7. Why does it seem that adjustable main jets are more susceptible to becoming partially blocked than the fixed main jets of later model carburetors?

While the total open area around the needle and seat of adjustable main jets is approximately the same as the open area of the orifice in fixed main jets, the cross-section of the annular space between the needle and seat of adjustable main jets is very thin, and therefore very vulnerable to trapping tiny particles of dirt - particles which will in many cases would move right through the orifice of fixed main jets.

In many cases, backing out the needle of an adjustable main jet a couple turns will allow these tiny particulates to move through the main jet and out of harm's way. Once the tiny particulate moves through, the needle valve will need to be readjusted to the proper setting. In cases where the pesky particulate is too large to pass through, the carburetor will have to be removed for more serious cleaning. - Updated: November 4, 2003