Popping and lurching

[Cantole]

Just bought a 2012 Kymco people 150 for my wife. The guy I bought it from said he put a new plug wire (it was junk) and carburetor in it. He also had a new final drive installed at the dealer to reportedly make it go faster. I have been having some problems with it and wondering if any one can help. It has 3 main problems and I don’t know if they are related.

Problem 1. When idling it will start to rev up for a few 10 seconds to a minute and then go back to normal.

Problem 2. It sometimes has a very light backfire.

Problem 3. When driving it will suddenly lurch when accelerating. As if it has a small loss of power. At the same time i can hear popping sounds but I think they are coming from the air box or cvt.

What I have done so far: replaced the plug wire, spark plug, fuel filter, fuel line, intake manifold, cleaned the carb, and adjusted the valves. I have also tried to adjust the carb but admittedly it was my first time adjusting a carb as all my other mopeds have been small 50cc that the idel mixture screw was covered. So I am not sure I did it right on this one.

I am going to try playing with the carb some more but not sure if that’s it. I will also check the weights for flat spots, and the belt for cracks. Any other ideas or help would be much appreciated. Thank you in advance.

[JJJoseph]

Don't change anything while you're sorting things out!  Just leave jets, weights, etc exactly as you found them.  Then change/clean one thing at a time, carefully.  The idle speed will change after startup since it has an electric "choke" that adjusts after a minute or so.  The weights and belt won't affect anything at this time. A fresh sparkplug and coil helps.  Most likely the ratty behavior is coming from the carburetor, and I'm guessing the pilot jet (not the main jet) may have a speck of gonk in it somewhere. It's a very fine jet, maybe .3mm in size, and the tiniest speck of fly-sh** will be a nuisance. If it has a diaphragm carb the diaphragm must be assembled carefully.  The Keihin carb is an easy assemble, but still care must be taken with the diaphragm, the accelerator pump, etc. and the bowl. 

[Cantole]

Good advice about not getting to many things changed at one time. I had a friend that used to own a scooter rental company stop by today. He helped me discover that I was not at top dead center last time I adjusted the valves. As a result my exhaust was valve was to tight. He showed me how to adjust both of them. It was a bit to late to start it up when I was done so I will try again in the morning. Hopefully it solved some issues.

[Iahawk]

good info to bank away...I guess it's assumed that all valve adjustments are done correctly..this is a good reminder! Hope it solves your issues.

[dwarnimont]

where can i find instructions on valve timing adjustment as you'd mentioned?

[mrmike]

where can i find instructions on valve timing adjustment as you'd mentioned?

        Look into a Cyclopedia manual.           https://www.cyclepedia.com/

[Iahawk]

where can i find instructions on valve timing adjustment as you'd mentioned?

or do a youtube search for 150cc scooter valve adjustments...or 50cc or whatever size you're looking for. Tons of videos out there to walk you through it.

[JJJoseph]

where can i find instructions on valve timing adjustment as you'd mentioned?

The bottom line is there is no valve timing adjustment on a GY6 engine.  The timing is fixed, so you have one less thing to worry about

[Stig / Major Tom]

The bottom line is there is no valve timing adjustment on a GY6 engine.  The timing is fixed, so you have one less thing to worry about

He meant adjustment of the valve lash. Not timing.
Of one doesn't correctly find TDC, valve adjustment will be incorrect.
Stig

[JJJoseph]

Of one doesn't correctly find TDC, valve adjustment will be incorrect.

You don't have to look for TDC, there's an indicator right there on the camshaft sprocket.