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Zenith carb: I don't think this is OK
#1310399 Sat May 11 2019 06:43 PM
Joined: Feb 2013
Posts: 243
E
Shop Shark
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

Re: Zenith carb: I don't think this is OK
Eureka Jim #1310409 Sat May 11 2019 08:14 PM
Joined: May 2005
Posts: 9,146
G
.
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.

Last edited by Grigg; Sun May 12 2019 06:34 PM. Reason: oops

1951 GMC 250 in the Project Journals [stovebolt.com]
1948 Chevrolet 6400 [stovebolt.com] - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
1952 Chevrolet 3800 pickup [stovebolt.com]
---All pictures [picasaweb.google.com]---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-
Re: Zenith carb: I don't think this is OK
Eureka Jim #1310416 Sat May 11 2019 10:00 PM
Joined: Feb 2000
Posts: 4,193
J
Shop Shark
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.

Re: Zenith carb: I don't think this is OK
Eureka Jim #1310484 Sun May 12 2019 01:35 PM
Joined: Feb 2004
Posts: 21,245
H
Boltergeist
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Zenith carb: I don't think this is OK
Eureka Jim #1310503 Sun May 12 2019 05:57 PM
Joined: Jan 2008
Posts: 4,688
C
Shop Shark
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.

Re: Zenith carb: I don't think this is OK
Eureka Jim #1310508 Sun May 12 2019 07:08 PM
Joined: May 2005
Posts: 7,530
B
General Purpose
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?



Watch out for careful drivers!!!
I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.
You can't teach a new dog old tricks.


Re: Zenith carb: I don't think this is OK
Eureka Jim #1310509 Sun May 12 2019 07:19 PM
Joined: Feb 2004
Posts: 21,245
H
Boltergeist
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Zenith carb: I don't think this is OK
Eureka Jim #1310511 Sun May 12 2019 07:24 PM
Joined: May 2005
Posts: 7,530
B
General Purpose
OK this rookie would like to know if he got two identical carbs, why you can't swap bases.


Watch out for careful drivers!!!
I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.
You can't teach a new dog old tricks.


Re: Zenith carb: I don't think this is OK
Eureka Jim #1310515 Sun May 12 2019 07:47 PM
Joined: Jan 2013
Posts: 1,663
S
Shop Shark
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.

Last edited by sstock; Sun May 12 2019 07:48 PM.

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
https://www.youtube.com/watch?v=Bli...n05i04t1aokgm4p04jiwgffwhyyih5xbk0h00410
1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration
Re: Zenith carb: I don't think this is OK
bartamos #1310524 Sun May 12 2019 09:14 PM
Joined: Nov 2002
Posts: 2,068
C
Carburetion specialist
Originally Posted by bartamos
OK this rookie would like to know if he got two identical carbs, why you can't swap bases.



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

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.


Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify
If you truly believe "one size fits all" try walking a mile in your spouse's shoes!
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