I found this on the web. Mikuni's site to be exact. I know we have Keihin carbs, but same difference. If the following is indeed what's happening, do you think this could wear out the needle or needle jet faster?
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1) Slide rattle:
Most HSR carburetors make a soft "ticking" noise at idle. This ticking is the result of the normal pulsing of the air in the intake system. This pulsing is caused by piston and cam action. The ticking is not a sign of wear or incorrect tolerances. It is normal.
During the intake cycle, when an intake valve is open and the piston is moving down, a partial vacuum is formed in the cylinder, intake manifold and behind the carburetor's throttle valve. The normal pressure on the out-side of the carburetor's slide then pushes the throttle valve back in the carburetor body. This movement can cause a "click".
Near the end of the intake cycle and below a critical rpm, which varies with cam design, a positive pressure forms in the cylinder, intake manifold and behind the throttle valve. This positive pressure forces the throttle valve forward in the carburetor body. This movement can cause another click.
So, the throttle valve being "pushed and pulled" by the variations in manifold pressure at low (idle) rpm results in some ticking. As soon as the throttle is opened and the throttle valve is raised, the ticking stops.
Cams that have been designed for high rpm performance usually close the intake valves much later (in crankshaft degrees) than the stock cams. The later the intake valves close, the greater the pressure on the back of the throttle valve and the louder the ticking may become.
Exhaust systems with no baffles, like long open straight pipes, also tend to increase the reverse pressure on the throttle valve.
By the way, it is the late closing of intake valves that cause "reversion" in high performance engines. Reversion is the word most commonly used to describe the fog of air/fuel ejected from the mouth of the carburetor when the throttle is opened at low rpm. Reversion disappears when the engine rpm becomes high enough that the intake valves close before the piston can push air/fuel mixture back through them.
Factors that affect slide rattle:
1) Cam design --- the later the intake valves close (in crankshaft degrees) the higher the rpm before reversion and therefore slide rattle stops.
2) Exhaust systems without baffles, particularly long, straight open pipes, increase reversion pressure.
3) Windshields, fairings and unrestricted air cleaners (like the Screamin' Eagle) echo any noise coming from the intake system.
4) Throttle valve to carburetor body clearance. There is some minor variation in the amount of play between the throttle valve and carb body. A "loose" throttle valve and a "big" cam can combine to make the normal ticking sound louder.
