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Matt, Engineer
Category: Motorcycle
Satisfied Customers: 20389
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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My bizarre problem is actually with a motor scooter and not

Customer Question

My bizarre problem is actually with a motor scooter and not a motorcycle but engines are similar if smaller.
My Yamaha Vino 50cc two stroke engine starts quickly and runs well with power on a level sloop. But when going up a hill (on which it previously had no problem) the engine abruptly (not gradually) looses power but continues running at lower speed (but not with enough power to climb the hill). It will immediately run normally if turned downhill or onto a level slope.
- I have carefully cleaned carb twice and replaced float needle valve.
- have replaced the centrifugal clutch which is not source of problem
- transmission is in good condition
- ignition system, coil and spark plug in good condition
- engine compression pressure is about 110 psi which is normal
- vacuum operated fuel valve functioning normally
- battery in good condition
- my guess is problem is fuel related but throttle system working wellI have run out of ideas to investigate and need help.I stress again that the problem occurs quickly and not gradually.
Phillip Hawley
La Jolla, CA
Submitted: 1 month ago.
Category: Motorcycle
Customer: replied 1 month ago.
My phone is a land line so please do not answer by trying to text me>
Expert:  Matt replied 1 month ago.

Hello Phillip

I'd say that the issue is with fuel being picked up, and being delivered out of the tank to the carb

as it only suffers on an incline then I'd say its possible the pipe exit is either being blocked by something or its moving around

I'd also check that the filler cap valve isn't sticking and so creating a vacuum inside the tank

if you loosen the cap when the issue occurs and it helps then I'd replace the cap

Expert:  Matt replied 1 month ago.


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