Help with ATC 70

[Mike O]

I'm having trouble getting my 85 ATC 70 running correctly. I put in a new piston and rings, new points and condenser, rebuilt the carb, etc... The bike starts and runs but will not idle. The idle speed screw and pilot screw have no effect on how the engine runs. I can turn the idle screw and pilot all the way in and it still runs but will not idle. I thought something must be wrong with old carb so I bought a new factory Honda carb and still have the same problem. There are no vacuum leaks at carb or manifold. What could possibly be wrong? I did set the point gap with the flywheel on the T mark and have been told this is incorrect. Could this have anything to do with it? Any ideas or help would be greatly appreciated.

[HairyJR]

Point gap should just start to open at the "F" mark, size of gap isn't all that important. Recheck cam timing could be off one tooth.

"HJ" 265592 :beer

[coolpool]

If what Hairy said checks out fine, what about checking your valve lash again? What's compression at now after rebuild?

[Mike O]

Thanks guys, will check cam timing and valve lash today. I don't have a compression tester with a small enough thread to fit the small spark plug hole. Will have to buy or borrow one.

[Mike O]

If cam timing is off one tooth could the valves interfere with piston and cause damage? I hope not

[Mike O]

Cam timing and valve lash right on. Engine failed the leak down test and compression test. 90 psi dry and 150 wet. I guess it needed a rebore job instead of a quick hone and standard piston and rings. Lesson learned. Anyone know of a good machine shop to do the rebore?

[Jim mac]

was it a std bore? did you mike the old piston vs new piston? make sure you didn't install a too small piston and ring set
did you fill the ports with alcohol to see if the valves might be leaking? jim
edited the post. jim

[Mike O]

The bore didn't really look that bad but I didn't measure it. Old piston was original Honda so I assumed it was standard size. I wire brushed the dome and didn't see any over size markings. Unfortunately, I already tossed the old one. I replaced the valves and springs with new Shindy parts and the seats looked good. I lightly lapped the new valves then saw in the shop book that you're not supposed to lap them? Hard to believe the little bit of lapping I did would make that big of a difference. I will say that the bore had a strange low spot about halfway down. I couldn't really feel it with my finger but it was visible. I honed it until it was gone which took a bit of doing. Maybe that was enough to mess things up?

[coolpool]

Sounds like it needed a bore right from the start. At least you know why it's doing what it's doing and you're not chasing ghosts. As for lapping, I may be off but I've done it lightly
on a couple of small Honda motors and had no ill results on machines still running fine after a number of years. Apparently the factory seat hardening is microscopic thin and subject to removal by lapping?