylinder engine

[Guest]

Hey everyone! I tried asking this question over at TT on the XR forums but I never get many responses over there and seems how you all are so awesome at answering motorcycle related questions I thought you could help me out.

I picked up an XR80 4-stroke dirt bike for my son. Got the points cleaned up and set, now she starts great and has good spark. However, it doesn't like to idle and the RPM's will all of a sudden go WAY up and stay there for a while and then maybe come back down. So my first thought was.....ahhhh dirty carb. So I took the carb apart and gave it good cleaning and put it back together. Honestly it didn't look that bad anyway.

Still doing the exact same thing. Now the guy I bought it from said the compression was a little low. So now I"m wondering is a valve not seating all the way occasionally? If it was the intake valve that might cause the surge right? Also when it's doing that it smokes like crazy!

Thoughts? I know I probably need to replace the rings and plan a full top end rebuilt for the winter but I'd like to know what's causing this before I go any further.

Thanks for any help you guys can offer.

[Nessism]

Air leak into intake system. One of the most common GS problems too.

[Jeepfreak81]

Ya I had replaced the air boots on my GS850 because it ran like crap. Hmm maybe I'll try starting it and spraying carb cleaner around the intake boot to see if that's the issue, there is an O-ring there. Would a worn out air screw cause that? when it revs up I can crank the air screw down and it calms down.

[Nessism]

Spraying stuff in search of intake leaks will only find drastic leaks.

[chuck hahn]

Bluish white oil smoke or blackish grey fuel smoke???

[Jeepfreak81]

Definitely blue oil smoke. Actually had it running off and on this evening for a total of at least 15 or 20 minutes and never had the surge problem, idled half way decent too. I reset the air screw to factory setting (I believe). Not sure if that's what helped, and I'm not counting my chickens yet.

Will do a full top end rebuild this winter but hoping my son can get a little time in on it before the weather turns because it was his B-day present. Not sure how long this bike has sat either, didn't run when I got it.

[chuck hahn]

Run the snot out of it for a while....rings may have been stuck from prolonged sitting. They will release and if it still has oil smoke after a week or so it will most likely be rings when the rebuild takes place.

[Cassius086]

Yeah, see what happens when you run the poop out of it for a couple hours. Maybe use a bit of seafoam in the fuel?

And when you do the rebuild, let him help. Coolest Dad and son project ever

[Jeepfreak81]

Planning to do the rebuild together for sure. He rode it for a while the other day, runs pretty good just smokes a bit. You'd think it was a 2-stroke if you plugged your ears. I put in a hotter plug and she idles much better now and it doesn't seem to be fouling the plug too bad. Good enough for the little bit of riding he can get in before it gets cold. Then we'll put it in the basement and rebuild it over the winter.