It’s always gratifying to pass along a technical note that can be implemented at practically no cost, and which has the potential for improving the performance of a large number of engines in the Atomic 4 fleet. One such technical issue recently came to our attention, which could well explain why many late model carburetors (through the late 1970’s) might produce a richer than normal fuel mixture.
I ask your patience in sorting through this rather lengthy note, and the good news is you’ll find that the fix is much easier than the explanation. In fact, if you’re on a really tight time schedule, you can skip to the last paragraph and blindly follow the suggested fix without ever understanding the problem.
BACKGROUND: All Zenith carburetors produced since the mid 1980’s have a fifth retaining bolt in the front of the upper and lower housings. Prior to this generation of carburetors, there were only four bolts holding the two housings together, and all four bolts were located around the float chamber. This configuration left no fastener to pull the front of the housings together. How this fifth bolt (or the lack of it) relates to a rich running condition can best be understood by a quick review of how the internal pressure within the carburetor is regulated to maintain inlet pressure instead of ambient atmospheric pressure.
INTERNAL PRESSURE DISTRIBUTION WITHIN ZENITH CARBURETORS: If you look in the upper part of the intake throat of a carburetor with the flame arrestor removed, you’ll see a small hole just in front of the choke valve. You can see this hole in the schematic of a Zenith carburetor on page 4-3 of our service and overhaul manual or in the attached image.
This opening in the top of the intake throat connects with a passageway between the upper and lower housings which continues around the venturi tube, into the area of the top of the main well vent jet, and eventually entering through the top of the float chamber. This passageway has the effect of delivering inlet pressure to the main well vent jet and the inside of the float chamber.
NOTE: It’s very important to note that while inlet pressure varies with RPM (and how dirty the flame arrester element might be); inlet pressure is always somewhat lower than atmospheric pressure.
EFFECT OF INTERNAL PRESSURE IN THE HIGH SPEED SYSTEM: During normal operation, suction created by the venturi draws fuel from the bottom of the main well just below the float chamber, and as the fuel level within the main well lowers, it uncovers one or more of the small holes in the bottom of the main discharge nozzle. At this point, internal air is drawn into the discharge nozzle through the main well jet to mix with the fuel moving up through the nozzle.
A sort of “tug-of-war” develops between the venturi (low pressure) on one end of the discharge nozzle and inlet pressure (low, but not quite as low) within the float chamber and well vent jet on the other end of the nozzle. If atmospheric pressure (somewhat higher than inlet pressure) is allowed to enter the float chamber and the area of the main well vent jet, the venturi will have an easier time on its end of the “tug-of-war”, and a slightly richer fuel mixture will usually result. As long as inlet pressure is maintained within the float chamber and the top of the main well vent jet, a proper fuel mixture will result at any power setting within the normal RPM range.
INSPECTING THE INLET PRESSURE PASSAGEWAY: The simplest way to inspect the inlet pressure passageway in a four-bolt carburetor is to attempt to insert a thin feeler gauge (.002″ or so) between the front of the upper and lower housings. If you discover a space between the fronts of the housings, atmospheric pressure will be allowed to enter the float chamber and the area on top of the main well vent jet.
REMEDIATING CARBURETOR INTERNAL PRESSURE REGULATION: The good news (finally) in all of this is that all you have to do to bring the front of the housings together is to disassemble the carburetor and file the top and bottom of the two housings until the surfaces of the housings are flat and meet all around their edges. I like to use a broad (rather coarse) file, but some folks prefer to use a piece of sand paper on a flat surface.
CAUTION: The tendency for the front of the two housings to pull apart is apparently so great that a few folks have reported that they were fearful of removing so much material that the venturi might no longer fit between the two housings. Our advice in these cases is to stop removing metal from the housings and fill in the remaining gap (usually small at this point) with a small amount of sealer on both sides of the gasket in the area of the inlet passageway.