You can't "govern" an engine with the carburator. The rpm at wide open
throttle is controlled by the pitch of the prop. You can't go to a 21 pitch
since you're already at 4200 with a 17 pitch. You might be able to go to a
19 pitch but the 17 is probably good, particularly if you did not have the
boat fully loaded when you tested the wot rpm. Boat rpm is high compared to
cars. Best cruising is probably around 3400.

"TJF" wrote in message
. ..
All,
I have a 1971, 21' Fiberform that has a 165 Mercruiser (250 cid inline
Chevy 6 cylinder). WOT on the tachometer registers @ 4200 RPM. I believe
that the Rochester 2 barrel carburetor is governed for that RPM. I am
running a 17 pitch aluminum prop which came with the boat, but have
contemplated a more aggressive pitch as well as stainless steel. The
outdrive is geared @ 1.68 to 1. This all equals to 32 mph according to the
fish finder/gps unit @ WOT.
The original owners manual states that "3900-4300 RPM lets the engine
turn at the recommended RPM and gives best performance". It also states
that I could go up to a 21 pitch prop and still be in this window. The
boat currently has very good hole shot. I am more about getting to the
fishing hole than pulling a skier.
My question is, what is a good RPM for cruising without thrashing on
the motor and leading to premature engine failure? The recommended RPM
seems a little high compared to what my truck's RPM is cruising down the
interstate (2800 RPM). Any suggestions?

Tim

d

If anything I'd suggest that your truck needs an overdrive to get its
rpm down on the highway unless its towing or a 4 cyl.