Low engine RPM

Tom J · Feb 7, 2014

I would double-check the actual RPM's with a hand held tachometer that reads the rev's from a mark on the crankshaft pulley. Compare that to your tach on the instrument panel. My tachometer goes hoopey (technical term) on occasion when the points on the calibration screw get oxidized.

Stu Jackson · Feb 7, 2014

The tach might stick at the start because you're plugged in all the time, I'm guessing and your batteries are full, so the alternator's regulator is not requiring any power output, which is what makes the tach work.

We have lots of M25 engine info on our C34 website, here:

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

A 2 bladed prop shouldn't affect your engine rpms. What is your WOT out of gear?

weinie · Feb 7, 2014

Check to make sure the throttle linkage is moving freely at the engine itself. Make sure the cable isn't binding.

You can confirm the rpms with a photo sensor tach. You can get one from harbor freight for about $30. The rpms should be checked in neutral to eliminate issues with the prop.

Are you having any other symptoms like stalling or black smoke?

Since the boat is new to you, I would change the primary and secondary fuel filters asap. Note the electric fuel pump has a mesh screen/filter to clean or replace too. Change the oil while you are at it.

Remove the fuel pickup tube from your tank. There may be a mesh screen on it which is probably blocked up. This should be removed.

Check to make sure your air intake is not blocked and the air filter (if it has one) is clean and intact.

If you are still having issues, you will need to inspect your exhaust elbow for blockage.

I would also check the valve clearance and fuel timing at this point (though probably not the culprits, it is necessary maintenance).

If the exhaust side is clear, you may have clogged injectors. You can get new ones for about $50 each. It's cheaper than having them professionally cleaned.

If you are STILL having problems, you might have a compression problem. (But if this is the case, you will have a hard time starting the engine to begin with). You can buy a diesel compression test kit from harbor freight. An adapter can be purchased online.

If your compression is good and everything else is fine, adjust the high speed throttle limit screw. If you can get your rpms up to spec with the engine running smoothly, then the problem might be a broken governor spring or the injection pump.

Old Skool Neil · Feb 8, 2014

atpjim said:
It has a 2 blade folding prop.
It runs good but I can only get about 2000 RPM's out of it. The speed seems okay, but the manual says normal cruise is in the 2600 range.

If there's little or no improvement after following the previous suggestions you might want to revisit the prop. It's interesting that no mention has yet been made that it isn't the number of blades but rather the diameter and pitch that determines the load applied to the engine. That load affects max. RPM directly. So for your 2 bladed prop, what are the diameter and pitch?

atpjim said:
The tach sticks when first started but seems to be work okay if I goose the throttle a little to unstick it.

This is typical of many alternator driven tachometers. You have to get the alternator RPM high enough to get it producing for the tach to read. I suspect as well your tach does not zero itself upon shutdown. This means when you shut the engine off the tach needle remains at the last reading therefore appearing to be stuck.

Picflight · Feb 8, 2014

Great advice here, please post your solution when you find it, I am curious to know.

Congratulations on the new boat. Pictures?