I recently made the stupid mistake of fixing a Chinese scooter we got for free, qmb139 "gy6" 50cc. It reinforced why folks distrust those china motored things. so much wrong. it still doesn't run right despite fixing SEVERAL smoking guns and replacing EVRYTHING.
Had to replace the tank and petcock off the bat, rust city.
Then was the carb - just wrecked but that was from a year or 2 of neglect, and a whole new one was 20 bucks shipped. had it almost cleaned except the accelerator pump, and wasn't confident on the idle.
It'd idle fine and run to about 20-25 and then just start surging, like someone flicking the light switch on and off. rev die rev die rev die. about 1 second cycle. I thought it was jetting and clogged jets so I got the new carb, slapped it on, still only went 20-25 surging. checked the variator and that's all good, variates fine and isn't surging from the vario. figured valves maybe were bad and then hoke just ordered a whole kit for it. head cylinder valves rockers CDI, ht coil, new intake- everything, even belt and variator weights for $80 shipped lol.
The only thing it didn't have was the spacer under the carb which it turned out was cracked out and airleaking massively. Figured that musta been the problem, but with the spacer replaced and whole new top end it runs EXACTLY the same. surging on/off as soon as it hits 20-25.
Played with jetting from 70 to 95 Also played with airbox and pipe, the jetting affects how it runs but in all cases it'd still get either sputter or boggy or whatever it'd hit 20ish and start surging on and off. it's like a 1 second cycle.
So I replaced the CDI and coil and plug and still exactly same. THEN! I noticed the pickup for the CDI trigger had a broken bolt! I fixed that bolt and still no change. but then noticed the pickup itself looks cracked so I just ordered a new stator and pickup. not arrived yet
*New gas, tank, lines and filters
*New CDI, coil and plug
*new 80cc cylinder, head and valves, neatly set and timed
*New intake, carb and gaskets, jetted from 70-95, now "best" at 85
It runs exactly the same as it did with a clogged stock carb, broken intake gasket, loose valves and a broken intake pickup. Exactly. the. same.
Any thoughts? I've had fun seeing if my diagnoses match the MA OT on other vehicles, whattaya got?