VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 Zenith T-4X Carburettor

I have a 1920 FB Roadster fitted with a Zenith T-4X Carburettor. I would like to know the correct sizes for the Main jet, Idling jet and Compensator jet.

In checking the 1928 parts book it gets rather confusing.

Main Jet: 80 or 90 and no distinction

Idling Jet: 35-40-45 Again no distinction

Compensating Jet: 18 or 19 No distinction

Checking the Model "FB" Combined Parts List. Dtd Mar 15, 1921

Main Jet: In sizes by 5 from 60-110

Idling Jet: In sizes by 5 from 30-50

Compensating Jet: In sizes by 5 from 60-110

Some of the figures don't jive and I have no explanation.

Thanks Ray for the information, I checked my parts list and came up with the same figures as you and I agree the figures don't jive.
There appears to be a broad range of jet sizes available originally. I thought there would have been a standard set of jets that were fitted with sizes varing above and below, as per the later Cater Chev 4 carburettors. Maybe someone else has more information on these carburettors.

Actually the correct carburetor for your 1921 FB would be the Zenith O-4. The Zenith T4-X was not used until '22, most likely later in the model year.

Somewhere I have a chart that shows the meaning of the numbers on the jets. I have looked in the Zenith file but did not find it. Maybe in one of my other books. If I remember correctly the early numbers were the size in mm and the later were a fraction of the size that must be multiplied by a factor to get the mm size.

Thanks Chipper for the information. Is the Zenith O-4 an all brass carburettor similar to the earlier 490 carburettors.
If you can find the information on working out the jet sizes I would sure appreaiate a copy. You mention mm, were these carburettors all made to metric sizes.

Chipper I found a Carburettor information sheet which was printed in the Sept 1987 G&D, it states that F, FA & FB wre fitted with T-4XF Carburettors. This information is faily old, has other information come to light which has superceded this.

The most recent article on the early carburetors was published in the July 2001 issue. It cites the references used so future research can be used to update and correct the information. So far it is the most correct early Chevy carby information that I know of.

It is my understanding that Zenith was a French company. They were made in GB before the USA. If that is the case then internal dimensions in mm seems logical even when the bolt and bore sizes were in inches.

From my 1919 Zenith Instructions:

The chokes are numbered in millimeters according to the size of the smallest inside diameter.

The Jets and Compensators are numbered in hundredths of a millimeter. A one hundred jet has one millimeter hole and is smaller than a one hundred five jet. They are graded by five hundredths of a millimeter apart.
KenK

Thanks Chipper and Ken the information supplied has been a big help.

I will update my information from the sources supplied.

Derek,
Yes the O-4 Zenith is has an all brass body. They are similar in design and internals to the T series carbs that were used later. It is my opinion that the cast iron parts were used in the T series to lower the cost.

Still have not found my reference to the later specs. Actually have not spent enough time looking. Will get more time later this week end.
Ken posted the specs for the earlier parts. But sometime later they changed the number system.

For what it's worth, the T4X in my '23 has a 78 main jet and the compensating jet is an 18.

I have found my reference. It is from a 1928 edition "Dykes Automobile and gas engine Encyclopedia" page 1295 (actually a separate supplementary book entitled "Carburetors").

“In the older types of carburetors such as Models L, O. and HP, its size is marked in 1/100ths of a millimeter.

In the later types the mark signifies the number of 1/100th of a millimeter divided by 5.”

Later it states “ The main jets are made in various sizes progressing in steps of 5/100ths of a millimeter.”

“Model L (old model) main jets come in sizes as low as 60 up to as high as 250, and in this order … 100-105-110, etc.”

“Equivalent sizes of Model U, ST, SV (later model) main jets would be from 12 up to 50, and in this order- 20-21-22-23.”

I hope that this ends any confusion on the bore numbering system used by Zenith for the O, T and V series carbs used in early Chevrolets.
I have found my reference. It is from a 1928 edition "Dykes Automobile and gas engine Encylopedia" page 1295 (actually a separate suplimentary book entitled "Carburetors")

Gents
I have been following this thread with interest, could someone provide any info on the model number Zenith used on the 1915 Amesbury Special, is an all brass Zenith or part brass and part cast iron like the 490's
Chris

According to an article in the July 2001 G&D the 1914-16 used a Chevrolet part number 21168B or Zenith O-4. That would be and all brass bodied carburetor. The internal part specifications might be in the owners manual which I don't have, or maybe a repair manual. Maybe some of you that have model H stuff can add more information.

Thanks Chipper I thought it was an all brass type and was pretty sure it was O-4, mine has a 490 carby at present and would like to get it right
Chris

Thanks Chipper when you said you had to refer to your reference material it spured me into looking through what I have. After searching through a stack of information I found a book that I forgot I had and it contained the following specifications for O-4, T-4 and T-4-X carburetors.

The following information was found in a reprinted copy of the Chevrolet Repair Manual 1918-1924 "490"-Superior G-FA-FB-T Third Edition January 1923

The carburetor jets which we find to give the best results are as follows:

model carb choke main jet Comp idle jet needle va
490 O-4 19 90 95 40 44
Superi T-4X 19 18 18 40 44
490-G T-4 19 17 18 40 44
FA O-4 21 95 100 40 44
FB O-4 21 95 100 40 44
FB T-4 21 19 20 45 38
T 21 95 110 45 38

The readings on Model T-4 carburetor, main and compensator jets, can be multiplied by five to determine the comparison with jets used in Model O-4 carburetor.

Thanks again Chipper and Ray as always both of you have been a great help.