How to measure RPMs?

[SeaHarlequin]

Hi - I'm trying to adjust my engine to the appropriate idle speed.

I have a 1974 Ranger 29 with a late model Atomic 4 engine. I bought a tachometer today and added reflective tape to the shaft. I took some baseline measurements and was surprised to find the tachometer registered ~100 rpms.

The Moyer manual says the idle stop screw should be set to approximately 800 rpm at idle. I turned the screw twice and got the rpms up to about 140 which is still a far cry from the expected 800. I can keep going in turning the idle stop screw but something seems off to me so I wanted to calibrate before proceeding.

While the tachometer may not be accurate, I've eyeballed the revolutions for about 15 seconds and extrapolated to ~100 rpm so I don't think it's entirely off.

When the Moyer manual talks about RPMs, are they talking about measuring elsewhere (maybe the flywheel? Is there a 1:1 relationship?) or is it the shaft revolutions that's to be measured?

In case it makes a difference, I have a folding prop -- I don't know if the OP did something to the RPM speed (assuming I'm not measuring the wrong thing) to accommodate that.

Thanks!

[sunnnnseeeker]

tachometer

something wrong with Tach. An engine will not run at 100 rpm.

[edwardc]

Hi - I'm trying to adjust my engine to the appropriate idle speed.

I have a 1974 Ranger 29 with a late model Atomic 4 engine. I bought a tachometer today and added reflective tape to the shaft. I took some baseline measurements and was surprised to find the tachometer registered ~100 rpms. ...

Unless you have a V-Drive (which includes reduction gearing), a direct drive A4 is 1:1 in forward. So the shaft RPM should be the engine RPM (unless the clutch disks in the reversing gear assembly are slipping badly). You can elininate this possibility by measuring RPM on the accessory drive pulley. It is geared 1:1 with the crankshaft, and is ahead of the clutch pack in the reversing gear assembly.

...
While the tachometer may not be accurate, I've eyeballed the revolutions for about 15 seconds and extrapolated to ~100 rpm so I don't think it's entirely off. ...

My experience with a cheap optical tach from Harbor Freight was completely unsatisfactory. It was NEVER accurate. YMMV Get an electronic tach that attaches to the coil "-" wire.

When the Moyer manual talks about RPMs, are they talking about measuring elsewhere (maybe the flywheel? Is there a 1:1 relationship?) or is it the shaft revolutions that's to be measured? ...

Yes, engine (flywheel) RPMs is what is being described. But as mentioned above, in a direct drive A4, the shaft RPMs should be the same (in FWD only!)

In case it makes a difference, I have a folding prop...

Doesn't make a difference.

[SeaHarlequin]

Unless you have a V-Drive (which includes reduction gearing), a direct drive A4 is 1:1 in forward. So the shaft RPM should be the engine RPM (unless the clutch disks in the reversing gear assembly are slipping badly). You can elininate this possibility by measuring RPM on the accessory drive pulley. It is geared 1:1 with the crankshaft, and is ahead of the clutch pack in the reversing gear assembly.
...
Yes, engine (flywheel) RPMs is what is being described. But as mentioned above, in a direct drive A4, the shaft RPMs should be the same (in FWD only!)

OK, thanks, that helped me understand the system better. I did not know about the accessory drive pulley option, thanks for that!

My experience with a cheap optical tach from Harbor Freight was completely unsatisfactory. It was NEVER accurate. YMMV Get an electronic tach that attaches to the coil "-" wire.

That's the exact unit I bought... I guess I'll be taking it back today. Thanks for the suggestion on the electronic tach.

[Al Schober]

By any chance, are you measuring shaft RPM with the reversing gear in neutral? Just the oil drag in the clutch pack could be giving you ~100 RPM with the engine turning many times faster.
I use an old dwell/tachometer that clips to coil (-) for RPM - works well. I also have one of the HF reflective things that I've used for other applications. I've had no problems with it. Both devices have their pitfalls and learning curves.

[SeaHarlequin]

Yes, that's what I'm doing - both the gas lever and drive lever are in neutral. I thought that was what we were measuring when measuring idle speed. It sounds like I should have the lever in forward gear and the gas lever in neutral?

[edwardc]

Yes, that's what I'm doing - both the gas lever and drive lever are in neutral. I thought that was what we were measuring when measuring idle speed.

Normally, it is. But you then have to measure on the flywheel, the accessory drive, or use an electronic tach.

It sounds like I should have the lever in forward gear and the gas lever in neutral?

Yes, but only if you are measuring on the shaft. Note that this will be a "loaded" idle, which may be slower than an unloaded idle, which is usually what people are discussing.

[SeaHarlequin]

Thank you very much; I'm now calibrated! I appreciate the patience in explaining these things.

[ndutton]

Good news.

Selfishly I was glad to hear of your success with the Harbor Freight optical tachometer tool that others gave mixed reviews. It's the same one I've had that has been a proven performer for many years.

[edwardc]

Good news.

Selfishly I was glad to hear of your success with the Harbor Freight optical tachometer tool that others gave mixed reviews. It's the same one I've had that has been a proven performer for many years.

Guess you won the quality control lottery!