Corvair Rochester Carburetor Float System
After presenting a program on carburetion in October, 2005, I was asked by longtime NTCA and CORSA member Fred Johnson if I had written any articles about my program. So to honor Fred’s request, I am writing this one. Many books have been written about carburetion, but I’ll examine areas which to the best of my knowledge have not been covered in much detail.
The first area to cover is the float system and more specifically, the needle and seat. The Chevrolet parts catalog, P & A 30C, lists two needle and seat assemblies for the Rochester carburetors. Part number 7023814 is for the ’60 to ’63 models and has a .058” diameter fuel orifice. Part number 7023803 is listed for the ’64 to ’69 models and has a .084” diameter fuel orifice. I have NOS examples of each and have verified these diameters. However, without actual product drawings I can’t say what the tolerances were on these hole diameters.
This hole diameter has great importance in the proper operation of the carburetor. There is an easy way to identify these by the number of grooves machined into the outside of the body of the seat, as shown in the two examples pictured.
’60 to ’63 Seat (two grooves)
’64 to ’69 Seat (one groove)
There are other Rochester needle and seat assemblies that have other grooves cut to identify their size. Currently replacements that are available have two styles and are pictured below. The seat diameters are either .096” or .101”. For most normal applications, this will raise the dynamic float level in the bowl above the static level adjustment. I personally would not recommend this for the “normal” stock street operation, unless you are willing to experiment with various float level settings, but this large size may be warranted in specialized racing applications. Larger sizes also allow for purging fuel vapors.
Aftermarket Seat (no grooves)
Aftermarket Seat (two grooves)
Another item worth mentioning at this time is the “caged” double-ball type seat, which is great for shutting off the fuel supply. It is so good that if your car or van is not driven regularly, fuel “varnish” can actually make the seat stick shut. It can be very difficult to start and keep the engine running when one carburetor will not fill with fuel. Another problem is the inlet size. The ones I bought have about a .067” diameter – too small for a 164 CID engine and just slightly too large for the ’60 to ’63 models, unless you are willing to play with the static float level setting. The original source that made this, Grose Jet is no longer producing these for whatever reason. I have used these on other applications and was very pleased with the results as long as the proper inlet size was available and as long as you could keep oxidized gasoline from causing them to stick shut.
I have said much about how the inlet size of the seat affects the float level. Now let’s examine the importance of the float level in the carburetor. The level of fuel in the bowl is critical because its height determines at what engine speed gasoline will start flowing from the venturi cluster. A float level set too high starts fuel flowing from the venturi cluster while the off-idle, or progression, circuit is still flowing fuel. This creates a rich overlap between these two circuits. If the float level is too low there is a “lean hole” between the off-idle circuit and the main metering circuit. The off-idle circuit’s flow is progressively slowed, but the main metering circuit supplied through the main jet and venturi cluster hasn’t started up yet.
Symptoms of too high a float level are black smoke flowing from the exhaust, higher than normal fuel consumption, and difficult engine restarting after a hot-soak of 15 to 30 minutes. Conversely, if the float level is too low, symptoms could include severe hesitation on acceleration, overheating while the engine is under heavy load, or lower than average fuel consumption at moderate speeds.
You are probably saying by now, “I already know I very accurately set the float level the last time I rebuilt the carburetors!!” But setting the float level during a rebuild is a “static” setting. We all know, or should know, that the float, along with the needle and seat assembly, is anything but a static application when installed on an engine that is running. Gasoline is metered into the engine all the time it is operating, even during deceleration, although fuel is not needed during this mode of operation. The float, along with the needle and seat, acts to allow fuel into the bowl to replace consumed fuel, and to stop the flow of fuel when the float forces the needle against the seat. This alternate opening and closing of the needle and seat is continuous as the engine is running, and works to maintain a constant fuel level.
Now you may ask, “What does the seat hole diameter have to do with anything? The float has a buoyancy force from the gasoline in the bowl and shuts off the fuel flow, just like in a toilet.”
The answer is yes and no. Let me explain.
