Why does the tachometer pointer move up and down after washing the throttle?

The fluctuation of tachometer pointer should be the phenomenon of unstable idle speed caused by cleaning throttle.

1 air inlet pipe and various valves are leaking. Air enters either from the intake pipe, gasoline vapor enters from the carbon canister valve, and exhaust gas enters the intake pipe, which will cause the mixture to be too rich and too thin, which will make the engine burn abnormally. Resulting in unstable idle speed

There is too much dirt on the throttle and air intake. If the air intake suddenly increases in a short time after cleaning, the idle speed will be unstable.