QMB139 Idle issues.

[chris23413]

Hello Everyone! I have been having a bit of trouble as of late with my qmb139 powered scooter. It runs fine on first startup when the auto enricher is active, and anytime you're off idle, but as soon as the auto enricher cuts out or you drop RPM to the point the idle circuit comes into play, it refuses to run on the idle circuit for more than 10-15 seconds. RPM gradually drops off until it sputters out and dies. I've already:

-Adjusted the idle mix and idle speed screw so that they are close to the stock configuration.

- Changed out the spark plug.

- Adjusted valve lash so both the intake and exhaust are @0.003".

-Cleaned and been through the carb multiple times, changed out the main and pilot jet from stock to see if it possibly wasn't getting enough fuel and that made no difference, so i went and shoved the stock jets back in. Even changed out the carburetor for an entirely new unit with no difference.

Only things that I have yet to replace are the CDI box and the ignition coil. I'm at my wits end here, any help would be heavily appreciated. I will note that it has been getting cold recently, perhaps the jetting is still not quite right for the weather.

[GrumpyUnk]

The symptom is a fuel flow problem in my opinion. Check the flow to the carburetor inlet, disconnect the fuel line and apply vacuum to the petcock. It should flow full stream as long as the vacuum is applied. If not, check for internally damaged fuel line, clogged filter, or clogged outlet screen in the tank.
There is an opening into the venturi behind the tip of the idle mixture adjustment screw. If you poke it with a bread bag tie denuded of paper, it should allow thw wir to poke in freely. You can then spray carb cleaner through the tiny hole back to where the mixture needle area and flush any obstruction. The pilot jet has a tiny hole that you should be able to see light through if it is unclogged. A new carb should not bring trouble with it, so my bet is on fuel supply being low... but if it runs at high speed, the flow should be sufficient. In that case, I would check for air leaks in the intake gaskets between carb and elbow and between elbow and cylinder head. Or vacuum lines that are cracked or damaged.
tom

[chris23413]

Hi GrumpyUnk. Thank you for the suggestions. I did test the fuel petcock this moring with my vaccum gauge and hand operated vaccum pump, It seems to be holding vaccum just fine and flow is strong and consistent when there is vaccum present. Took the carb off (again), went through all the small passages with a fine wire, including the idle mix orfice and the pilot jet and blew them out with compressed air after a clean, replaced the intake elbow gasket, and replaced the vaccum line going to the fuel petcock. After all this, the issue still persists. I'm lost at this point.

[GrumpyUnk]

Did you check the elbow O-ring gasket to the intake port? Check for tight nuts on the two studs, and the O-ring for protruding above the plane of the elbow. The O-ring must seal, and they can get deformed and hardened even when never removed. It would/could allow air that makes the mixture too lean. Also inspect for hairline cracks, etc.
If you have some RTV - room temperature vulcanizing - silicone sealant, you may want to put a smear on the surfaces to insure there is a good seal. I do not mean a blob, just a dab that is used to coat the surface, including the O-ring, and then allowed to set before being run. It will help with any imperfections, and also help a tired O-ring that has lost its round, become flat, and doesn't want to push out any more.
tom