How to test RPM using a laser tachometer

[Jokani]

Having installed a new to me VP MD2020B, the rev counter appears to be reporting on the low side, ie tickover 400 rpm.

i have a laser tachometer to test, but i am unsure where best to place the reflective tape.

Would on the side or the water pump pulley be best?

I have had a looked at the parts explosion but can't work out whether the pump is geared in any way or if it will be a true indication of RPM

 

[Giblets]

Is the RPM of an engine not the revolutions of the crankshaft? Therefore it would be best to use the lower crankshaft pulley in the pic.

 

[pvb]

Crankshaft pulley, unless the water pump pulley is an identical diameter.

 

[RichardS]

Is the RPM of an engine not the revolutions of the crankshaft? Therefore it would be best to use the lower crankshaft pulley in the pic.

+1

Used the silver tape gizmo on my engines last week as I felt that they were not running at the same speed even when the two tachos were showing identical speeds.

I first stuck a piece of tape on the alternator fan as that was the easiest to access with the gun just to see if there was a difference. I tested at 1000, 1500 and 2000 indicated and could see the problem immediately with a difference in reading of about 250rpm at all speeds.

I then removed the engine cowling and stuck another piece on the crankshaft pulley and re-tested and got a difference of around 125rpm with one tacho reading high and one reading low. I've now adjusted the throttle controls so they are level at the same real engine revs.

Richard

 

[Jokani]

Many thanks, glad I checked.

 

[sarabande]

how much does it cost you to be allowed to keep the boat engine nice and warm in the kitchen ?

 

[Jokani]

how much does it cost you to be allowed to keep the boat engine nice and warm in the kitchen ?

I never asked as I knew I would not be able to afford it, luckily I have a home-office where i could tinker with it prior to fitting.

 

[pagoda]

+1

Used the silver tape gizmo on my engines last week as I felt that they were not running at the same speed even when the two tachos were showing identical speeds.

I first stuck a piece of tape on the alternator fan as that was the easiest to access with the gun just to see if there was a difference. I tested at 1000, 1500 and 2000 indicated and could see the problem immediately with a difference in reading of about 250rpm at all speeds.

I then removed the engine cowling and stuck another piece on the crankshaft pulley and re-tested and got a difference of around 125rpm with one tacho reading high and one reading low. I've now adjusted the throttle controls so they are level at the same real engine revs.

Richard

My purpose was to check revs accurately for a Kiwi prop. Moving from neutral to in gear with lower than ideal RPM will not operate the blades properly. I found my 850 on the engine gauge was actually just over 700RPM. I adjusted the engine idle back to specification and all is well. It depends of gearbox ratios, but slightly over is preferable to not enough!
The reflective marker works reliably on the crankshaft pulley only - the other pulley wheels are very often differing diameters, and run at speeds to suit the alternator etc.

 

[Skylark]

how much does it cost you to be allowed to keep the boat engine nice and warm in the kitchen ?

+1

Respect, Gary. You are my hero :encouragement:

 

[nigelmercier]

It's also quite informative to measure the alternator RPM for a given crank RPM.

 

[Jokani]

It's also quite informative to measure the alternator RPM for a given crank RPM.

That is a good idea, particularly as I am in discussion with the alternator supplier over pulley size (I am attempting to optimise the alternaor output for my average cruising RPM). They have asked for, pulley diameter and belt length, I guess thay would use this to estimate alternator RPM, but I could supply an exact figure at varying crank RPM's.