139QMB engine surge

[Mark...]

My scooter runs like crap! Its a chinese four stroke 139QMB with a walbro slanted bowl carb. everything is okay except for the fact that it surges. From a dead stop I can full throttle and get 2 and a half seconds of acceleration and then it completely acts as if I went to zero throttle(without changing the position of the right handgrip), then after about an equal time of no power, it goes to acceleration again. I have fought this problem for quite some time now. I have tried cleaning the carb, disconnecting the air cleaner, changing the cdi unit, replacing the vacuum actuated fuel valve with a ball or shut off valve and plugging the vacuum port, changing the float level, changing the mixture setting, and numerous experimentations with the carburetor. I connected a vacuum hose to the vent port on the carb and found that if I apply vacuum to it, the surging subsides. I have also found that too much vacuum and the engine bogs, also that during acceleration vacuum is sometimes self produced, and if I close the vent port during this event that the scooter will accelerate ok until too much vacuum is present, in which event opening the port seems to help, or occasionally insufficient vacuum is present, in which case it again bogs and responds to induced vacuum. Finally I replaced the carb. the thing ran worse than ever. I pulled the whole engine and swapped it with another of equal value. I attained a third carburetor that ran okay for about a week, then the surging returned. now back to manually induced vacuum and venting. I have taken the bike to a few different scooter mechanics here in Vegas, but all I get is "I've never seen that type of problem", or "I've never heard of anything like that" and of course the occasional "its probably cuz you need to clean your air filter" or "have you checked your belt and pullys?"!!!!!!! PLEASE!!! I have been rebuilding carburetors on cars and trucks ever since 1990 and the carbs on these scooters are way less compicated. Can someone with knowledge please help!!!!!
Driving this thing in Las Vegas traffic with this condition is neither safe nor fun.
Mark...

[cb50j]

sorry ft,

like the scooter mechanics, i've never heard of such a problem

"I have been rebuilding carburetors on cars and trucks ever since 1990 and the carbs on these scooters are way less compicated. Can someone with knowledge please help!!!!!"

sounds like you're more qualified than most of us

sorry again

..................... C

anyone help this man?

[ally1756]

dont ahve a scooby on that one bud, is it the same carb that came with the bike

[Mark...]

I am thinking that the carb is loosing vacuum(leaking) somewhere. Maybe the rubber diaphragm has a pinhole? Still shooting in the dark.
Yes, Ally its the same style and size, the original carb is on the shelf with yet another twin. In other words I have three of the same counting the one on the scoot.
The scoot has been running a lil better the past few days, once up to about 17-20mph the surge stops and I can usually get around 38mph on flat. I guess thats not bad, maybe well enough left alone?

[daddyeby]

I would check the flow of gas into the carb. A good fuel filter ect It sounds like my scoot after drilling the jet. It don take much to slow the gravity feed of gas in as small of lines as they use.