The Carter YF as designed for Chevrolet 235 CID six cylinder

General information

There is quite a story as to why Carter produced "bolt-on" aftermarket type YF carburetors for Chevrolet. That story is not the focus of this article. We will concern ourselves with the fact Carter did in fact produce type YF carburetors as aftermarket for the 235 CID six cylinder. Carter also produced YF carburetors as aftermarket for the Chevrolet 216 CID six cylinder; but they will NOT be covered in this article.

Carter original YF versions for the 235

Carter released 7 different type YF replacements for the 235. They are:

788s 1950-1952 truck manual choke (a significant calibration modification made, resulting in the number 788sa)

789s 1950-1952 w/powerglide (automatic choke) (a significant calibration modification made, resulting in the number 789sa)

966s 1950-1952 truck manual choke

967s 1950-1952 w/powerglide automatic choke

2100s 1953-1959 truck manual choke

2101s 1953-1956 w/powerglide automatic choke

3211s 1957-1952 passenger, 1960-1962 truck automatic choke

All 7 carburetors are an S.A.E. size 3 (2.9375 inch center to center mounting holes) and 1.3125 inch venturi. Thus both physical and airflow sizes are identical.

Calibration

Below is a table of original calibrations for these carburetors:

Number	Status	Idle Jet	Main Jet	Rod (Economy)	Rod (Midrange)	Rod (Power)	Comment
788	S	0.033	0.095	0.068		0.052	2 step rod
788	SA	0.033	0.1015	0.073		0.044	2 step rod
789	S	0.033	0.095	0.068		0.052	2 step rod
789	SA	0.033	0.1015	0.073		0.044	2 step rod
966	S	0.032	0.1015	0.073	0.059	0.050	3 step rod
967	S	0.032	0.1015	0.073	0.059	0.050	3 step rod
2100	S	0.028	0.0945	0.067	0.058	0.050	3 step rod
2101	S	0.028	0.0945	0.068	0.060	0.050	3 step rod
3211	S	0.028	0.0945	0.071		0.048	2 step rod

Note that there is a major calibration change from the first generation 788s and 789s to 788sa and 789sa. After that, calibration changes were basically "tweaking" for improved drivability and fuel economy. Although, I do not know why Carter reverted to the 2 step rod when the 4th generation 3211s was produced. For those who enjoy mathematics, remember that the main circuit calibration is the AREA (not the diameter) of the main metering jet, less the dynamic area of the step of the metering rod currently in the jet.

Restoration of the Carter YF

As with the restoration of any carburetor, some homework PRIOR to disassembly of the carburetor is in order. This article should aid in the identification of the carburetor in question. Acquire good instructions (a Chevrolet Shop Manual, or both the parts and service pages from a Carter Master Manual) AND READ THEM! The generic instructions found in cheap rebuilding kits should NOT be used. Now carefully disassemble the carburetor. If you are unsure of your ability, try mounting a camcorder on a tripod, direct the camcorder at your work, and talk to the camcorder. Now you have a reference when reassembling the carburetor.

Acquire the PROPER kit. While many parts will physically interchange from carburetor to carburetor, they may not correctly function. For this reason, we offer SIX DIFFERENT repair kits for the 7 Chevrolet YF aftermarket carburetors mentioned in this article. If you really believe “one size fits all”; try walking a mile in your spouse’s shoes. But the biggest issues with the cheap kits are the fuel valve. The vast majority of YF carbs were used by Ford, and the Ford 300 CID engine requires more fuel than the Chevrolet 235. Depending on which of the cheap kits one purchases, one will find the orifice of the fuel valve to be 0.098~0.101 inch. The CORRECT orifice for the aftermarket Chevrolet YF's was 0.081 inch. If one does the math (remember area is a function of pi, one will find the fuel flow through a 0.098 orifice is 46 percent GREATER than through a 0.081 orifice). Additionally, the fuel valves in the aftermarket Chevrolet YF carbs were the more expensive spring-loaded valves. Not saying one does not exist, but have never seen one of the cheap kits with the spring-loaded fuel valve. Why is this a concern? With the wrong orifice, and the fixed valve, the carburetor will often flood at idle, plus fuel economy will be noticeably reduced.

Worn steel arms and rods should be replaced (they are readily available at this time).

Metering rods very rarely go bad without help from Dr. Goodpliers (the evil twin of Mr. Goodwrench). The "power tip" will bend, ruining the rod. We can machine new rods, but they are expensive.

There has been much discussion on the internet concerning the use of neoprene fuel valves and deathanol. While deathanol creates many problems for the restorer, the problem with the newer fuel valves lies in an “economy” move by the manufacturer. The “staking” process has been eliminated. The effects of this process may be duplicated by the restorer. Prior to installing a new neoprene fuel valve, acquire the following: small wooden block, steel ball from a modern Q-Jet, drift punch, and a hammer. Set the thread of the fuel valve seat on the wooden block, insert the ball inside the seat (replacing the plunger), set the drift punch on top of the ball, and whack the drift punch with the hammer. The wooden block protects the threads from damage. The pressure from the steel ball will form a chamfer around the orifice, eliminating the sharp edges which cut into the neoprene, and providing a larger area for a better seal.

We have, with written authority from Carter, reproduced the original Carter service instructions for the Carter YF: Carter YF Service Instructions

Mounting gaskets

Carter specified mounting gasket # 1A-31 for all of these carbs except the 2100s and 2101s in the bill-of-material. Carter did not specify a mounting gasket for either the 2100s or the 2101s in the bill-of-material; but did specify gasket set 240 for the 2101s and gasket set 241 for the 2100s. These Carter gasket sets contain mounting gasket 1A-31. I have no information as to why 1A-31 was not specified in the bills-of-material for these carburetors.

Gotchas

The major "gotcha" with any of the Carter YF's is the use of the neoprene diaphragm used for the accelerator pump. However, once neoprene has been exposed to ethanol AND THEN ALLOWED TO DRY, it turns hard. A diaphragm has to flex for accelerator pump action, and the hardened neoprene will crack, and subsequently fail. Thus the solution is to either (A) use non-ethanol fuel, or (B) start the engine sufficiently often to not allow the fuel bowl to evaporate dry. I am occasionally asked why the fuel valves don't also have this issue. The fuel valves do not have to flex, thus hardening is not an issue. Neoprene is an acceptable material for use with ethanol, according to government sources. Carburetors are seeing less usage annually. Thus the part manufacturers have no incentive to do the research to determining the thickness of Buna N with fiberglass reinforcement that could be used for the diaphragms, and redo their dies to produce diaphragms from this material.

Other uses (multiple carb set-ups)

The YF utilizes a vacuum power system. For this reason, they are less easy to tune than carburetors with a manual power circuit. While many enthusiasts have succeeded in using YF's in a multiple carburetor environment, I would suggest to someone building a multiple carb set-up from scratch that the Carter W-1 series carbs are MUCH easier to tune.

Comments

If I were going to use any of the first generation carbs (788s or 789s) I would definitely upgrade them to the "SA" calibration by changing the metering rod and main jet.