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

You have really answered your own question. There are MANY differences in the Zeniths used on a 228 and a 270 other than the main venturi.

Find someone with a 228 that might want your Zenith, and look for the correct size.

Jon.

I may have a 228 carb for sale and be in the market for a 270 in the very near future. I appreciate your candor. Would any Zenith 270 carb work from any year or would it have to be 1950 specific? I'm wondering if late 50's-early 60's intake manifolds have different spacing for the mounting studs? What year range would work?

What you describe is a plugged idle circuit. No fuel is getting to the engine through the idle circuit. Spray carb cleaner, small wire through this circuit until fuel can flow. Mtneermike

Sounds likely. Both answers sound likely. I've got nothing to lose but time by trying to unplug the idle circuit.

There is a passage in the GMC maintenance manual under "Cleaning and Inspection" of the downdraft carburetor that states:

"...Be sure all carbon deposits have been removed from throttle bore and idle port. It is advisable to reverse the flow of compressed air in all passages to insure that all dirt has been removed...CAUTION: Never use a wire or drill to clean out jets."

I'd better use a small wire as a last resort. The diagram of the idling system in the shop manual has some sharp turns in it and some of it seems completely unreachable by means of a wire.

I have run 228 carbs on 270's. No problem idling, no problem with throttle response, but I did give up top end power. I agree you should look for a 270 carburetor, but all of the problems you describe are not caused by the mismatch. I started driving a GMC with a 228 to school in 1958 and have been messing with GMC ever since.

Until you find a 270 carb, you might try revving her up and then holding your hand over the carb to cut off the air supply. Quite often you can suck the trash through the carb with the high vacuum.

Here's what I have discovered so far:

1. Due to my careless assembly of the carb during rebuild, the idling jet wasn't tightened down all the way to contact the jet calibration seat. That couldn't have helped the idling any. No sign of blockage in the fuel passages.

2. The idling mixture screw supplied with the rebuild kit didn't have the exact same profile as the old one that I took out. The new one wouldn't screw in as far. I put the old one back in.

3. The pump piston supplied with the rebuild kit with some sort of plastic diaphragm on it stuck in the well and wouldn't depress. Hence, no functioning accelerating system. Maybe the ethanol in modern gas caused it to stick, maybe it's just old. The kit came with directions to rebuild a 1974 Opel. I still have the old metal pump piston that I have epoxied onto a new pump rod. Same overall length as the original.

I don't know yet that these modifications will fix all of the issues with this carb, but I doubt that it will run worse. I haven't had a chance to test drive yet but I will report back. For the record, this was a Walker rebuild kit from Carburetor Factory. I'm not sure that I'd buy another one from them.

The carb only performs marginally better. The used metal pump piston fits too loosely to give a good jolt of fuel when depressed. I beat it up pretty badly when first disassembling the carb. The idling system is a little better but not much.

I installed a Rochester carb on it and the engine is running much better. Better idle and acceleration. I have multiple main metering jets that I can try in order to get the right balance between power and fuel economy for my altitude. For the time being, I'm going to run with the Rochester.

I appreciate the help with this. I need to find the balance between throwing additional time and money into this particular 228 carb or biding my time until the right 270 carb comes along. Dave