idle at 5000rpm or stall, nothing inbetween?

It's a 66cc China Girl:

I have a problem with the engine either revving like crazy or stalling when the clutch lever is pulled. I took the carb apart, everything looks normal. For laughs I used a spare carb and tried that, exactly the same problem. With the carb off the bike but connected to the throttle cable, I can see the needle and sleeve moving up and down nice and smooth. Occasionally I can get a more reasonable idle rpm, but then the rpm's creep lower and lower until the engine stalls, or I blip the throttle and it goes up to 5000rpm and stay there. (I have a rpm gauge). The carb idle adjust screw is frustrating because I either get high rpm's or a stall, there is no happy medium. The engine runs fine when it's driving the rear wheel, it's just aggravating to pull up at an intersection, pull in the clutch lever, and either the engine rpm's go to maximum or 0. The clutch is not dragging, because with the clutch lever pulled in and the rear wheel off the ground, the wheel doesn't move. Any ideas?


FYI - the magneto coil has been replaced too with a spare, no improvement. I didn't try replacing the ignition block or throttle cable because I don't have a spare one of either of those to hand.

Not much progress this week, too much rain outside. I removed the standard inlet tube and installed a ManicMechanic straight inlet with rubber seals. That didn't improve anything so I doubt there is an inlet air leak. Next I'll check the exhaust seal to the engine and tighten the head bolts down a bit.

Worked on the bike today. First tried some silicone sealer where the throttle cable sits in the top of the carb, no improvement. Then thought I could detect small puffs of air at the front of the engine. Checked the exhaust nut tightness, nice and tight, no improvement. Checked the head bolts, already tight, no problem. Replaced the spark plug, no improvement. Took off the head and it looked like the gasket may have been leaking, but the gasket got damaged in the process of removing the head so really wasn't sure. Cleaned up the mating surfaces and used a new gasket, bolted the head back on, bingo. Now there is a nice stable idle rpm, and I'll take a longer ride tomorrow to be sure all is well. Thanks for all the suggestions, and it was the air leak ones that hit the target.