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I have a TaoTao ATM50-A1 carburetor based 50cc GY6 Chinese 4-stroke scooter, made in 2012, itself a clone of the wildly successful Japanese Honda Elite GY6 engine from the early 1980s, as everyone knows.  The 139QMB engine isn't it?

The problem is, it's really hard starting, and stalls out, especially in colder weather when it's nearly impossible to start.

It usually stalls out the first 5 or 10 starts before it'll stay running, IF it stays running. This is really annoying because the electric start battery usually runs down before the scooter can get running and stay running. This forces you to kick start it 20 times in the hopes it'll start and stay running.

The only adjustments on the carb are:
1. idle rpm throttle cable screw which supposedly controls the idle - 1/4 throttle, along with the idle jet's hole opening size.
2. carb needle: the vacuum-operated, rubber diaphragm-suspended, needle, with add-on rings which decide how far down into a metal hole air passageway it hangs and therefore how much air it blocks, this supposedly controls the 1/4 - 3/4 throttle mixture.
3. air/fuel ratio mixture screw which apparently controls the 3/4 - wide open throttle mixture, currently this is adjusted according the procedure in the taotao 50cc 49cc chinese scooter 139qmb engine service manual, basically it tells you to start it and run it rich at around 3000 rpm, and gradually close the screw (lean it out) until it gets to peak rpm, then give it a quarter or half turn richer, to stay safe.


I've tested the usual things and ruled them out:
1. spark is reliable,
2. spark plug is new and decently clean enough,
3. there seems to be no air leaks around the rubber manifold because spraying starting fluid there makes no difference in the engine start behavior,
4. the vacuum operated petcock is good,
5. fuel supply is flowing and entering the carb OK,
6. new gas in the tank,
7. the original carb I removed and replaced with a new one of the same type with screw heads still there so on this new carb you can open it and clean it, unlike the original factory carb which had its screw heads cut off to prevent you from opening it and cleaning it.

Anyway, I just want the thing to start up the first time and stay running, not fade and stall after 10 seconds, and no more instant stalling when I try to rev the engine past idle!

Any suggestion on how to pinpoint the cause of the problem and fix it??