Carburetor Issues

Question:

Dear Motorhead,

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I have a 1990 Yamaha Champ. Previous owner rebuilt the carb and it runs good when hot, but not cold.

I put a new carburetor on and it starts good but seems like the choke doesn’t work correctly (doesn’t rev up and run on choke like other 4 wheelers)

I also won’t idle cold very well have to feather the gas. don’t know carb
settings got any ideas. ps smokes a little, rings or valve guides.

keith harvey

Reply:

Thanks for your email!

You have exceeded my historical knowledge of ATV’s with your Q. However,I will offer some thoughts.

First, I suspect this engine is using a butterfly choke which are notorious for hard cold starts and poor idle.

Essentially the butterfly blocks all but a slight amount of airflow while allowing the carb mainjet to respond to the increased vacuum of the piston sucking against a brick wall and looking for something to eat.

Essentially, the mainjet “floods” the engine in a crude way and the motor eventually fires. The problem is there is little atomization of the fuel – it just pours into the mill, washes down the cylinder walls and creates smoke.

If you had an enricher circuit style choke the fuel is drawn into the engine in a much more orderly manner and as long as you do not move the throttle flipper, fuel is atomized and sprayed into the intake resulting in a quick firing of the mill. When you shut off the enricher idle drops and when it’s on idle increases.

Without knowing for sure what you’ve got I would say steady as she goes. If it runs warm, you’re at least running – right?

Thanks,

Motorhead mark

Dirt Trax Online
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