The Stovebolt Site Forums The Shop Area Engine Shop Zenith carb won't idle properly-1950 270 engine

I put a 1950 low-mileage 270 into my 1952 half-ton pickup. It came from a salvage-yard firetruck and was missing its carb when I bought it. I later found a Zenith carb elsewhere but it's from a 228. So I have some mismatch as far as venturi size and some of the jets. Add into the mixture that I live at 7600' elevation. This engine has the standard PCV set-up which seems in good condition. I rebuilt this carb with as many new parts as I could obtain after a long soaking in solvent. I was told that the 33 venturi is no longer available. Currently using a 30 main jet. Carb shows condensation at its base when first running which tells me that its really sucking high-velocity air through its smaller venturi. The engine runs fine down the road at medium to high RPM.

This carb won't idle worth a darn. Drop the engine RPM down from driving speeds and it will hesitate/stall unless I keep the throttle lever stop screw cranked up to a higher level than is desirable. The idle mixture screw doesn't seem to change the idle at all. Whether it's tightened all the way in to its seat or backed out 3 full turns, no difference. This carb spent many years exposed to the elements and was rusty when I bought it. I wonder if there is some debris of some sort completely clogging the idle fuel passage somewhere.

I'm aware that not having a correct 270 carb isn't a great start. But I've got $250 into this 228 Zenith and would like to give it every opportunity to work before I change it out for a Rochester which might have its own set of issues. I could also change out the whole PCV setup for the road draft option if that would simplify things.

Where would you guys go from here? Thanks! Dave