Trouble Shooting

Carburetion

Aside from improper installation of the supercharger the biggest problem with a supercharged engine is carburetion.

The following may help to rectify problems with carburetion.

Rochester Carburetors

This carburetor offers less pressurization problems than any of the others. Sealing it is somewhat more difficult due to the accelerator pump, but otherwise it performs well throughout the range, if the proper jet sizes are installed. There are, however, occasional problems, and there are some important steps in the carburetor modification that need to be checked:

1) Fuel leaks-All gaskets should be in good shape, but there is one, between the throttle body and the main body, which almost always requires replacing. This gasket is of very thin section and hard composition, and the surfaces which it is expected to seal are usually somewhat rough; therefore, it forms a poor seal even when new. If this gasket is not replaced, fuel leaks (which appear to come from the idle adjustment screws) will often occur. To resolve this make a new gasket of soft Vellumoid about .0625 thick and use instead of the stock gasket.
2) Fuel leak-1956 Oldsmobile - The throttle body to main body gasket in the 1956 Rochester carburetor has cut outs in it which vent directly to the atmosphere. A new gasket will have to be made to replace this, which does not have the cut outs.
3) Flat Spots-Due to excessively rich mixture - Occasionally carburetor modifications can result in a car which stumbles with a flat spot under full throttle before the supercharger gets into "high blower".
Usually this is caused by the power valve opening too soon and dumping an excessively rich mixture into the engine before the supercharger builds up enough air volume to utilize it. The power valve opens more quickly with the supercharger than without it because air pressure from the supercharger acts with the power valve actuating spring to overcome engine vacuum. To resolve this make an air/fuel ratio check to identify this condition. If present, then weaken the power valve actuating spring (this is not the spring inside the power valve itself) by cutting off part of a coil at a time, and running air/fuel ratio checks until the condition is eliminated.
4) Flat Spots-Due to lean mixture - Occasionally, a car will hesitate, and spit and backfire at the carburetor upon opening the throttle from a closed position. This is an indication of a lean mixture (which should be confirmed with an air/fuel meter) and is usually caused by an air leak above the float bowl area. Even at idle speed, the engine will get rough if the carburetor housing is loosened sufficiently to allow a leak. To resolve, check that the atmospheric valve above the accelerator pump has been removed and the resultant openings have been plugged. Check and resolve any other leaks from the pressurized carburetor.
5) Engine Dying - With dual quad carburetors-One some cars with dual quad carburetors the engine will die when rounding a corner, or when a quick stop is made. To resolve this lower the float bowl level about .1875 from stock position. Cars with a single carburetor can usually use the extra fuel which slops over under the aforementioned conditions, but the extra fuel from two carburetors is usually too much for the engine to absorb, and as a result it floods and dies.

Carter Carburetors

1) Lean Air/Fuel Ratio - At Full Throttle-Due to the design of the carburetor it is difficult to get this carburetor rich enough at full throttle. No matter how large the secondary jets are made the condition remains. We could of course enlarge the main jets even more then recommended in order to overcome this problem, but this would ruin fuel economy under cruise conditions. If an air/fuel ratio check shows this to be true (full throttle high blower ratio should be at least 10.5 to 1) then remove the float bowl cover and install a baffle under the screw which holds the accelerator pump jet in place ( this screw is between the two front barrels and slightly in front of them). This baffle will shield the main jets from the supercharger air blast and tend to allow a richer mixture to flow through them without increasing their size to a point where cruise economy would suffer. If this does not work then fasten a longer copper impact tube to the impact tube which sticks out of the sidewall of the air horn (that is if the carburetor has an impact tube) and bend the new copper tube so that it faces into the air stream as it enters the carburetor housing. A short piece of .3125 diameter Copper tube will fit over the existing tube, if the shank of a .025 drill is used to slightly enlarge one
2) Leaks from the Accelerator Pump Operating Shaft-Some Carter carburetors will leak at this location under some conditions.
The solution is to remove the loose copper washers from their position on the main jet metering rods. These washers trap fuel above them as it sloshes up on sudden stops, or acceleration, and it has no where to go except top leak out along the pump shaft.
3) Flat Spots, Stumbling, Rich Mixture on Full Throttle Application from Closed Throttle-This may be caused by the power valve opening before the supercharger can supply enough air to take care of the additional fuel. The valve, which is operated by a spring, vacuum, and a positive mechanical linkage, is sometimes blown open too soon by the pressure from the supercharger. To resolve this remove the vacumeter piston spring from below the vacumeter piston and discard. The valve will still open with the assistance of vacuum, air, and mechanical linkage, but in effect, will not open entirely until blown open by supercharger pressure.
4) Flat Spots-Due to lean mixture-Occasionally a car will hesitate, and spit and backfire at the carburetor upon opening the throttle from a closed position. This is an indication of a lean mixture (which should be confirmed with an air/fuel meter) and is usually caused by an air leak above the float bowl area. Even at idle speed, the engine will get rough if the carburetor housing is loosened sufficiently to allow a leak. To resolve check that the atmospheric valve above the accelerator pump has been removed and the resultant openings have been plugged. Check and resolve any other leaks from the pressurized carburetor.
5) Poor Performance-General-One cause is a failure to plug all holes leading to the atmosphere; one hole, in particular, is sometimes overlooked. It is located in the end of the dust cover over the main metering rod jets (in some carburetors). If left open, the car will perform as though it had ignition trouble. Another trouble spot is the throttle body to main body gasket which is sometimes directly vented to the atmosphere. An example is the 1956 Corvette (some, not all). Solutions are to recheck for holes leading to the atmosphere and plug. In the case of the 1956 Corvette, there are three elongated holes along one edge of the gasket, the center one of which causes a leak to the atmosphere due to a counterbore below. Make a new gasket with two holes rather than the center elongated hole. In other words, separate the hole to the vacumeter piston from the main body to throttle body screw.
6) Engine Dying - With dual quad carburetors - On some cars with dual quad carburetors the engine will die when rounding a corner, or when a quick stop is made. To resolve this, lower the float bowl level about .1875 from stock position. Cars with a single carburetor can usually use the extra fuel which slops over under the aforementioned conditions, but the extra fuel from two carburetors is usually too much for the engine to absorb, and as a result it floods and dies.

Holley Carburetors

This carburetor performs as well as any other when pressurized, but requires more modification around the power valve area.

1) Leaks - General poor performance-Some late Holley carburetors have a hollow accelerator pump shaft which is open at the top end. This, of course, allows air and fuel to leak to the atmosphere. To resolve this plug the top end of the shaft with a soft left ball. Also plug the hole in the side of the shaft as per the instructions in the example on the installation page.
2) Flat Spots, Stumbling - (Rich Mixture) on Full Throttle Application from Low RPM-This could be an indication that the power valve is opening too soon and causing an excessively rich mixture before the supercharger can get into "high blower" operation.
- Too resolve this make an air/fuel ratio check to make sure that the mixtire is rich, If so, the size of the holes in the power valve may be decreased if, after doing so, the air/fuel ratio is still at least 10.5 : 1 at full throttle high blower operation. If this ratio cannot be attained, then the hole size must be returned to the size called for in the original modification, and then you must weaken the power valve actuating spring (this is not the spring inside the power valve itself) by cutting off part of a coil at a time, and running air/fuel ratio checks until the condition is eliminated.