low engine rpms

I have just recently replaced the motor on my mom's push mower. The original engine was worn out so bad that it to operate at 4000 rpms to function. the new (used) motor only operates at about 900-1100 rpms and it cuts through thick grass. What's wrong with the normal 3600 rpms. Will it last longer at this speed?

Engine speeds were reduced to meet government standards for blade tip speed, but 1100 rpms sounds low. If it's doing what you want it to do, ok, if not find the lever that pulls the governor spring and "bend to adjust".

Yes the government has lowered blade tip speed on new machines, but a replacement engine should reach the rated speed for the machine. What make engine is it? I know B&S had to rerate their quantum series engines to 4.5 HP @ 3200 RPM, as they had come to the end of the line as to how much power they could get out of that old style 2&3/4 HP block! At 3600 RPMs they were blowing up the Quantum 5 HP's left and right. They had to detune to keep the engines together! I agree that the 1100 RPM figure is too slow though. How are you measuring engine speed? Is the replacement engine a direct replacement - short or long block, or is it a generic engine replacement (like one out of the Northern catalog? Andrew

This is one those Classic 3.75 hp B&S motors.

I can only guess the engine speed and it is darn right low. I can see the flywheel fins clearly. However it still puts out horsepower because cuts through thick grass with no sweat. It doesn't burn much fuel, but will the engine last longer at this speed.