I wish to use a hand held tachometer to set slow idle and fast idle on my tractors. Except for the JDs, access to the end of the crankshaft is limited on most tractors. This, obviously, leaves the belt pulley and the PTO. Can anyone give me a ball-park figure for the number of engine revolutions for each revolution of the PTO shaft? Also, the belt pulley. Checked the archives and got only general information. Thanks.

The gear ratios would vary between models.

Make it simple.

Get a "photo tach".

What tractor????

Post back and I can give it to you if it is a M, 130, 504, or 656

Belt pulleys are all over the spectrum due to the differences in the diameter of the pulley, and the application specifications. There are ratings in the manuals for each at the pulley.
The RPM at the PTO is again a ball park figure in practice, but specific when rating a tractor's HP at its rated PTO speed wide open throttle. The PTO speed in practice is probably between 600 and 450 depending on the tractor, load, and throttle setting. Nobody cares in practice with these older units in the field it is pull the throttle and make hay, or throttle back, and do it "gently"
all an operator's call.
To your question, three answers.
If you want it exact, use a strobe tach (buy, borrow, or rent one) and be sure by flashing it on the front pulley till it is stopped in its tracks, read the LCD numbers.
You can set both High and Low idle that way.
The second is to use a shaft tach on the PTO to set low Idle and High idle. The ratio is the ratio between the rpm on the Serial number plate, 1600 for instance, and 540 rpm This is the ratio in the drive for the PTO. 540/1600.
So at low idle figure the ratio, and make it work.
However!! at the slow speed the shaft tach might not be very accurate. A digital shaft tach will be better, but it should be in the middle 50% of its range to be considered a critical number.
A tach dwell meter used on the ignition (distributor wire usually) will also give a reasonable number, but not great.
The last answer is to set the idle down as low as it will go and not die/stumble/cough/or run rough with out possible smoothness, then take it up a bit and call it good. High Idle is found when the throttle lever pulls the governor lever all the way to the stop, and the carb is opening all the way when the governor demand it. (unless it is broken, or someone has tweaked it, it will be just fine. If it has stock sized tires, an M will go between 16 and 18 MPH in road gear wide open. JimN

I think you could also get an automotive tach (VDO, Autometer, etc) and use the point trigger signal.

Whether you mount this permanently is another story, but I believe it will give a decent result.

I think "low rpm" models exist, for example with 3000 rpm max. Trying to discern 1000 and 1250 would be more difficult on a 7000 rpm tach.

Just an idea.

TractormanNC: Presently working on an M. But thought there might be a very general ratio for all tractors.
For Bob: Don't own a photo tach!!!

Well, they aren't very expensive nowadays, and typically VERY accurate.

Bert,
You will be better off with an electronic tach/timing light combo. Hard to take a reading on the back of tractor, go to front, adjust, go back for another reading. Figures are different for every tractor,i.e. JD A engine 950RPM/PTO 545RPM, Case800=1800/540. Operators manual gives idle adjustment procedure, low idle depends on to many things to be anul about a specific RPM; condition of valves/engine in general,air temp/humidity,timing variances,type of fuel,etc.

JimN: As usual, thank you for the useful information. After researching photo tachs, I think I will purchase one.
Thanks also, to Bob and mattofvining for alerting me to the photo type.
To all others, Thanks for responding.

Dr. Bert,

For an M

537 on PTO equals 1450 full load rated engine speed.
591 on PTO equals 1595 high idle speed at engine.

ratio is 20:54

HTH.

Dr. Bert - For the M the engine to PTO ratio is 54:20 (= 2.70:1----

For old tractors with battery ignition (not magneto...) an old style tach-dwell meter will read RPM directly from a connection to the distributor primary. Old tach/dwells can be picked up for almost nothing at garage sales, auctions, etc. (I have a nice Mac unit given to me by a guy who no longer used it.)

BobM: Thank you--just the information I wanted!!

TractormanNC: Thank you: With the information you sent I will be able to set my engine speeds close enough "for who it is for".

My manual calls for 989rpm's on the belt pulley at 1595rpm's no load on the engine for the M. Hal

I use an old "dwell/tach" from the 50's. Just clip it on coil and ground and you are right there where you want to be for adjusting.
Maybe you can find one on ebay.

Gordo