Runs rough stalls at idle

[fekim]

I have a CY6 150cc in a JMSTAR scooter. I own 2 of these scooters. I've had these for 2 years now. One of them began to run rough while accellerating from a stop, but runs fine after a constant speed. After a few days it began to stall at a stop or when I try to take off from a stop. It seems to do this after its run a few blocks. If I let it idle the RPMs go up and down and sometimes the engine will die.
I went through the carb twice. I switched the electronic choke. I replaced the O-ring between the carb and the head. I swapped carbs with the other scooter. Replaced the fuel pump. Checked all vacuum and fuel lines and it still didnt fix the problem.
Any other ideas? please.

 

[mavio]

If a jmstar is a two stroke check the reeds. An air leak would be the obvious but you seem to have tried that already.

 

[fekim]

Its a 4 stroke air cooled.
When idling it seems to have a miss to it every few seconds, and sometimes will die.

 

[StepVino]

Do you have a service manual for it? Does it have an EGR
sytem? Egr system takes some exhaust gas, and recirculates
it into the intake. In cars, if this goes bad (too much egr), it can
have very similar symptoms.

 

[fekim]

I've read through the service manual and don't recall seeing anything about a EGR system.

 

[StepVino]

Ok. At this point,since you have tried switching a lot of parts, I
would still be suspicious of something emissions related. Of course,
as someone mentioned, there could be an air leak somewhere.
Study any strange hose coming into the intake. For example,
there may be something going to a charcoal canister.. a system
to keep the pressure low in the gas tank, and sucking out fumes
from there into the intake.

 

[fekim]

There is one hose from the intake.It looks good. I used carb cleaner to spray around all hoses and carb and I detected no signs of a leak anywhere while idling.

 

[StepVino]

There is one hose from the intake.It looks good. I used carb cleaner to spray around all hoses and carb and I detected no signs of a leak anywhere while idling.

My point is it may not be a leak, but a malfunctioning emissions
system. You wouldn't detect any leaks, it could just be sucking
air from somewhere in the system when it's not supposed to.
I don't want to send you on a futile hunt though. I would just
study the intake system from the service manual, if there's
enough information there to do that. See what other systems/
parts are connected to it, and when they are supoosed to be
on or off. If you are not sure, you could even try removing
the hose you mentioned from the intake, and blocking off
the hole. If that helps, find out what that hose is supposed to
do.

BTW Have you tried replacing the spark plug?

 

[fekim]

That hose tees of and goes to the vacuum fuel pump and to the carb.
Yes, I replaced the spark plug yesterday.

 

[StepVino]

Ok, the only other things I can think of if everything is
working, and intake air leaks have been ruled out, is
to check for cylinder head, or exhaust gasket leaks.
Wouldn't hurt to do a compression check as well, just
to look for more clues.

 

[fekim]

OK.
I adjusted one valve to .05mm the other was fine. As far as compression I will have to wait til I can get my friends guage.

 

[fekim]

It was the coil.

 

[StepVino]

Congratulations . Sounds like it was a really
frustrating hunt.