One of the most common problems that people complain about is the spit-cough-pop from the carburetor as a Harley Davidson motorcycle goes from idle to ¼ throttle. This can occur on any motorcycle, any carburetor, but seems to happen most often with the stock CV Keihin carburetors used on the newer (90 up) Big Twins and Sportster models. This condition is not only annoying, but can cause the Harley Davidson motorcycle to stall dangerously at stoplights, and can damage intake seals parts. It is even possible to ignite oil or gas residue in the air cleaner causing a fire! If your Harley Davidson motorcycle has these symptoms, there are several causes and several easy fixes.

Common causes of motorcycle carburetor spit are as follows:

Problem:......................Solution:
Fouled plugs................Change motorcycle plugs, check gap.
Intake leak..................Spray manifold/head seals with carburetor cleaner, listen for RPM change.
Timing incorrect...........Check timing with light, lube/replace advance weights if used.
Idle mixture incorrect....Adjust low speed mixture screw/re-jet low speed circuit.
Accelerator pump bad....Remove air cleaner, check squirt, check diaphragm.
Bad coil or wires...........Check coil & wires with an ohmmeter, replace if bad.

Any of these can cause the problem, but luckily all are easy to fix.

We have found that the EPA lean setting of the carburetor is too lean for most applications. Through trial and error, we have settled on the #45/46 pilot jet as the best starting point for most engines.

The #42 used in most Harley Davidson motorcycle models is to lean to adjust out with out turning the screw so far out that it is no longer affecting flow.



Source: Directparts.com