The Stovebolt Site Forums The Shop Area Engine Shop Zenith carburetor question

Can an adjustable main jet be installed in a carburetor with a fixed jet?

i sent in a carb to my guy for rebuilding. i told him its off of a 228 and wanted to put it on a 270 in time. he said the main jet wouldnt be large enough for the 270. he said if i had an adjustable jet (which i just happen to have) to send it and when the time came all i would have to do is dial it into the 270. when the carb came back the fixed jet was laying loose in the box. in my case i would say yes.

We used to buy adjustable main jets for Zeniths when we repowered a GMC from a 228 to a 248. We got them at the GMC house, but I don't remember if they had a GMC part number or if they were aftermarket. We always ran our GMCs a little on the rich side to keep the valves cooler.

The answer is a qualified yes. Zenith produced dozens of different TYPES of carburetors; and each type has at least one adjustable main metering jet which differs from all of the other types.

WITHIN TYPE, fixed jets may normally be replaced with adjustable jets.

However, replacing a fixed jet with an adjustable jet is not going to change the air flow of the carburetor. You cannot rejet a 228 carb to work on a 270 engine.

Jon.

It's also not a good idea to use adjustables in dual carb installations. You'll never get the flow to match without extensive testing.

I think by "work" you mean produce acceptable power and the same fuel curve. The power is obvious, but the mixture curve could probably be corrected with some work to the air bleeds etc.

The carb in question is for a 270 and it has a fixed jet, I figured if I rebuilt it and swapped the adjustable main metering jet it would be just right for a chevy 261.
Am I thinking correctly?

Thank you,
Ray

thanks i wasnt told that part.

The larger engine requires more air.

Just to illustrate this point, I checked the internal venturi size (controls air flow) on both the 228 and 270 carburetors for 1952.

The 228 carb has a 29 mm venturi and the 270 carb has a 33 mm venturi. Since area is a function of the square of the radius, there is a 29 1/2 percent increase in the airflow for the 270 carburetor.

The smaller venturi will actually run RICHER on the larger engine (up to a point). When that point is reached, the engine will starve for air.

Jon.

[quote=carbking
The 228 carb has a 29 mm venturi and the 270 carb has a 33 mm venturi. Since area is a function of the square of the radius, there is a 29 1/2 percent increase in the airflow for the 270 carburetor.

The smaller venturi will actually run RICHER on the larger engine (up to a point). When that point is reached, the engine will starve for air.
Jon. [/quote]

ok, then here's what else i did but some time ago figuring how much could it hurt. i took a zenith off my 248 and put it on a 270 upgrade for my 51 350 firetruck. whats the difference here if you have the info? just curious.

The venturi size for the 248 would depend on the actual carburetor number, but probably 30~31mm.

Please understand that not all 228 carbs have a 29mm venturi; nor do all 270 carbs have a 33mm venturi. I simply picked the two carbs used in 1952 as an illustration.

A GENERAL RULE OF THUMB (and when I was in the military, I used to tell the SGT that "general" rules were made to be broken by us "privates"):

When migrating an O.E. (original equipment) carburetor from one engine to another; BEST RESULTS will be obtained if the donee engine is within plus or minus 3 percent of the displacement of the donor engine.

Thus 3 percent of 228 is roughly 7; so a 228 carb could generally be used successfully on an engine from 221 to 235 CID.

Using the 248 example: 3 percent of 248 is about 7 1/2 (round to 7); thus the 248 carb could probably be used successfully on engines from 241 to 255 CID.

The engines need to be of similiar design and RPM for this rule of thumb to work.

Also generally:

Placing too large a carb on an engine will cause LEAN running (due to decreased air velocity in the venturi area).

Placing too small a carb on an engine will cause RICH running up to the point of where the carburetor cannot supply the required air for the engine.

For best results, always adhere to the "5 P's" of engineering!

Jon.

Using a 228 carb with an adjustable metering jet on a 248 worked just fine for us 50 years ago on rural Texas roads. Running a slightly over rich mixture gave us much longer valve life and actually did a little better on fuel economy because the 20 cu in saved a gear on a lot of hills. We used C45 plugs to compensate for the richer mixture. Carburetor icing was however more of a problem, but it was only around 38 degrees and high humidity. I agree, however, that a 228 carb would not work on a 270 with good results. I have put Stromberg WW carbs from a 56 Plymough on a 270. If you have a 3 bolt manifold, a Holly from a 272 Ford works well on a GMC 270 also.

Great info, Jon! Now, please tell me how to jet my 800 CFM Holley spread-bore carb to work right on my 1955 Corvette 265 engine! It's gotta work, because my second cousin who used to work at Jiffy Lube says it will!

(Now, where's that sarcasm icon?)

Jerry

carb king, great stuff you laid out here.

Carbking,
I have a 270 GMC. I have access to a Zenith 228BV12C (Out line 10168) that I am thinking of buying. Problem is, this Zenith has a 28mm venturi and from your posts I learned I need a 33mm venturi for this carb to work properly. Also this carb has a 29 Main Jet, 13 idling jet and 13 accelerator pump jet. Is there a source of replacement venturis and replacement jets? Would you recommend this carb? It is NOS and $125.

Thanks,
David

David - the price is "dirt-cheap", but NOT for a 270. You would have to change at least:

(1) venturi
(2) main discharge nozzle
(3) main metering jet

And possibly
(4) idling jet
(5) idle air bleed
(6) idle restrictor
(7) idle bypass
(8) main airbleed
(9) pump jet

Do you have a jewelers lathe, and lots of free time?

Look at the second line in my "signature".

Jon.

Carbking,
You saved me a lot of pain! Thanks. For anyone else that may want the item described above, here's the link. The title of the listing doesn't match the tag on the carb so look carefully.



http://rover.ebay.com/rover/0/e11050.m43.l1123/7?euid=845d1c89364e4f49b6081f706faf64eb&loc=http%3A%2F%2Fcgi.ebay.com%2Febaymotors%2Fws%2FeBayISAPI.dll%3FViewItem%26item%3D121814921673%26ssPageName%3DADME%3AX%3AAAQ%3AMOTORS%3A1123