The Corvair fuel pump is supposed to pressurize the gasoline to 4 - 5 PSI (pounds per square inch) between idle and 1000 RPM. Raising fuel pressure beyond this point will also raise the float level. Raising the fuel pressure by just 1.5 PSI above normal can raise the float level over 1/32”. Weber carbs, for example, can not tolerate more than 3.5 PSI fuel pressure. Below is an example that can be surprising.
P/N 7023803 has a .084” diameter fuel orifice. Using 5 PSI fuel pressure, force exerted on the seat may be determined by the following:
(.084”)² x π
______________ X 5 = .0277 Lbs. (force) or .4433 Oz. (force)
Not much force, you say. Maybe it isn’t, but let’s look at the ’60 to ’63 seat with the same 5 PSI fuel pressure.
(.058”)² x π
______________ X 5 = .01321 Lbs. (force) or .2114 Oz. (force)
Examining the pressure difference between the two seat sizes reveals a whopping 52.3 % difference. I haven’t tested the buoyant force of the float, but I have unknowingly installed the smaller diameter seats in my ’64 Monza coupe (110 w/ PG and A/C) and had a perpetual flat spot whenever I accelerated the car from a dead stop. Full throttle from a dead stop would completely stall the engine. No amount of additional volume output from the accelerator pump/ larger discharge jets size or bigger main jets would cure the problem hesitation. Replacing the needle/seat assemblies was the only cure necessary.
Our past NTCA President Garry Parsley had a similar hesitation problem, but not nearly as bad. Investigation showed he had one small and one large seat in the carbs in his Greenbrier, which runs a modified 140 engine with +.060 overbore and PG. Replacing the smaller ’60 to ’63 seat eliminated the hesitation and improved his power and drivability.
I have to admit to having no experience with ’60 to ’63 models that might mistakenly have a ’64 to ’69 needle and seat assembly, but I can imagine them having continuous rich running problems and difficulty restarting during hot-soak. In severe cases, there might even be continuous fuel dripping out of the venturi cluster and very poor gas mileage. The only cure is physically lowering the static float level.
The float has a finite amount of buoyancy force that will be exerted against the needle and seat. There are two different sizes of floats, differing only in height. The ’60 to ’63 has the larger volume. The ’64 to ’69 float is smaller, but also has a float assist spring (commonly called a “bumper spring”) which is used to stabilize the float and float level on rough roads. I personally have never seen two of these springs that exert the same force, even new replacement parts!! Some of these springs can exert enough force to actually hold the float up against the seat while measuring the float drop dimension. Of course, this is an extreme case, but I mention it only because too much force from this spring will prematurely close the needle and seat, and also result in a lower float level.
So let’s summarize things that will affect the float level:
1. Seat diameter
2. Fuel Pressure
3. Float type (’60 - ’63 or ’64 - ’69 style)
4. Float assist spring strength
5. Static float level setting
I have left the fifth item for last because I want to put emphasis on how you set the static float level. The Kent –Moore metal gauges are great if you have them, but if not, how can you measure the float level accurately without a scale? There is a simple solution that is probably in your tool box. It happens to be a common drill bit.
The ’64 to ’69 can use a 1/8” bit and the ’60 to ’63 uses a 9/64” bit. The drill bit is placed between the air horn (fuel bowl) gasket and the top of the float. Because you have the top of the carburetor cover inverted, the top of the float is now on the bottom. Adjust the float using the correct drill bit until it touches the soldered edge of the float halves.
A hint to seal the top of the carburetor and prevent gas leakage is to soak the air horn gasket in motor oil over night, or longer if you like. Then remove excess oil and install. The oil will make the gasket swell slightly and give a good seal. It also prevents sticking and tearing of the gasket during removal.
Another hint to prevent the gas line metal-to-metal seal at the carb from leaking is to lightly oil a 2-010 O-Ring, install it in the large fuel filter nut and lightly screw in the fuel line. DO NOT OVER-TIGHTEN the fuel line since it only takes light force to affect a seal. If you do over-tighten this line you will shred the O-Ring, and this can cause flooding if rubber gets past the bronze fuel filter!!!! Don’t ask me how I know this!!!
The last hint is use LOW-strength Loctite on your airhorn screws to prevent them from loosening up during operation and causing leaks. If you mistakenly use high-strength Loctite, you will not get the carburetor apart for a very long time, if ever!!!!