The Stovebolt Site Forums The Shop Area Engine Shop Zenith 228 tuning question

I have a fresh rebuilt Zenith 228 carb (model 11963 that orig came on the GMC 270) that I am ready to set on my stock-setup 261 to replace a junky non-tagged Rochester B. Looks fairly straight forward, but I have a couple of questions:

1. How do I need to set the idle screw? I believe I read in a manual to screw it all the way in, and then back out 1 - 1 1/2 turns. This carb does not have an adjustable main jet.

2. There is a vacuum adaptor, I assume used to connect to the spark advance. However, I am running an HEI distributor from a 250, and the vacuum line from the spark advance is plumbed into a large brass nut connected to the intake manifold. Do I need to plug the carb vacuum inlet with a bolt?

3. My carb doesn't appear to have a bracket for the hand-operated throttle cable. Not even sure how I would find one.

4. Anyone have pictures of a Zenith installed?

To me, this looks like about a 20-minute swap out, but I'm sure there's something I'm missing...

Try the idle mixture control screw at 1 1/4 turn on a fresh engine. Once you get 500 or so miles on the engine, should be able to get it down to 1 turn.

Hand throttle cable brackets are tough to find; since this is a non-stock application - make one, not that difficult.

Jon.

Do I need to plug the carb vacuum inlet with a bolt?

Depends on:
1. does the carburetor have ported or full vacuum?
2. is this what you want?

Panic:
1. I am not sure, how would I confirm? There is a very small hole on the bottom side of the base, where the carb sits on the insulator. Does that make a difference?
2. I figured a small bolt would work if I did not need the vacuum connection.

Don't use a bolt.

Generally (not always) the threaded hole for vacuum is 1/8 pipe. If so, neat appearing 1/8 pipe hex plugs are readily available, if needed.

Jon.

The key is inside the bore.
A small hole through the casting in the side of the bore: is it below the (closed) throttle disc? Or higher up in the bore?
This determines whether the ignition gets a signal all the time, or only when the throttle passes a certain point.
For stock engines both work well, but I favor "ported" (part time) with engines with any hot cam.

Patrick, you're over-thinking this situation, as usual. Give me a phone call!
Jerry

I always over-think, I'm just thankful I'm not the typical under-thinking millennial! The vacuum port is outside of shaft when the throttle flap is closed, and has a small vacuum hole on the bottom of the carb flange where it sits on the insulator.

I'll give you a call Thursday Jerry, didn't want to bother you today, you're always such an enormous help for us amateurs

Turns out...

A Zenith 228 has a 2 1/2" hole spacing, and a 261 intake manifold has a 3" carb stud spacing. Jerry and I talked on the phone earlier, and I will weigh my options on an adapter plate, or selling the Zenith and buying a Carter that would feed enough air to my bored 261. I could slot the Zenith base holes 9/16" but I'd rather not do that...

The Zenith type 228 was made in SEVERAL DIFFERENT flange sizes.

Start with the correct size, and no slotting is necessary; PLUS the internal venturi will be closer to being correct for your 261.

Jon.

I thought I was starting with the correct size, being that it is a "11963" model number, which was used on the GMC 270, but I was unaware that there were intake manifold carb stud spacing differences between the 235 and 261 engines! There's always the "gotcha".