Motorcycle Repair: 1999 yamaha zuma II, yamaha zuma, jet nozzle

Question
I'm sorry it's not in your specialized field but your the only person it seems willing to take other questions haha but my zuma is a 50cc 2 stroke and it seems to run pretty good around 50% throttle but when you give it full throttle it takes off then seems to lose fuel i have kinda gathered that this may be the bowl running out of fuel and im really just wondering how to either make the bowl fill faster or maybe im just totally wrong any help would be appreciated

thanks
robert


Answer
Robert, I am in a good mood, so I'll take a shot at it....

Mid-range to top end power is determined by:

Main jet size, selected to match the intake and exhaust and porting designs of the engine. If you have changed anything in that regard, the main jet size could be too small. If the bike has been sitting and the gas has evaporated inside the carb, then there could be both air and fuel passages/jets that are blocked, altering the fuel/air ratios.

Needle position and taper are responsible for mid-range fuel metering, as well. If the clip falls off of the needle, the needle will be stuck down in the main jet nozzle.

Spark advance.. critical for 4 strokes, but not always needed on 2 strokes. If the bike has CDI or electronic ignition, then it could be faulty.

Spark plug/spark plug cap issues. Plug could be fouling due to poor fuel mixture or too much oil from the injector system

Contamination of the fuel in tank/carburetor. Fuel doesn't last much past 2-3 weeks without a fuel stabilizer added in. Spark plugs do die in service. Ohm meter should be seeing around 5k ohms from one side of the plug cap to the other connection side.

Exhaust system coked up. Sometimes the exhaust port will get plugged up with carbon. Mufflers can plug up too, sometimes, on 2 strokes. If exhaust can't get out, then the fuel mix can't get into the cylinder to burn and make power.

Fuel flow restricted. Petcocks usually have a screen to keep out the debris. Any rust, water or dirt buildup inside the fuel tank will clog the fuel filter and slow down fuel delivery to the carburetor.

Piston seizure! 2-stroke pistons heat up fast and seize even quicker, when fuel mixture isn't right or air leaks at crankshaft seal leans out the engine.

Air filter plugged up! A clean air filter ensures a correct fuel-air mixture at the carburetor.

Fuel to the carb is gravity fed. Yamaha knows how to make a motorcycle function properly, so if you are having starvation issues, it is probably a maintenance failure of some kind.

Happy motoring.... when you find the problem.

Bill "MrHonda" Silver