Engine runs very fast, then slow, before cutting off

[Khendal46]

Alright guys, I need some help. I bought this bike off of craigslist less than a week ago, I have not had a peaceful ride on it since. Ever since I bought the bike, it had an issue where it will start going really well and driving perfectly, before it starts speeding out of control and then finally shuts off. When it starts doing this, throttle has no control over the speed of the engine, and it just goes out of control. For the first 500 feet or so, it will run perfectly, the throttle controls the speed, and everything goes fine, then all of a sudden after that's over it starts doing what it wants and ignoring the throttle's control, before sputtering then shutting off. It used to leak gas, but just today I fixed that, by taking the bottom of the carburetor off and bending down the fuel prongs so that the carburetor does not overflow.
The seller told me I needed to change the spark plug, because it would probably be covered with oil, as the bike was only bought on the 10th of September. When I took the first spark plug off, it was covered in oil and I changed it. I thought this would fix the problem, but it didn't. I was still having issues with the gas line. It did not seem to be flowing properly, and getting to the carburetor, but I fix this by flipping it around, since the original person that built it put the fuel filter on upside down, as I came to find out.
So far, I have changed the spark plug, put the fuel filter on correctly, unscrew the fuel line from the gas tank to check for any debris, and bent the prongs in the carburetor to make sure that the fuel tank was not overflowing anymore. I just took the spark plug off five minutes ago, and saw that it was covered in oil. I've them took the old spark plug which I had replaced, and mostly dried out since replacing it, and put it back on the bike. I thought that maybe since the spark plug had just been changed that the bike would work properly. This is not the case and it still continued to do the speeding out of control then stopping problem. Does anyone have any suggestions?? I am so frustrated with this thing!!

 

[Khendal46]

It is an 80cc chinese motor.

 

[Toadmund]

Maybe the fuel supply is not sufficient enough to keep the engine from a lean condition.
Your float bowl may be starved because you bent the prongs too far.

It shuts off when the fuel supply goes beyond lean, meaning, not enough fuel.

That's IMO, I could be wrong.

 

[2door]

What carburetor do you have and what fuel to oil ratio are you using? We need to know this before offering any definitive advice. My first guess would be you have a massive air (vacuum) leak in the intake circuit. You could also be looking at a throttle cable/carburetor issue but more information would make troubleshooting easier.
Have you checked your throttle function? Is the cable routed so there are no sharp bends or kinks?

Get back to us.

Tom

 

[crassius]

first, try running it without the gas cap on the tank - if that cures it drill a vent hole in the cap

if that doesn't fix it, then put a rag under the carb, open the drain plug on left/bottom of bowl and watch if fuel runs out nicely

 

[maniac57]

Sounds to me like you bent the float prongs too far and are starving for fuel.
Your flooding problem was probably from trash in the float needle rather than float level. Put it back the way it was and try using a fuel filter.