C70 Challenges

[rymburt]

I’ve got 3 C70s. I love all three equally but one is a problem child. It’s a newer acquired 1980 with 2000 miles and always garaged.
I swapped tires and tubes, cleaned the carb repeatedly. Installed new battery, points, condensor and even coil since it was cracked. I’ve adjusted the valves and timed the ignition and cam chain. After not helping I tested compression at 70 lbs. So I swapped jug, piston rings and all seals with OEM.
It has a couple issues. It idles weak until it craps out after a couple minutes but pulls like a mofo with throttle.
Why the heck can’t I get it to idle consitently?
I’ve even swapped out to a dratv knock-off carb with the same results.
The other issue is the tranny. It clunks from gear to gear nearly snapping my neck in doing so. I tried to adjust the clutch but no good so I swapped the discs and seals which showed minimal wear and I still have terrible shifting luck with it.
Any thoughts would be appreciated. My next thought is to pull the right side cover again and go looking for broken or bent parts.

 

[69ST]

Have to talk in general terms, since I'm not a C70 expert. Fortunately, the engine is basically a garden-variety 72cc horizontal single.

Assuming that your carb swap test has eliminated the carburetor, itself, as part of the weak idle problem, then the next things to consider are: point gap, vacuum leak, worn valve & seat. I'd start with the point gap, since it's easy, quick & costs nothing. I doubt that's the issue but, wtf...may as well be thorough. Vacuum leaks usually manifest in hard-starting, especially when cold and weak/erratic idle. They can be tricky to diagnose/localize. If the intake manifold has never been off the engine, start with the carburetor flange O-ring & insulator + gasket. Again, cheap enough and easy enough to take a shot throwing parts at the problem. If you want to be thorough, however, pull the intake and look inside the port. If it's not completely clean, the valve isn't sealing. The leakage comes on gradually, which can explain the weak idle/strong high-rpm performance. Compression is bleeding-off slowly enough that rpm can overcome it...for now. It's possible that the rings aren't sealing properly. Once you pull the head, you'll be able to see the cylinder walls and go from there.

As for the balky shifting, definitely strikes me as a clutch issue. Shift the engine into gear, from idle, don't release the shift lever then feed the engine some throttle. It should freewheel, same as if you were grabbing a clutch lever on a manual. If the bike moves, there's your problem...clutchless shifts. Then it'd be a matter of figuring out why and making the necessary correction(s); could be as simple as an adjustment.

 

[rymburt]

Both helpful suggestions. I may pull the head again and really go after the valves. I’ve carb cleaner the hell out of it and no leaks are found.

I also think I may have to pull the trans cover and dig in since the clutch swap made no difference.

 

[69ST]

IDK...somethings here just aren't quite "adding-up", mentally. If there's little enough mileage that the rings haven't yet seated, then splitting the cases and going through the entire motor, top-to-bottom could pay dividends. IOW, you'd find some answers to your questions. IF...and that is the key word...the clutch is correctly assembled, in good working order and properly adjusted, then the poor shift quality may be the result of worn shift forks & pins. Gotta split the cases to check these items. As for valves, if they're leaking, there's no real substitute for a proper valve job, i.e. cut the seats, new valves + lapping.

All of that having been said, it IS possible that you've missed something with the clutch & linkage assemblies and that's where I'd start...far less work than a complete teardown. And, those parts have to come out first before the cases can be split.

 

[Karl81]

You do know these little engines have a cam adjustment screw you set while it's running. Left side up front on the bottom. It tightens the timing chain. Could be your issue. As well as knowing how to self adjust the clutch.

 

[red69]

I think you mean the cam chain tensioner. No adjustment on the cam itself except for jumping the cam sprocket a tooth either way off the head mark.