I recently acquired a freshly rebuilt Zenith carb and a parts carb for my '48 GMC with a 228 engine to replace a old, leaking and tired incorrect Rochester. The rebuilt carb did not have the base drilled and tapped for distributor vacuum advance though the parts carb does.

The machine shop correctly drilled and tapped the boss on the carb but for the brass vacuum fitting but the small diameter hole that is drilled completely through into the carb throat is much larger than the one on the parts carb. I eyeball the hole diameter in the parts carb at 1/16" while the freshly drilled hole in the rebuilt carb is 5/32". I won't know for sure until I install the carb but it looks as though the top of the butterfly will be visible through the hole when at idle setting. That can't be good.

In my never ending quest to learn all sorts of stuff I never knew I needed to learn I have three questions. Does the larger diameter drilled hole greatly affect the vacuum at idle? Is there a related problem because the butterfly doesn't close completely above the drilled hole? Can I fit a restrictor into the threaded part of the carb boss to fix this?

I think I know the answers but I welcome, as always, your collective help! Thanks, Jim

The throttle plate needs to close above the port for vacuum advance, providing vacuum to that port (engine side of throttle plate) when at an idle.

EDIT I may be wrong above.. the throttle plate should close and cover the vacuum port.. best bet to look at a good carburetor and see how it's done.
I have Zenith that from new (it was NOS) had the throttle shaft installed upside down, causing the throttle plate to close in the wrong place in relation to the vacuum advance port, as a result the vacuum advance didn't work as it should. Reversed the throttle shaft and all is well.

Hole diameter will not effect vacuum or amount of said vacuum. Just be sure the throttle blade operates as it should and the hole isn't at a point that the throttle blade closes right on it. Above the throttle blade is ported vacuum ( you only have vacuum when the throttle blade is open ). Below the throttle blade is full time vacuum ( you only have vacuum when the throttle is blade is closed ). If a smaller hole is preferred, an easy fix is to press a piece of soft lead into the hole, then drill out the lead to the size you want. If it was to ever come loose, the soft lead will go right through the engine with no problems.

So, depending on where the hole was drilled, you may or may not have vacuum at idle, but you will have vacuum while driving no matter what.

You should only see the hole on one side of the throttle blade, if you can see it from top and bottom, then its located wrong.

Somebody ruined a good carburetor! The vacuum advance should not get any vacuum at idle, so if you can see any part of the hole from inside the carb throat with the throttle blade at idle the hole is too big, and/or drilled wrong. I'd suggest plugging the hole with a top hat shaped piece of brass so it can't get sucked into the air stream, and finding somebody else to drill a correct sized hole. Machining the brass plug shouldn't be difficult for anyone with a small lathe and a little common sense.
Jerry

If your carburetor had no provision for vacuum control, it was meant for a larger truck that uses centrifugal advance only. I don't even see the need for vacuum advance on smaller trucks. It is just another thing that can go wrong. Back 60 years ago when we used GMC to work with, we never had vacuum advance. I would plug the hole and not use vacuum advance.

My reading of the post is that the hole in question is thread. So to plug it or make it smaller, use a screw/plug or a screw/plug with a hole in it. Why all these lead/brass plugs?

Also, why can't you just swap the bases?

The threaded hole in the throttle base for the vacuum line is a 1/8" pipe thread, with a tiny hole drilled behind it. That's why we're recommending plugging and re-drilling. Swapping carburetor bases is a rookie mistake that people who don't understand how carburetors are manufactured attempt, rarely with any degree of success.
Jerry

OK this rookie would like to know if he got two identical carbs, why you can't swap bases.

I know our engines get no vacuum advance at idle, but not too many years later, Chevrolet went to full vacuum advance at idle and as manifold pressure drops through throttle opening and increased load, less vacuum advance until zero at full throttle, then back the opposite way in the emission years to ported vacuum. It has gone every direction, run what works best on your application, a nice bit of vacuum advance for cruising makes for fuel efficiency.

If one base had the vacuum hole, and the other did not, then the carbs are not identical

Joking aside, there may be other consideration rather than the hole.

Assuming this is a Zenith 28 or 228 series, there are dozens of varieties in each of 6 different sizes. And there are different throttle bores within the same S.A.E. size.

Unless the carbs are identical by tag number, BAD idea to swap castings (unless you have prints for each carb, and the castings in question have exactly the same part number). Obviously, in this case, they would not have the same part number.

Yes, sometimes it will work. More often it will leave one with a carburetor "gremlin" that ends up with an eighty post thread on one of the automobile forums.

The position of the hole is more important than the size of the hole.

No comment about whether of not vacuum advance is necessary, not my area of expertise.

But if I were still doing carbs, and someone sent it to me like this, I would take it to the local welding shop, and have the entire hole filled with brass; then correctly machine a new hole. And charge accordingly.

Jon.

The carb base is on its way to me. When I return it, it's going to work properly.
Jerry

The pipe which goes to the vacuum advance is 1/16". The fitting for that pipe in the carburetor has a 1/16" hole in it. Any variation from that design may or may not cause a problem.
I have 99 problems, but a too big hole ain't one. I wouldn't want to have one and always wonder if it is causing "that noise, or that stumble".
Carl

Thanks for all the helpful and educational responses. I now know more than I did the other day about vacuum, carburetors and machine shops that don't always get things right. I was apprehensive about doing this job myself---with good reason as it turns out---that's why I had a machine shop try it. The best decision I made was to post my issue here on Stovebolt and talk with Jerry about it. I'm very grateful for his offer to help. Across the ocean it goes!

Ok- - - -the part arrived. Two initial impressions. Yes, it's drilled wrong- - - -badly wrong! Yes, it's fixable. It should be on its way back to you in a few days.
Jerry

OK- - - -all fixed and headed home! There was a hole approximately 1/8" in diameter that was directly in line with the throttle blade when it was fully closed. That would have applied straight intake manifold vacuum to the advance at idle. Also, there would have been an air leak past the throttle blade that might have raised the idle sped too high, even with the butterfly fully closed. Bad juju!

I tapped the existing hole for a 10-24 flat head machine screw, which I inserted from the outside after grinding the head down a bit to allow it to pass through the hole drilled for the 1/8" pipe thread. I installed the screw with a drop of blue Loctite after cutting it off so just a couple of threads would protrude into the carb throat, then smoothed off the projecting stub with a Dremel tool and a very fine grit grinding stone. The screw slot was left vertical, and I drilled a 1/16" hole near the top of the slot. Now the hole will be slightly above the throttle blade at idle, and will allow the vacuum advance to work as designed- - - - -no advance at idle, then it comes in at off-idle and light throttle cruise. On acceleration or a hard pull with a wider throttle opening, that advance goes away due to low vacuum to prevent detonation when the engine is pulling hard.
Jerry