I have a Zenith carb that fits a 270 GMC engine.It has an adjustment screw up top near where the air filter mounts at.What is the basic "starting out"point for the screw?I am familar with adjust screw near vacum hookup.Should I set it at 2 turns out?

the only adjustments on my two zeniths is the idle mixture adjustment screw. one of them i put an adjustable jet which is right above the vac advance connection. the idle mixture adjustment screw is on the rear of the carb facing the fire wall. not sure of which one your asking about i cant give you an answer.

A Zenith 28 has no adjustment near the top. The only adjustments are as Mike stated. What model Zenith do you have?

Where would I find the model #?

The 28 has no markings IDing it as a 28. It is the carb used in most GMC sixes of the '40s and'50s. They look like this.

next to the big nut that cletis's link shows is a round tag with a rivet head holding it in place. on that tag is the info for your carb. fyi the under the big nut is/was a gas filter. my carbs didnt have them and were removed earlier it is life.

My carbs are missing the filters also.Okay I willl post the model after I go write it down.Thankyou

This is what is on that round tag::61621 HL or HE;9439D

That carburetor is MUCH TOO SMALL for a 270. I would do some additional looking.

Jon.

It is identical to what I had on the 270 except the adjustable screw is in a different location.