Scooter newbie having 50cc revving problems....

[laterider]

Hi there,

I’m new to the forum, but have been nosing around a bit hoping that I could find an answer to my problem. I did see that someone on the site had a similar problem and the thought was that maybe there was a torn or broken diaphragm in the carb, but I’m not sure about that here….I have a 2008 Motobravo 50cc scooter with about 2300 miles on it. It has your basic GY6 engine setup in it. It’s been driving fine all summer, until about a month or so ago when my wife took off to go to work and the engine just died. It would start and then die. She ended up having to walk it home and I took a look at it, and fiddled with the carb a bit and got it to at least idle. But give it gas and it barely revs any higher and sounds like it’s struggling to get enough air. Or maybe it’s not getting enough fuel. I ended up changing the spark plug, fuel filter, bought a replacement carburetor (I was going to pull it and rebuild it, but the replacement was only $25, so I figured that would be easier. I’ve replaced the CDI and ignition wire this year already. Cylinder head compression is about 120 psi. When I began work on this scooter, I noticed it was really low on oil, so I changed it, and it’s now back where it needs to be. And when I run the engine, it sounds okay, but when I rev it up, it sounds like there’s a “squeaking” noise, kind of like the noise a loose fan belt makes, but other than the drive belt, I don’t think there’s such a belt in this engine at all. I’m sure I have all the vacuum hoses attached properly, and I’m wondering if adjusting the valves would help (it’s never been done to my knowledge) or is this engine just plain cooked? Any ideas?

[kz1000st]

Adjust the valves. You're losing compression. Valve clearance is .003 inch for both. More likely you'll find the exhaust valve is tight.

[laterider]

Well the valves were perfect. Right where they needed to be. My dad's a mechanic and he verified there's no vacuum leaks. Engine idles great. But give it some throttle and it dies. Fuel filter is new. Any ideas? Maybe a fuel tank issue?

[kz1000st]

Where is the fuel tank? Is the carburetor fed by gravity or a fuel pump? If you get the scooter running have you tried spraying starter fluid in it and revving the engine. There could be residual crud in the carburetor if the starter fluid works.

[laterider]

I think you're on to something with the fuel tank. The carburetor is fed by a vacuum petcock. The tank itself is in the rear, above the rear wheel. We verified that there was fuel flow, and we decided to look inside the carburetor. I had the original carburetor sitting on the garage floor and I took the bowl off and took a look inside. There was crud, which kind of looked a bit like little balls of clear jelly. We removed the new carburetor and found the same thing. Cleaning that gunk all out resulted in perfect running and revving scooter! I popped the seat on it, and took it for a ride, and it was great, for about 2 minutes, then it began bogging again.....

I'd bet that if I removed the carburetor again, removed the bowl, and looked inside, Id find the same thing....that gunky clear jelly-like stuff.

So a tank drain and full clear-out of the tank, and new fuel lines?

[laterider]

So I drained the tank, put in new fuel lines (clear yellow ones) and I can now see that when I rev the engine, there's air bubbles in the fuel line and it bogs.

[kz1000st]

Bogs while riding or bogs in general? You should spray fluid in the carburetor and see if the bogging goes away. Also make sure your intake manifold isn't cracked or loose. The best test for that is to start the scooter and spray WD-40 around the manifold. If it changes sound you have an air leak there.

[skyrider]

I recently had the same problem after I siphoned gas out of my tank into a dirty container.then poured the same gas back in. Engine cut off at acceleration. Pulled out the petcock and their was dirt around the end of it and all around the inside of the petcock filter going into the tank. Replaced the petcock and filter, (first poured a gal of hot water thru the tank and let it drain where the petcock was) and problem went away.