My 50cc Chinese Scooter GY6 QMB139 hard start when cold, anything below 65 degrees F. Last spring when I got it I had nothing but trouble, fixed the following and it ran perfect all summer until it got cold. 4 stroke. Cleaned carb (many times) bypassed the fuel petcock with a shutoff the screws on the top of carb that cover the diaphram were too short that caused air, fixed that with longer screws replaced the intake manifold, (had a tear) I always use high octane gas with a cap of that blue enzyme stuff

When the scooter is warmed up it runs great but I need to stay on the throttle for 20 minutes cause it will stall at idle as well. Today was 55 degrees out, when I got it running good I went to the store, 15 minutes it was not running, started and ran good for 1/2 mile then bogged down and stalled at my 1st light. Takes 20 seconds to start and it takes right off but will do the same until its warm. Usually I drive it 2 miles on back streets until its warm because it will stall 3-5 times

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    have you ever had the valves adjusted on the scooter? Commented Mar 8, 2016 at 23:02
  • As well, when the scooter is warmed up does it experience any strange idling issues like a high idle that hangs and then drops considerably. Any issues like that? Commented Mar 8, 2016 at 23:25
  • Never adjusted the valves, it ran great during the summer, but when it got below 65 it runs like poop. When I finally get it running it idles fine. Yesterday I got it warmed up and driving after about 15 minutes, parked it for 10 minutes started, took off for half mile then started dying then stalled. Started up and took off strong for the next half mile. and idled fine when I got to my destination. Commented Mar 13, 2016 at 19:30

1 Answer 1


It depends on the oil that you use on your scooter, If you are using heavy oil then the cold start is going to be a problem, I have a 150 cc Gy6, I use lighter oil like 5w-40 and I had no problem, but if you are using any other oil that has higher viscosity than 5w-40, you better switch back to the 5w-40.

The manual suggests 75w but most people use 5w-40. I use the same and never had any problem, my scooter has about 5000 miles. So save the heavier oil for summers and lighter oil for the winters.

  • I have always used 10w-40. This scooter has 2300 miles on it, its a 2014 Commented Mar 13, 2016 at 19:31
  • That sounds reasonable I am in Florida and I am using 10/40 . It gets into the 60s here and my scooter is hard to start in the beginning of the day the only logical thing I can come up with is that the oil is too heavy . Although in the summertime I can run 15 to 20 miles a day to and from work and I've been running 50 weight 2050 rather because of the heat factor . I'll try the 540 dude thanks for a reasonable tip
    – Richard
    Commented Mar 3, 2019 at 17:21

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