M25 Tachometer on Universal won't calibrate high enough?

[patrickkidd]

Hi there!

I have a universal M25 in a 1985 Catalina 30, and I am trying to calibrate the tachometer. When the calibration pot is at the maximum the tach still only reads about 2700rpm at full throttle where I understand the maximum rpm to be about 3000. At idle it reads 500 rpm. So in general it seems like it is set low.

Now, I know that there is a high speed governor on the M25 (I assume it is just a set-screw) and I have not yet tried to check the mechanical readout with a digital tach (have to order one), but it seems like it should at least reach 3000rpm. For what it's worth, I recently had to replace the crimp connector on the alternator for a bad connection. The governor screw has not been altered since the engine was last painted. The tachometer seems original like all the other instruments on the panel.

Does anyone have any thoughts on this problem? Does poor conductivity affect the actual top speed of the tach? The outer wire strands just have that typical dark surface corrosion but the inner strands are fine. The contact on the alternator seemed imperfect but conductible.

Thanks!

 

[Stu Jackson]

Now, I know that there is a high speed governor on the M25 (I assume it is just a set-screw)

Yes. Forget about "governors" and all that stuff, it is a simple idle set screw on the top forward starboard of your engine. They usually have keeper wires on them from the factory.

You may have seen me post these before:

Engines 101 - The BIGGEST & BEST collection of M25 Series Universal Engine Information on the Internet, plus some M35, too

http://www.c34.org/wiki/index.php?title=Diesel_Engine

Critical Upgrades http://c34.org/bbs/index.php/topic,5078.0.html

If you can "calibrate" your tach, then just do simple arithmetic in your head:

2700 / 3000 = % to apply to any other reading. It's not really all that important, since most of us find that our engines have "sweet spots" and one avoid the rough spots. My engine is finer at 1,200, 1,500 and 1,800 (and above), but rough in between.

Since the tach runs on electrical wiring, and since 99% of ALL electrical issues are connections, clean up the wires and the connectors, butt a new clean wire to whatever is suspect and try again.

Good luck, let me know if you have more specifics.

 

[patrickkidd]

Thanks for the tips Stu. I assume that the hi-speed set screw is set correctly. I imagine I should check it with a digital tach... Other than that yeah I guess the best thing to do is just do the adjustment in my head. I think the wiring is good enough

 

[patrickkidd]

I bought a $16 digital tach and it turns out my M25 is topping out at 3300rpm, which reads 2700 on the tach. So I took a few speed intervals and can do the math in my head.

But two questions:

What would cause a tachometer to not calibrate to the correct range? Is that a sign of wear?

I was under the impression that the M25 had a maximum rpm of 3000, or is that the max recommended rpm? Should I set the high end set screw to limit it to 3000?

Thanks!