Rotax 503 with single Mikuni carb

by Eddie Postell in General Tech Discussion 12 years ago

Hey all, Help needed. Rans S4 Coyote/Rotax 503 single Mikuni carb. Running way too rich. This carb has four adjustment screws. I don't know which to turn. There are no numbers stamped on this carb. What is stamped is Mikuni Japan. Anyone out there can tell me what model I have? Where might I find a breakdown of this carb?

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Re: Rotax 503 with single Mikuni carb

by Dennis Richardson » 12 years ago

Dennis Richardson

1 153 174

Carbs like this were common on older snowmobiles and smaller versions still see service on chainsaws and other small engine applications. They were discontinued on snowmobiles in the 1970's. To my knowledge there was never a Rotax 503 produced that ever ran a diaphragm carb.
There should just be two fuel metering adjusting screws, one will set the idle mixture (screw driver slot)and the other (cross tee)will set the high speed circuit. You probably do have an idle stop screw but I do not know what you are counting as the 4th adjusting screw.
If you google 'Mikuni Diaphragm Carburators' you will find a wealth of information and diagrams.
I can not remember a single item that caused as many engine failures as these externally adjustable carburators, so use some caution and common sense.

Dennis

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