Digital tachometer?

[Geo-TH,In]

I have a digital tack, the kind you wrap a wire
around the spark plug wire.
My Jubilee has a mechanical tack. The digital tachometer is
reading half the Jubilee tack. 900 RPMS vs 450
RPMS. I don't think the Jubilee can idle at 450.

My Farmall C has a magneto. The RPMs on the
digital tachometer are about 2x more than they
should be. No way can the farmall turn 2800 PRMs.

In the past my digital tach was spot on when I used it on my Kaw mule.

What digital tach do you recommend to use on a
Magneto?

 

[MarkB_MI]

Most small engines fire their plugs on every revolution, 'wasting' one spark. Your tach is designed for this sort of ignition system.

It sounds like the Farmall fires twice per revolution.

 

[Loosehandle]

I tried to use one on a single cylinder Wisconsin engine with magneto, didn't work.

 

[John T]

George Im not sure about YOUR magneto, Im sure the IHC guys like Bob M or Janicholson etc can tell you about mags IHC and others used, but FWIW if it happened to use a Wico AP they are a gear reduction drive and only fire at each pistons COMPRESSION STROKE TDC unlike a Wico C or XH series (and many other mag brands) which fire at each pistons TDC BOTH power and exhaust....If its showing twice the RPM my guess is the mag fires at BOTH power and exhaust TDC's BUT NO WARRANTY ask the IHC mag men not meeeeeeeee lol

Sorry I have no advice for any digital tachs

John T

 

[RCP]

Regarding the Farmall, you are on a spark plug wire and not the coil wire? Could your lead be close enough to two sparkplug wires that it is picking up both of them?

 

[Geo-TH,In]

Yes, one spark plug wire only on both tractors.
If I used the coil wire I would have 4x the RPMs,

 

[Geo-TH,In]

2 cycles fire once every revolution.

Can't see how the spark can get to the plug when the both coil and mag use a distributor cab and rotor.

Please recommend a good tach that uses a wire that wraps around the spark plug wire.

The old tach I used decades ago you would connect one wire to the + of the coil. - to the ground.

 

[MarkB_MI]

>2 cycles fire once every revolution.

So does any 4 stroke engine where the ignition timing is determined by crankshaft speed rather than camshaft speed.

 

[wp6529]

As noted, those cheap inductive pickup tachs are intended for single cylinder 4 cycle gas engines. I don't believe any have provisions to select a number of cylinders or other options.

Get one of the tachs that uses a magnetic pickup and mount the magnet on a suitable crankshaft pulley or some such.

 

[Janicholson]

Here is a solution not dependent on the spark. they work slick and are reasonable cost. the normal Mag is driven at crankshaft speed in the magnetic component, then geared down 2:1 in the points compartment with a timed gear set. so it should have one pulse per compression stroke. Jim
no contact reasonable tach

 

[Hemmjo]

A lot of 4 cycle engines fire the plugs every revolution. It is called wasted spark ignition.

Most digital tachs I have dealt with have a setting that allows you to tell it how many sparks per revolution or revolutions per spark

In regard to idle speed, my Ford 860 will idle all day at well under 500 rpm.

 

[UncleTom]

The one i have there is a choice of how many cylinders your engine is in the settings.

 

[T.R.K.]

I am thinking your tach is set up for small engines with magneto spark. Most small engine magnetos make a spark everytime the flywheel magnet goes past the module. (The exception is some with fuel injection. The computer is controlling their spark) The extra spark comes between the exhaust and intake strokes, so it doesn't really do anything (unless it is really retarded. Then it will backfire.) That is why it works on your Kawasaki Mule.

The jubilee distribuitor is only turning 1/2 as fast. It turns at camshaft speed because you only really need a spark once every two full revolutions of the crank, when there is fuel in the cylinder. That is why your tach is only reading 1/2 the rpm it should.

On the Super C, I bet your tach is seeing a pulse for the opposite cylinder in the firing order as well as the cylinder you are testing. I am not real familiar with that style magneto, however.

One of my tachs can be set for 2 cycle or four cycle as well as the number of cylinders, up to 8. Sometimes it still multiplies or divides the speed the engine is actually running.

Also, on my tach at work, I found that in some cases, the tach reads totally wrong unless you turn it completely around, or use the pickup lead that came with it. Since it is reading off the electrical field of the spark plug wire, I think the energy is just too great for it to give an accurate reading. Most 2 strokes just read off the pickup corner of the tach.

 

[Geo-TH,In]

After thinking about my Jubilee, when the mechanical tachometer is showing 900 rpms there are only 450 sparks going to each spark plug every minute so I need to double the digital tach's reading.

I can't explain why the Farmall Magneto is showing about 2x the RPM's. Perhaps I'll measure the number of sparks coming from the mag wire going to the center of the rotor cap. Divide that number by 2 to get the RPMS.

In the case of my Kaw mule. The spark is generated the same way the spark is produced on an air cooled engine. There will be a spark each revolution when the magnets pass near the ignition module. Kaw is a 2 cylinder and it has 2 ignition modules.

 

[teddy52food]

IHC mags fire only at TDC compression . Nothing between ex & intake.

 

[teddy52food]

Depends on how the wires lay next to each other. A hot mag may induce enough in another wire to trigger your tach. Try putting the pickup on different wires & in different places & see what it does.

 

[Geo-TH,In]

I think you are right, the mag is producing a lot of magnetic radiation. That's why I plan to wrap wire around the wire coming out of the mag and measure all spark pulses, then divide by 2 for RPMs

 

[Geo-TH,In]

I used the wire coming from the mag. Divided by 4 and got 840. The max RPMS in the tractor's ID .Tag .