I use an APC 15x8 size prop on my Saito-125a and the rpm
is about 9000-9200 for fuel with 10-15% nitro.
Different brands of prop often give quite different rpm
for the same size prop. The weather also has some effect (high
temperatures and/or high humidity will reduce the rpm).
So it is hard to say whether 8100rpm
is OK for your Saito-125 with the 16x6 prop.
It sounds a bit low. You should also check the valve clearances
because if the clearances are too large that will also slightly reduce the
rpm.
If you are getting 10,500 rpm with your Saito-72 I suggest using a larger
prop to get the rpm in the 9000-9500 range which in my opinion
is better for a 4-stroke engine.
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