Motorcycle Repair: compression psi, vacuum leaks, piston ring

Question
hello-I hope you can help me Im repairing the cylinder head-cylinder honey-new piston and ring-carburetor rebuil-ok-the guestion is the compersion test on cool is 171psi on front and rear but when hot it is 160psi the bike starts very good, but i would like to know why it is diferent my other problem is the rpm gos up and down. 1000 to 1100 and gos down 1000 to 900. i hope you can help me and thanks for your time

-gamalier

Answer
Hi Gamalier,

There are many reasons for compression to
vary hot and cold.
One is that some engines have a slightly
tapered cylinder that expands straight when
hot so when cold there may be less clearance
on the piston and rings resulting in
slightly higher compression when cold.

Also, the oil is thicker when cold
so the cylinder to piston ring
seal may be better when cold.

If the compression readings are within 10 per cent
and the cylinders are also reading within 10 per
cent of each other then the compression is good.
Your readings appear to be quite good.

The idle going up and down could be a small vacuum
or air leak around the carbs or intake rubbers.
Vacuum leaks are the most common cause of idle
problems other than the carbs needing syncronization.

The carbs have to be adjusted so they are
all reading similar vacuum readings on vacuum
carb sticks or gauges.

If the carbs are not synchronized the idle will
not be steady.

Next time be sure to include the year, make and model
of your bike so I know what engine it has.

Good luck!
Wayne S.

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