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Adjusting high speed jet on Chris Craft "M"

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Adjusting high speed jet on Chris Craft "M"

Postby Don Ayers » Sat Jul 23, 2011 6:41 pm

Here is a good post from the Chris Craft forum on adjusting the high speed needle for "M" straight 6 engines.

here is my experience with adjusting the jets on the M engine with a single Zenith carb with an adjustable high speed jet. Later ones had a fixed high speed jet that was not adjustable.

Check the ignition timing first and have it set where you want it. To adjust the idle jet, I used to warm the engine up and then turn the idle jet screw in until the engine rpm dropped. Then I would slowly turn it out; the engine would speed up and I kept turning the screw out until the engine rpm again dropped. Keep track of how many turns it took. Then I would turn the screw back in a little more than half way. After checking the idle rpm (it should be set at about 500 to 600 rpm) I would speed the engine up and drop it back to idle. The final fine adjustment was for maximum idle rpm by resetting the idle screw, if necessary, to 500 to 600 rpm.

The high speed jet adjustment is accomplished by turning the T-screw on the bottom right of the carburetor, as you noted. This screw has a packing that is held in place by a nut to eliminate any gas leaks. That nut may need to be loosened slightly to allow you to turn the T-screw which is the jet adjustment.

Take the boat out and warm the engine up well at about 1500 rpm. I used to adjust my high-speed jet at about 2000 rpm while the boat was running (moving) and under load. It shouldn’t be done sitting at the dock. It didn’t really seem to make a difference exactly what the high speed rpm was as long as the idle gas circuit wasn’t in play. It takes two people to do the following: I would run the engine at about 2000 rpm with the boat moving and slowly turn the T-screw in until the engine rpm dropped. You can detect this both by ear and by the tach. Then I would turn the screw out, until the rpm went up and then dropped again. Then I would turn the T-screw in to achieve maximum rpm. After it was adjusted make sure that the packing nut is tight enough to eliminate any drips. Once set, this jet rarely needed readjusting.

Don Ayers
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1978 Riva St. Tropez
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Don Ayers
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