The Stovebolt Site Forums The Shop Area Engine Shop zenith carb question

I've been offered a Zenith 9046A carb for my recently rebuilt 261 engine. I have a Carter YF on there now, but it is too small for the engine.

So, will I be any better off with the Zenith 9046A? Or, should I continue to look for a Zenith 228? I've heard that the 9046A was a military carb. If so, will my truck run like a tank? :p

The Zenith is a fine carburetor, but this one is a wee bit large for your 261. Bore the 261 125 thousands, and stroke it a quarter of an inch and then this carb would be fairly close.

Jon.

Yikes!

I'm only bored .030 over, so I guess that this carb isn't a great idea.

I really appreciate the input and advice that this board gives.

Thanks!

53Therapy - look for a Zenith from a G.M.C. 270 CID. Same size externally as the 9046, but smaller internally.

The internal size is ALWAYS more important than the external size (one can always make an adapter for the external size), as the internal size is what controls the airflow.

Jon.

OK, so now I may have found another zenith carb #11658. But it doesn't have a vacuum advance port on the throttle plate body. :confused:

Can I drill and tap a port, or is there some other method? :confused:

Thanks,
Steve

You can drill and tap the port. Use your other throttle body for a guide as to positioning the hole. Do not yield to the urge to interchange the throttle bodies.

Jon.

My carb arrived today! On the throttle body there is a boss where it appears that the vacuum port should be, and I'll drill a 1/4" hole just deep enough to tap, then drill a smaller hole inside that one going into the actual passage.

But, when comparing the original Rochester B and the Carter YF, there is a difference in where the hole(s) come out on the inside. On the Rochester, there are two very small holes beneath the butterfly valve, but on the Carter, there is one small hole above the butterfly.

So which is correct? should the vacuum be below or above the throttle butterfly? Does the size of the hole(s) matter? I really don't want to mess this up, it took me a while to find this carb.

Thanks,
Steve

The hole above the throttle plate is for "ported" vacuum; the hole below the throttle plate is for "manifold" vacuum.

Which you use depends on which your distributor wants (time for an ignition guy to jump onto this thread).

Jon.

Your 261 will want ported vacuum for the distributor. The hole should be above the throttle plate when at idle but will be below it as soon as the throttle opens a bit.

Start small, about 3/32" or smaller, as you can always enlarge the hole if you don't get enough vacuum. It's tough to drill it smaller! The "ported" vacuum should come in just above idle, so drill the hole to be exposed as the throttle plate opens a little. Vacuum on the distributor at idle makes it difficult to slow the engine down enough for a slow, smooth idle.
Jerry

Thanks for all the replies. I really appreciate everyone's willingness to share their knowledge.

It sounds like this could be a rather delicate operation...getting that tiny hole placed just barely above a wafer-thin piece of brass. If I screw it up, there's always JB Weld, right?!?! :p

I'm wondering why this GMC carb didn't have the vacuum advance port in the first place. Is there some other way of advancing/retarding the spark besides the vacuum advance? Just curious...

Thanks again,
Steve

Some constant-speed engines, like for power generators, water pumps, etc., had only "centrigugal" advance, controlled only by engine speed. Vacuum advance is a fuel-economy thing, allowing a few degrees more advance at light load, part throttle. The advance goes away quickly when the throttle is opened to climb a hill, pass, etc.
Jerry