Recently we have noticed that some Zenith downdraft carburetors on Ford 300, which is also known as the Ford CSG649, industrial engines have had poorly adjusted automatic chokes. As a result the engine is hard to start, runs poorly, fouls plugs and emits smoke.

Here's how to adjust these Zenith model 33 carburetors:

1. If your shop's ambient temperature is 70° simply open the choke plate 1/32" so it is just off the choke bore.
2. If not 70° loosen the three plate screws to open the choke plate 1/32". Then adjust for temperature by rotating the choke cover one mark per each 5° of temperature difference from 70°. Rotate counter clockwise to lean the mixture if warmer than 70° and clockwise to enrichen the mixture if colder than 70°.
3. Re-tighten the cover plate screws and check the choke shaft for free operation.

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