Scooter bogs during initial take-off. Suddenly 'wakes' up

[dimarza]

Hello everyone,



I'm looking for some advice on an issue I'm experiencing with my 50cc Chinese GY6 scooter (139QMB engine). The scooter idles fine, and when it's on the kickstand, the throttle response sounds great. However, I'm having trouble when there's a load on the scooter.



Here's the problem:



When I'm trying to take off, the scooter seems like it can't accelerate. If I put my feet on the ground, I can actually stop it from moving forward. It doesn't rev up during this issue, so I don't think it's related to the belt or clutch. The scooter sounds like it's bogging down when this happens.



Interestingly, if I help it along for the first few meters (like by pushing with my feet or lifting myself up a bit), it suddenly "wakes up" and then accelerates normally, reaching its top speed without any further issues.

I don't think it is a transmission issue, since the behavior is strange.

I mean, when the scooter is on the kickstand, it revs good and the wheel runs normal and accelerates normal.



When I get it off the stand and try to accelerate with me sitting on it, it does not go forward with power. It does not rev up and the sound is like it is bogging.

If I push it a little bit and give it a slight momentum, it accelerates as perfect and reaches the top speed.



What I've done so far:



I've replaced almost every mechanical component I could think of, including:



New carburetor



New cylinder and cylinder head



New spark plug and ignition coil



New air and fuel filters



I've also checked for any vacuum leaks and inspected the variator rollers, which appear to be in good shape. Also performed a valve clearance check and they are within the specs. Despite all these replacements and checks, the problem persists.



Next steps:



The last component I plan to replace is the stator (magneto) because I'm starting to think this might be an electrical issue. I have tried 2 different CDIs and they both have the same issue.



Question:



Does anyone have any suggestions on what else I should check or consider? Could there be another electrical issue causing this bogging under load? Any advice would be greatly appreciated!



Thanks in advance for your help!

[FrankenMech]

This is a prime example of why one should never just "shotgun" replace parts. It is expensive and any one or more of the new parts may be defective making the problem worse. Please download a service manual and carefully work through the troubleshooting charts in the service manual without skipping ANY steps or checks. A lot of time and effort went into the development of the troubleshooting charts. Any other 'suggestions' we may have would be wild guesses liable to be as useless and expensive as your efforts so far.

[GrumpyUnk]

I would put all the old pieces back and start again. As Frank noted... adding new parts MAY add a new and different problem. Leaves you wondering what did I just do? THAT used to work, and now it doesn't, or perhaps covers/makes worse the original problem.
First thought was the slide in the carb being slightly sticky. Develop enough vacuum in the intake and it gets applied to the diaprhagm and FINALLY starts to lift the slide, and then rpms take offf and away you go.
That compounds with a new carb. A poor vacuum, caused by loose bolts(intake/carb), leaky vacuum lines, or more than 1,266,303 other things that can go wrong could also cause bogging. A too-lean pilot will cause a droop in acceleration, as would a faulty/mal-adjusted accelerator pump.
Read the words... and follow the checking procedure unless you don't mind following dead end rabbit holes.
Annndddd Away We Go!!! (fat guy named Jackie Gleason)
tom

[pili]

The fact that a small "push start" gets the acceleration going points to a clutch or possibly variator issue in my opinion. If you can get someone to help you, try taking the transmission cover off and having someone film what's happening with the belt when you're trying to take off and when you gently help it. The problem may reveal itself.

+1 on not making the assorted changes all at once. Easy mistake to make but not to diagnose.

[GrumpyUnk]

As I read the last post .. push start... made me think of a worn variator. The weights can wear away the metal of the slide area, and get the weights stuck in depressions. Given that, the variator will not move until you get things going by pushing(help the upshift?), or the rpms get high enough the weights can start to make their way up the ramp to get the variator to start variating...
I would suggest taking the variator apart and inspecting the gizzards. If the ramp areas where the weights reside are worn, time for a new one. Or, if adventurous and enjoy time wasting experiments, you could fill the depressions with J B Weld, making new smooth and flatter ramps so the weights move with reasonably even speed.
tom

[wiscootsin]

you can adjust the gap of the stator/magneto. You could also buy a $3 spark tester and verify the spark is strong or not. If you're thinking electrical, i'd add some redundant grounds before replacing or messing with other parts. distributors go bad, but i doubt would cause anything related to your problem.

I think the main suggestion would be to try more diagnosis before replacing parts - unless you just really enjoy tinkering and wanted new stuff on there to begin with. did you at least put an 80cc kit on it?

if you want a guess, i'd go with carb. if you replace the carb with a cheapo $20 carb, its just as likely crap and can take a few tries with the $20 carb to get a winner.