Modern Buddy

If I try to open the throttle up when I'm under 40 mph the engine sputters.

It's that clear throttle point where the main jet opens. Go past that and sputtters all the the way up through WOT.

Once I hit about 40 bmph, the engine smooths out.

So what says you? Fuel air adjustment screw? Slow jet?

I think it's a 112 main jet. Anything bigger and it drowns when the main is open.

Do you get the same symptoms if you run it on the center stand and work the throttle? Sounds to me as if you've got a clog in the main jet and it's not supplying enough fuel for the instantaneous demand generated when you crack it open. Could also be a torn membrane in the carb's slide assembly. In that case, you wouldn't be generating enough of an air pressure differential to raise the main jet needle high enough get added fuel into the mix.

I don't think that the mixture screw would cause these symptoms as it only governs the mixture at idling speed.

charlie55 wrote:Do you get the same symptoms if you run it on the center stand and work the throttle? Sounds to me as if you've got a clog in the main jet and it's not supplying enough fuel for the instantaneous demand generated when you crack it open. Could also be a torn membrane in the carb's slide assembly. In that case, you wouldn't be generating enough of an air pressure differential to raise the main jet needle high enough get added fuel into the mix.

I don't think that the mixture screw would cause these symptoms as it only governs the mixture at idling speed.

Same behavior on the centerstand. Once it running 45 MPH or faster all, it's right as rain.

I guess the needle valve could be the issue since that's vacuum operated.

Torn down the carb - no problems found.

Re-assembled and still have the same problems.

It's worst when the engine is cold.

Does it bog, or just miss-miss-miss until 45?

Spark, maybe? Check the cap is seated firmly, the wire's snug in the cap and coil, and that the spade terminals that connect to the coil are snug.

az_slynch wrote:Does it bog, or just miss-miss-miss until 45?

Spark, maybe? Check the cap is seated firmly, the wire's snug in the cap and coil, and that the spade terminals that connect to the coil are snug.

Bog, sputter, backfire.

Plug is good too.

CDI?

Auto choke stuck open?

I'm with charlie55. If this is a CV (constant velocity) carb, any flaw in the membrane or seal of the chamber above the carb would cause the jet needle to not raise to the right height for a given throttle opening. This would really mess up the mid-range.

OldGuy wrote:I'm with charlie55. If this is a CV (constant velocity) carb, any flaw in the membrane or seal of the chamber above the carb would cause the jet needle to not raise to the right height for a given throttle opening. This would really mess up the mid-range.

It could be CV diaphragm, but I had a thought: JettaKnight indicated that it does come 'round above 45mph. It he had a diaphragm leak, you'd think that he wouldn't be able to accelerate past a given speed due to vacuum loss.

Could be an air leak on the intake side (loose intake fasteners or a leaking vacuum line) between the carb and the cylinder head.

JettaKnight, any recent maintenance performed prior to this problem? Did it manifest gradually, or show up abruptly?

I thought I'd bring some closure to this.


Turns out it was the ignition coil.


Cheap Chinese "performance" part.



Also, yes, it's true I really didn't ride much from 2014 until the summer of 2016.

Thanks for posting the fix, it may help others with the same problem.

It was suggested in an earlier post that a leaking CV diaphragm might be the cause of the original problem, but in my experience, this would cause the opposite condition where the engine rpm's would be limited thus the top speed of the scooter would also be limited. In my case, the diaphragm was not damaged, but rather not sealing correctly around it's perimeter regardless of how carefully everything was assembled. This caused a vacuum leak which prevented the throttle slide from fully opening. A little silicone grease around the sealing surface of the diaphragm solved that issue for good.