The Stovebolt Site Forums The Shop Area Engine Shop Tips for adjustable main jet (1952 Zenith)?

Hi All,
Finally got the rest of my sheet metal back from the painter, and my truck is ready to go. It is a 1952 GMC long half ton with a 270 and a 4-speed. It is the original green with white grille. The carb is a Zenith 11275D, with an adjustable main jet (needle valve. Anyone know a method for setting this up properly other than trial and error?

The carb is a universal replacement, and an excellent one. Since the carb is not O.E., then trial and error are the only method. The recommended beginning value is 2 1/2 turns from lightly seated.

Jon.

Trial and error, unless you have access to a chassis dynamometer and an exhaust gas analyzer. If you haven't taken the carburetor apart yet, close the jet all the way, count the turns required to close it from its present position, then when you put the carburetor together, set the adjustment at the same place. From there, watch the color of your spark plugs after running a short time- 25-50 miles or so, doesn't necessarily have to be non-stop: Dark brown/dry black color indicates the mixture is too rich, lean the jet slightly; white tip indicates the mixture is too lean, enrich the jet slightly. A good color is slightly tan/off white. Also, when you are test-driving, listen for detonation/pinging/spark knock under both light and heavier acceleration. If the ignition timing is set correctly, detonation may indicate that the mixture is too lean- enrich the jet a little. The spark plugs are the best indicator of fuel mixture, but it takes a little experience (and miles) to get comfortable with what they are telling you. Regardless, clean, white is not the best condition. Also, burned electrodes or broken insulators are bad conditions, although those won't normally occur in a short drive.

Also, the spark plug color will be affected by the spark plug heat range you have. I'm not sure about your GMC, but AC42s or 43s probably are all right. AC44s may be all right for light duty- no heavy loads, but may be a little too hot if the original specification is for 42s. I'm sure someone else here can give the proper spark plug type.

Harvester

Thanks for the tips. It turns out that my biggest problem was some blockage in the idle circuit. I took th etop off the carb last night and took the jet out and cleaned it, and blew a little air through the passages. I had the main jet about 4 turns out, so I set it to 2.5 as suggested. Now it just purrs. I drove it to work this morning and was very pleased with the overall driveability. I will still watch the plugs. I also made a bushing for the throttle relay pin (that threads into the block), which was worn to the point that the bellcrank could get to the wrong side of the stop pin. All better now. Thnaks, Tom

Just to clarify: turning the main jet in = leaner, right?
Thanks, Tom

Usually, that is the case, unless your setup has left-hand threads. I'm not very familiar with that particular carbruretor, but others I have worked with had right-hand threads for the jet, and turning in was lean. On some adjusatble jets I've worked with, you could hear a difference in engine speed if you adjusted the jet while holding the engine speed at 1800-2000 RPM. In some cases, you could almost stall the engine that way. I don't know if that will work with your carburetor.

Harvester

Just to follow up: The jet has RH threads, so in = lean. So far, I have found that the best running is at about 3.5-4 turns out. I needed to seperate my desire for fuel economy from the need to make it run well. A mental block if you will. Runs well now.

I think you made the correct decision. Running well means better economy (no engine damage) in the long run.

Harvester

I hesitate to even calculate my fuel economy. I'm guessing maybe 12 or 13? I guess if that was my primary objective I would have restored a diesel rabbit pickup...

Twelve or 13 MPG would be reasonable for your truck. Check the spark plug color after a few miles.

Harvester