Idle climbs or engine stalls when adjusting idle screw By herringbone (Read 185 times)

[herringbone]

I have a 150cc Znen scooter (GY6). I bought it used from a single owner with 180 miles on it, but 4 years old, so the scooter did a lot of sitting. The day we went to buy it, it started right up and ran fine. The owner was a motorcycle mechanic, but he recommended we "take it in for a tune up".

Once I got it home, I immediately started having problems with it. The original problem was an inconsistent/cyclic idle. It was hard to get it started, and once started, it would rev up and down constantly every ~5 seconds.

As part of my troubleshooting I did the following:

- took apart and cleaned the carburetor

- checked and adjusted valve clearances. Intake valve was about right, set it to 0.003". Exhaust valve was basically fully closed, so I set it to 0.004".

- Eventually, found the vacuum hose from the intake manifold was disconnected from the other end, so I fixed that.

After reattaching the vacuum hose, the cyclic idling stopped and I began attempting to tune the carburetor to get it to idle properly. That's where I am having my current problem:

I can get it set up where it starts easily, idles about right with the back wheel not quite spinning, but then the idle will gradually increase until the rear wheel is spinning quite fast. I can back the idle screw down and the idle drops. I keep doing that until the rear wheel finally starts to slow down, but once I get it that far, the idle gradually drops until it stalls. I am making small adjustments and waiting a minute or so in between. The mixture screw doesn't seem to be altering the rpm all that much. 

I am new to these small motors/carburetors. I'm not sure what might be going wrong at this point. A carburetor is pretty cheap. I have been thinking about simply replacing it and seeing if that does it? Any one know of anything that might cause this sort of symptom?

Thank you for your help.