Universal M25 engine RPMs

Hey all,

I just finished upgrading the heat exchanger on the M25 in my '86 E35 mk3 to the 3" and moving up to a 3 blade prop. Now that my overheating issues are taken care of I'm moving on to the next question.

If I look in the manual my M25 should max out at about 3000rpm. But according to the engine panel tach, it maxes out at 2300 rpm. Is my tach out of whack or did some PO govern the engine down to keep it from overheating (with the old small heat exchanger it was always running about 185f even with the 2 blade prop)? Anyone out there with this same setup and what rpm's are you getting? Looking for ideas before I start tearing into things (again).

And yes Sean this is your old boat. So any history would be appreciated.

Thanks

Kevin Wright
s/v Hydro Therapy

The first thing to do is to check the accuracy of your tachometer. You can borrow a mechanic's shop tachometer or buy a digital photo tachometer for approximately $15 online. The multiple engine issues that could cause an RPM problem would need to be examined next...if necessary.
Mike Jacker

Well I found the throttle stop on the engine was the issue, probably set that way to avoid overheating. A few quick turns of a wrench and gained 500 rpm and cruising speed went from 6 to 6.5kts with top speed around 7kts. That seems about right to me for an E35. Old gal just go a bit more spring in her step.

Kevin Wright
E35 Hydro Therapy

Just motoring along...............

Kevin A Wright said:

Well I found the throttle stop on the engine was the issue, probably set that way to avoid overheating. A few quick turns of a wrench and gained 500 rpm and cruising speed went from 6 to 6.5kts with top speed around 7kts. That seems about right to me for an E35. Old gal just go a bit more spring in her step.

Kevin Wright
E35 Hydro Therapy

Click to expand...

It might still be wise (or at least good therapy) to check the boat's tach against an optical tach.

Our M25XP runs at 2500 for steady cruising, and WOT is about 2900 on our boat. With some slime on the bottom now we currently power along at 6.7 to 6.8. With a completely clean bottom we motor at 7.0. These speeds at 2500 rpm.
We are just back from a cruise down to the Astoria area and home again. Mostly motoring (sigh).

BTW, lots of outgoing tide against us on the return trip, and we were only doing SOG of 4.5 and 5.0 for up to 4 hours at a time. Ick.

Loren

Hey Loren, welcome back. I take off for two weeks at the end of the month, heading generally north until vacation is half over then generally south again till home. Unless I decide to retire early.

Yes when I find a buddy with an optical tach I will check it, but everything makes sense now. WOT is now about 2800 where it was 2300 before. Cruising was at 2000 now at 2500 and making 6.5kt with a bottom that hasn't been cleaned in a year. And the RPM's 'sound' right when listening to the engine.

Kevin Wright
E35 Hydro Therapy