Motorcycle Repair: Yamaha Badger, air fuel mixture, fuel droplets

Question
My son has a 1977 Yamaha Badger that is very hard starting when cold.  I was told by someone that the valves may need adjustment.  I was wondering if you could tell me how to do this and what the settings need to be.  Also could you tell me how to set the idle screw on the carburetor?  I took it out when cleaning the carb and want to make sure the initial setting is close and understand how to adjust it in.

Answer
Hi David,

The problem may be associated with a varnished carburetor, ignition problem, engine problem, or other.

I recommend that you get a copy of the service manual to learn about adjusting the valve clearances. The manual has illustrated procedures and important warnings to know about.

Adjust the idle screw in or out until proper engine idle speed is obtained. Basically, adjust the screw until engine idles a bit above stalling.\

Check out the ATV Frontier's forums:

www.atvfrontier.com

View microfiche parts at Yamaha's website:

I return tech information files by email. There was no email address with your question as requested and explained in my instructions. See sample file below.

www.yamaha-motor.com

Respectfully,
Mark Shively



Cold Natured Engines ~ Hard Starting
By Mark Shively

Cylinder compression and valve clearances must be within the manufacturer’s service limits for the carburetor to function properly and for the engine to run well. When either cylinder compression or valve clearances are out of service limits, the engine may start and run, but may run poorly or lack power.

Some engines are cold natured. This means the cool temperatures of an engine’s interior surfaces affects how the air-fuel mixture vapors travel from the carburetor, to the combustion chamber.

Normally, the air-fuel mixture remains atomized while en route to the combustion chamber. When the engine’s interior surfaces are cool, air-fuel mixture vapors may easily form fuel droplets. Fuel droplets are much harder to burn compared with air-fuel vapors. The flash point is much different between the two resulting with difficulty starting the engine.

Keeping the engine maintained as per the manufacturer’s periodic maintenance table may help prevent this phenomenon. Some engines are just cold natured by design.