Motorcycle Repair: Sheng Wey Carb, fuel air mixture, air vacuum

Question
I am stupid and have bought a chinese lexmoto xtr s 125 off a friend, i have had this problem where i will ride it, then after a quarter mile of riding it, it will die, i will sit there trying to start it up and after 15-20 seconds of turning it over with electric start it will then start and then it will run fine for a few miles, then come to a bog down again. i can hear, feel and see that it is running too rich and is flooding the engine with not enough air and can only find the idle screw on the carb, and i am at a complete loss with it and hope you can tell me how to find the fuel/air mixture screw on the carb. many thanks, brad.

Answer
Hi Brad,

NOTE:

The fuel screw is at the engine side of the carb straight up
from the bottom. Hard to see.
Clockwise is leaner.

Picture:

http://members.shaw.ca/wsamel/sheng.png

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The Sheng Wey is a fairly basic carb and is like a copy
of most of the small japan carbs like Mikuni or Keihin.

The problem could be one of a few things so I will start
with the easiest to check.

Check your spark plug color, this is a good indicator
of what is happening inside the engine.

Black equal too much fuel or too little air mixture.
Whitish equals not enough fuel or too much air.

A brown or tan color is when the fuel mixture is the best.

Next, try opening the fuel cap if it stops and see if you hear
any air vacuum or pressure. The fuel tank or cap
air vent could be plugged which holds back the fuel
from the fuel lines and starves the carb.

If it is flooding with too much fuel the float could be sticking
inside the carb. It has to close a small float valve needle
to stop the fuel when the float bowl is full.

If the float needle is sticking or leaking that can also
cause too much fuel in the carb bowl.

Check if your fuel is coming from the tank to the carb freely.
Sometimes the lines get plugged up. Use a inline fuel filter.

Check if your air filter is plugged as this can cause a lack of air.

Replace the spark plug occasionally.

These carbs have an acellerator pump which squirts some fuel in everytime
you open the throttle. If you keep the throttle steady it will
run leaner.

Lel me know if you find the problem, it might be flooding but could also be
just running out of fuel due to some of the reasons I listed.

Wayne S
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