Correct engine RPM range

[pbyrne]

Had the boat out on a nice day, but unfortunately had to motor as there was no wind. 

Engine ran fine, sounded fine, but it wouldn't rev past 1600 RPM, and the throttle was on the stop.  Everything seemed tight and correct in the throttle, and changing RPMs was smooth.

1600 seems too low to me, but the boat was doing 6.1 - 6.5Kts...

Something seems off to me, but I'm not sure what!

[Craig Illman]

What does the tachometer show in the slip in neutral?

Craig

[pbyrne]

What does the tachometer show in the slip in neutral?

Craig

At idle about 800.  Haven't tried to rev it at the slip.

[waughoo]

I would do as Craig suggests and get a no load RPM and see what that is.  Do the values of no load and load match?  The engine has a governor to keep it at a max RPM so you won't hurt it by pegging the throttle with no load.

You might also want to get a digital laser RPM meter to check the RPM at the crank pulley.  The Tach on our diesels uses the alternator AC pulse to determine the revolutions.  Problems with set up and the alternator itself can cause the tach reading to be inaccurate or fluxuate.

[Ron Hill]

pby : Another thing to check when the RPM is questionably low is your boat speed.

A thought

[pbyrne]

I would do as Craig suggests and get a no load RPM and see what that is.  Do the values of no load and load match?  The engine has a governor to keep it at a max RPM so you won't hurt it by pegging the throttle with no load.

You might also want to get a digital laser RPM meter to check the RPM at the crank pulley.  The Tach on our diesels uses the alternator AC pulse to determine the revolutions.  Problems with set up and the alternator itself can cause the tach reading to be inaccurate or fluxuate.

That's a good idea.  The RPM's sound correct for the engine noise though. It doesn't sound like its rev'ing like crazy.  Good to know though.

[Noah]

Had the boat out on a nice day, but unfortunately had to motor as there was no wind. 

Engine ran fine, sounded fine, but it wouldn't rev past 1600 RPM, and the throttle was on the stop. ...1600 seems too low to me, but the boat was doing 6.1 - 6.5Kts...

Something seems off to me, but I'm not sure what!

If you are going over 6 knots your boat is going over 1600 RPM. Get a laser tach on Amazon and check your true RPMs.

[pbyrne]

Had the boat out on a nice day, but unfortunately had to motor as there was no wind. 

Engine ran fine, sounded fine, but it wouldn't rev past 1600 RPM, and the throttle was on the stop. ...1600 seems too low to me, but the boat was doing 6.1 - 6.5Kts...

Something seems off to me, but I'm not sure what!

If you are going over 6 knots your boat is going over 1600 RPM. Get a laser tach on Amazon and check your true RPMs.

Looking at amazon offerings, do you really need to tape something to the belt for it to work!?

[Noah]

No.You stick some white reflective tape (included) to the main crankshaft pulley and aim the laser at it through the belt.

[pbyrne]

No.You stick some white reflective tape (included) to the main crankshaft pulley and aim the laser at it through the belt.

Ah! That makes more sense.  Do you put the tape inside the pulley groove or on the outside edge, front face next to the belt groove?

[Noah]

Wherever it will stick as close to center, and where the laser will "see it" Clean off engine with some degreaser and/or acetone first.

[pbyrne]

Ok. Laser Tach on order.

Reading this thread, I will say that nothing has been done to the engine https://c34.org/bbs/index.php/topic,11302.msg90447.html#msg90447. 

Oil changed last year, fresh fuel, starts immediately, and hasn't missed a beat the couple of times we had her out. In fact you can hear it here: https://c34.org/bbs/index.php/topic,11687.0.html

For completeness what/how should I be verifying the linkage, cables etc, to make sure I've inspected everything.  The engine sounds fine to me other than it's not rev'ing as high as I think it should; by sound anyways.

At the moment I'm going to:
1) check RPM at idle in slip
2) check RPM with Laser Tach
3) Alternator (which I'm not sure how to do)
3)???
4)???

I'm 2 hours away from the boat, so I have to check everything when I'm there.

[Noah]

It is easier to have a helper when checking tach with laser. One person on laser and one in cockpit at throttle. Run though idle to full throttle (in neutral)—communicating with each other, and charting the results every 100 RPMs or so— comparing actual (laser) to tach readings. Error rate/difference may vary and not be constant throughout the range. You may want to just make a deviation chart and use/memorize /refer to it instead of trying a adjust out the tach error.

[pbyrne]

It is easier to have a helper when checking tach with laser. One person on laser and one in cockpit at throttle. Run though idle to full throttle (in neutral)—communicating with each other, and charting the results every 100 RPMs or so— comparing actual (laser) to tach readings. Error rate/difference may vary and not be constant throughout the range. You may want to just make a deviation chart and use/memorize /refer to it instead of trying a adjust out the tach error.

Got it.

Anything else I can focus on?  If I find the RPM's are too low on the engine then what?

[Noah]

No clue, I am not a mechanic. Others may respond with more helpful info. However, if your boat goes 6+ knots (confirmed by GPS in slack water) with tach reading 1600, I suspect your tach is reading incorrectly.