70CC engine problems

[ghost civic]

I got my trike built from a 70cc kids atv
It was running good
I had to remove the carb to adjust the float, and at the same time I changed the oil. The engine had a 1/8'' layer of caked stuff at the bottom after draining the oil. Put new oil in and figured it would be fine.

very shortly after that, it started to die any time you gave it throttle. It would idle and always start back up fine but got so that it couldn't even move.

The carb on it was old, and I figured a new one would solve the issue. Put a new carb on today, it started right up but then died when I gave it throttle. I checked the valve lash...both were tight but since it was able to start and idle I figure they couldn't have been too tight. I gave them both a bit of play but the bike still would not move. I adjusted the mixture screw on the carb and off it went...but only for 100' or so, then started acting up again. No adjustments to the carb made it any better, it actually got so it would start, idle for 10 seconds or so and then die.

I went to check the valves again and noticed a hole in the engine which would lead to the timing chain cavity.

I am wondering, would a hole there prevent oil from being pushed up to the valve train? I don't honestly know what moves the oil to the valves/rockers...and I'm wondering if either the hole is preventing oil from flowing there, or if that caked up stuff I found when changing the oil ended up blocking an oil passage.

The picture shows where the hole was, I just put high heat stick weld over it.

 

[JTSpeedDemon]

That hole was certainly a problem.
But I don't think that was the cause of your erratic running.

I have weird erratic running issues with my kart, too. I now know that it's because of a worn throttle shaft causing a vacuum leak.

Obviously your throttle shaft is fine as it's a brand new carburetor, but check the intake manifold and all gaskets. That's probably where your problem lies.

Oh, and try to find the ACTUAL spec for the valve lash instead of guessing.

 

[Texan]

A small hole on the Cam cover would not cause erratic behavior.

These engines have an oil pump, so if it was really dirty as you say, the pump may be clogged and causing your valves to overheat from lack of oil.

 

[ghost civic]

That hole was certainly a problem.
But I don't think that was the cause of your erratic running.

I have weird erratic running issues with my kart, too. I now know that it's because of a worn throttle shaft causing a vacuum leak.

Obviously your throttle shaft is fine as it's a brand new carburetor, but check the intake manifold and all gaskets. That's probably where your problem lies.

Oh, and try to find the ACTUAL spec for the valve lash instead of guessing.

You wanna guess what part of the old carb I reused? throttle shaft/slide. Put the new one on and she ran great...for about 5 minutes...then she died at idle. Had trouble restarting. Tried messing with the carb a bit and hit the starter button and suddenly the engine sound changed...starter motor was spinning faster.
Put a compression tester on it, reading only 25psi!
Checked the valve lash, both seemed good.
Looked at the timing chain, looked good.
BTW, both the exhaust rocker cover and the timing chain cover showed fresh oil...so the top end looks to be getting oil.

Was about to give up, thinking a ring must have gone...put the spark plug back in, hit the button and she fired back up. My son drove it around for 10 minutes with no problems.

Now this engine has smoked some since I got it, and while the problem I first posted was almost certainly the damaged throttle cable causing a vacume leak (which at times could go away based on the strain on the cable shroud) the engine must have worn valve guides...I'm betting that is why it burns oil and when the compression dropped that the exhaust valve stuck slightly opened and it freed itself up.

I will have my son give it a good thrashing tomorrow...more than 10 minutes to see if it holds up. Higher temps could cause the valve to stick again. Not sure I can do anything for it without pulling the motor out.