ATC 90 was running good till it started smoking really bad and popping and backfiring

[81ATC185S]

I have a 1977 atc 90 that ran great now its smoking really bad and backfiring from the carb and exhaust.

[barnett468]

Is it lite blue smoke or black?

Is it constant and a fairly big amount?

If its lite blue, your top end is probably wasted.

If it's black, you might only have a carb problem.

Check the valve clearance.

Post a photo of the end of the spark plug.

Do a compression test with the gas off and throttle wide open and kick it 5 times.
.

[John Tice]

ATC; odds are that your piston rings are worn out, time to tear it down & take a look.

John Tice
503-593-2908 Alternate 541-508-3944
www.smallenginemachineworks.com & www.nwsleeve.com
Turning Custom Cylinder Sleeves Since 1971

[ps2fixer]

It could also be related to timing, such as a timing chain jumping a tooth. I'd throw checking the crank and cam timing on the list of things to check. Seems kind of odd it is all of a sudden, might want to check your air box as well, maybe some mice made a home there.

[1979atc]

Check timing, valves, and spark plug. Could be your points plate that moved or that the timing chain jumped a tooth or so.

[81ATC185S]

timing was off but its good now. I think the rings stuck again, because when you pull it over with the plug in and then take it out it will be wet with oil.

[1979atc]

Ok probably be a good thing to take top end off and put in new rings. Or could possibly be your valve seals. But probably not if you say it's only when you pull it over then has oil on plug, be a good thing to replace valve seals when putting in New rings. Good luck