Starting engine carburetor adjustment (Late modle D 2, Zenith TU4C carb)

One thing you should do before putting the top on the carbon bowl is to ensure that the little 90* crank that connects the high speed adjustment screw (horizontal in top of bowl) to the metering needle (vertical) is not bent off of 90 degrees. If it is crunched, it can be impossible to get the carb to work.

Then look at the way the crank translates the motion of the screw. I believe you will find that turning the high speed screw out will actually close the high speed needle by lowering it into the seat. In this case, you will gently back the screw out until the needle seats and the turn back in the 1 turn to lift the needle slightly. IF THIS IS NOT HOW YOURS WORKS, then screw the screw in and then back out the 1 turn to lift the needle off the seat to allow fuel to flow down through the seat into the horizontal passage across the bottom of the bowl. This horizontal passage is a famous trouble spot for clogs and is cleaned most effectively by drilling out the soft plugs and then gently running a drill bit through it across the bottom of the bowl toward the high speed metering well (retained by a hex head plug). Carbs with a clog here typically require choke to run at all times and have little power production.

The idle screw is actually metering AIR into a fixed fuel metering passage. So turned IN is CLOSED/RICH and backing the screw out 1/2 turn will lean the mix slightly. If your carbon won't idle, the jet may require cleaning. Should be able to look back and find ae explanation by Eddie B (edb) as to how this is done.

I found a great option for getting the carburetor to work correctly on my D2. I tried to rebuild the Zenith TU4C myself with an $80 kit but could not get the starter to run without the choke pulled closed. I believe one of the gas passageways was still clogged. I shipped the carburetor to Guaranteed Carburetor in Largos, FL. For $188 they completely rebuilt the carburetor, cleaned all the passageways, and bench tested the unit to make sure the two needles were set correctly. It took about two weeks. I highly recommend them.

like cc says above, make this bell crank 90* and you won't have to keep it choked to run. in the past the screw was adjusted in too far and bent the b-crank out of 90*. also when cleaning passages find some small stainless steel cable, untwist a strand to feed into the passages. its stiff just right to chase thru. i posted these photos in a post in august 2002 about this issue.

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