Let me guess: You're taching the Rossi engines at home using an AC powered light source, but you met your friend at the flying field to test your tach against his engines.

Hold your tach up to a light in your house. It will read 3600. I don't remember all the mathematics involved, but I am pretty sure that the spinning prop and flashing light will combine to give you weird results.

Use only DC light sources, such as a flashlight, or natural sunlight.

If the engines were really turning 25,500 RPM, you wouldn't be here to make the claim, IMHO. The engine would've exploded and killed you while you were taking the readings.

The only other possibility is that the RPMs are out of your tachometer's range.