Carburetor Troubleshooting

Even with the simple fuel system of early gas engines, carburetor troubleshooting is a good skills to develop if you want keep them running well.

| May/June 1968

Gas Engine

This is an unrestored Emerson2 1/2 hp, No. 8558. The original igniter hadbeen replaced with a spark plug system and is missing. It is a wellmade engine with sturdy castings.


The fuel system of the early gas engine is generally simple but can cause its share of problems. The carburetor is likely to be one such problem area.

The engine piston on its intake stroke creates a vacuum in the cylinder. Outside air, under atmospheric pressure, will try to push into the cylinder. If we provide a small passageway through which this air may enter and add a fuel jet, we have a simple carburetor (I'll abbreviate it to carb)—a device to vaporize liquid fuel and mix it with air in correct ratio for combustion. Better economy, variable speeds, acceleration, and power demands may require added parts, but the basic carburetor remains essentially the same. With that understanding, let's do a little carburetor troubleshooting.

Edgar Johnson of Flemington, N.J. has a 1 1/2 HP Fairbanks-Morse Z engine with Bosch Hi-tension oscillator ignition. The compressor is good and he has checked the spark plug, timing, and carb. The engine runs OK under load but misses or stops with no-load. His question: Could the mag be at fault? Or if not, what is wrong? 

I believe the trouble is in the fuel system, Ed. To check the mag, remove the wire from the spark plug and place the end about 3/8 inch from the engine. Trip the mag lever. If the spark will jump to the engine the mag should be OK. The fact that it fires the engine under load when voltage demands are higher, indicates it is OK.

To the above basic carb, FM has added the following: 1) A restriction (venturi) in the carb throat for better mixing and fuel suction; 2) A governer-operated throttle valve to control fuel-air flow into the engine; 3) An automatic auxilliary air valve at the air entrance to the carb, to improve fuel air ratio throughout the load range of the engine; 4) A gasoline reservoir at the carb for starting and 5) A fuel tank under the engine for running (with kerosene if desired); 6) An extra jet for the tank fuel and a hand-operated needle valve for each jet to adjust the fuel feed; 7) A ball-check valve at the bottom of the fuel supply tube in the fuel tank to hold the fuel level in the tube at or near the level of the carb jet.

Since items 1, 2, 3, cause severe impedance to air flow, air will try to enter the engine where it is not supposed to. Cheek for leaking gaskets or loose connections between the carb, throttle valve assembly, and engine. Check for a weak exhaust valve spring or too strong a spring on the intake valve. Be sure the ball-check valve (7) is there and that it is free. Clean and check the gauze filter screen on the same valve. Be sure the pressed in jet and needle valve assemblies (6) haven't worked loose. (A good present day fix for this is epoxy glue.)