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Farmall Cub • View topic - Engine Speed

Engine Speed

Thu Mar 07, 2013 5:44 pm

Can a mechanical tachometer be used on the PTO shaft to determine engine speed? Can an electronic tach be used with a 6 volt, mag fired Cub? I am not interested in a permanent installation, but just to help learn the proper speed to run the tractor. Thanks

Re: Engine Speed

Thu Mar 07, 2013 5:52 pm

thiggy wrote:Can a mechanical tachometer be used on the PTO shaft to determine engine speed?

Yes, since the PTO runs at engine speed you can determine the engine RPM this way.

Re: Engine Speed

Thu Mar 07, 2013 6:02 pm

thiggy wrote:Can an electronic tach be used with a 6 volt, mag fired Cub


I don't believe you can use an electric tach on a magneto, because the mag is independent of the battery, therefore there is no power source.

Re: Engine Speed

Thu Mar 07, 2013 6:45 pm

You can use an electronic tach, as long as you have a battery close by. My timing light has a tach built in and works great on both battery and Mag ignitions.

Re: Engine Speed

Thu Mar 07, 2013 7:12 pm

Im prety sure that if you have one of these it will work. I used to use this to set the timing on my 70 GTO. Back befor the HEI distributors took the place of distributors with points.
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The green clip goes to the points tower on the mag and the black one would go to the positive on the six volt positive ground cub.


BUT I COULD BE WRONG
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Re: Engine Speed

Thu Mar 07, 2013 7:48 pm

Looks like bythepond88 is right. Just went out and tried it on my 48 with mag. I have mine set up with an external coil so mine is going to be wired differently than one with the coil in the mag.

Re: Engine Speed

Thu Mar 07, 2013 10:05 pm

I'm not sure how the dwell tach would work, but I'll take a look at it. It's been a long time since I pulled it out of it's box, but my recollection is that there are three leads. Two go to the battery and the small one clips on the distributor side terminal. Will this work on the mag, or will it kill the engine?? We'll see.

Re: Engine Speed

Fri Mar 08, 2013 7:39 am

Thiggy, I think the type with 3 leads will work will work on the mag, I'm pretty sure I've done that. The type with 2 leads will not. Give it a shot, there's nothing to lose and you might find you have a new use for that old obsolete tool.

Re: Engine Speed

Fri Mar 08, 2013 10:14 am

An electronic tach that gets its signal from induction on the #1 spark plug wire will work regardless of ignition type. All you have to do is give it 12V power from an external battery if the Cub is 6V.

Re: Engine Speed

Fri Mar 08, 2013 10:22 am

thiggy wrote:my recollection is that there are three leads. Two go to the battery and the small one clips on the distributor side terminal. Will this work on the mag, or will it kill the engine??
Magneto voltage on the side terminal is considerable higher, 10 to 20K higher, than the nominal 6 or 12 volts used in a standard points ignition system.

Re: Engine Speed

Fri Mar 08, 2013 1:22 pm

I agree with Gary Dotson. My initial comment was for a tach with only two leads. The three lead tach, connected to a battery, should work with a tach.

Re: Engine Speed

Sat Mar 09, 2013 10:28 am

thiggy wrote:...... Can an electronic tach be used with a 6 volt, mag fired Cub? I am not interested in a permanent installation, but just to help learn the proper speed to run the tractor. Thanks




Here are a couple of forum links that may help you. The first is a back door way to measure engine speed by determining ground speed and the second link is for an inexpensive electronic tach/hourmeter that is self powered measuring rpm by an inductive wire wrapped around the first plug wire.

I use them and they work great, especially keeping track of engine hours.

viewtopic.php?f=1&t=73889

viewtopic.php?f=1&t=75561

Re: Engine Speed

Sat Mar 09, 2013 5:58 pm

Thanks for the suggestion, driver. However, as you noted, not everyone has a smart phone. (My cell still has a dial!) As you suggest, perhaps one of my grandchildren could assist with their phone. :D