139QMB 50cc screw adjuster square top measures around 3mm . . nut is 9mm . . so the H9 tool should be compatible.
Per
this parts listing, all the GY6 50/125/150cc engines use the same adjuster screws and nuts.
Not really a great value, but the Motion Pro gauges work very well . .
.002 inches for cold 50cc . .
.004 inches for cold 125/150cc.
Since GY6
are a frequent gremlin and while access is open, loosen clamp and slide carb back from manifold, scrape/file any mating casting bumps, and apply a very light smear of silicone grease (not RTV sealer) on the join surfaces to deter leaks. Replacement
resource (single vacuum port).
Also only takes a few minutes to test battery
and
charging volts to monitor those gremlins.