Critical considerations for proper operation

  • The inlet needle and seat must be in good condition or else the carburetor will flood due to leakage or result in erratic operation due to uneven gas flow. If there is any evidence of damage, these parts will need replacement. The 4 edges of the needle should not be worn (the sharpness would change about 2/3 of the way from the pointed end). If the edges are noticeable rounded, replace the needle. There may be varnish deposits on the needle, rubber seat, and the metal casing in which it is installed. These must be totally removed using carburetor cleaner and soft (wooden or plastic) tools.

  • The float height adjustment should be fairly accurate. With the float and inlet needle reinstalled (and the seat replaced if it was removed), invert the carburetor - the float should sit just about horizontal. For more precision, a .210" (#4) drill bit should just fit between the body and the non-hinge end of the float.

  • The machined passages must be free and clear and not damaged - never use wires to clean them. Use compressed air, carburetor cleaner, wooden sticks, etc. However, do make sure that they are fully open. There are no blind passages in these carburetors so a strong light should permit you to see that they are unblocked (the following are typical - your model may differ slightly):

    • Air bleed, inlet side angled down toward main jet.
    • Passage to primer chamber, inlet side.
    • Slot towards center at edge of welch plug (may not be present).
    • Pair of main fuel passages in central cylinder in main body.
    • Main metering hole in bowl bolt.

  • The hole in the bowl bolt is the main metering orifice and it is critical to the proper operation of the carburetor. This area also tends to collect a lot of crud. It will yield to repeated use of carburetor cleaner, cotton swabs (Q-tips), and wooden sticks. Continue cleaning until you are down to shiny brass. Just don't become impatient and use any wires or sharp tools to speed the process!

  • Any primer should be air-tight for it to function properly and for the engine to run properly. Any leaks will result the primer being partially or totally ineffective. In addition, the engine will run rich and contaminants may enter the carburetor. Check for damaged rubber parts or hoses that have fallen off.

    If the primer bulb is on the carburetor, there is a 'welch plug' (a metal disk pressed into a mating cavity) sealing the primer chamber. On the side toward the center, there is a tiny rectangular hole that must be open - it often gets clogged and may not even be readily apparent. Do not attempt to remove the welch plug unless you seriously suspect something is inside. If pressing the primer bulb results in a blast of air out of the hole, it's probably fine. Where the primer is separate from the carburetor, there is usually no welch plug.

  • The float must be air (and gas) tight. Shake it - if there is any gas inside, the float will need replacement. (It's possible there may be metal particles or other debris sealed inside at the time of manufacture - this will cause no harm.) Put the float under water - there should be absolutely no evidence of bubbles and leakage. Pinholes sometimes develop in the thin brass and while these can be soldered, this practice is not recommended.

  • The large O-ring must seal properly. If it leaks, the engine will run rich and contaminants may enter the carburetor bowl. Replacement is usually recommended whenever the carburetor is disassembled. However, if it is in perfect condition, you can try to reuse making sure that the mating surfaces are clean and smooth. Use some engine oil on the O-ring to assure a tight seal.
    Note: When tightening the float bowel, take care that the O-ring doesn't get pinched or kinked resulting in an inadequate seal or no seal. A narrow stick may be useful to help it along. If the carburetor hasn't been removed from the engine, a mirror will be needed to inspect it.