spark tachometer

[doug v]

I bought a electronic tachometer/houmeter from Northern Tool mailorder-it has a sensor lead that is supposed to wrap around the spark plug wire. When it arrived, it said its for 1 or 2 cylinder engines-of course the catalog didn't mention that! If I put it around any given spark plug, I figure it would only show 1/4 of the actual revs since each plug fires once in 4 revs; however, if I put it on the hi tension lead between coil and distrbutor, it should show true engine revs- 1 spark per rev? Is my logic right or should I just return the dang thing. Thanks! DV

[cjpenny89]

Sounds like it would work like that to me. It would then think that the engine is a one lunger? Let us know how it works!

[Jim Becker]

It will either:

1) work correctly on a plug wire and show 4x on the coil wire.
or
2) show 1/2 speed on a plug wire and 2x on the coil wire.

It depends on whether the tach is intended for an engine that throws away a spark at the end of each exhaust stroke.

[George Willer]

Doug,

The engine fires TWICE on each revolution. Each cylinder fires EVERY OTHER revolution. This will help you figure the RPM correctly.

As Jim said in different words, many single cylinder engines have a spark on every revolution simply because it's more difficult not to. In that case, only the one at the end of the compression stroke is useful.

Chances are that the tack will work but will read double actual speed on the coil wire.

[WKPoor]

You must have purchased the Tiny Tach. They are common on most newer equipment. My mower has one. Also for yrs now electronic ignitions fire on every stroke including automotive wether its ditributless type or not. Thats because the reference pulse is taken from the crank. Its most likely reading half rpm on the plug. Its would be easy to check however since high idle is approx. 1600 on a Cub and I've found that to be true within 100rpm most of the time.

[400lbsonacubseatspring]

It will either:

1) work correctly on a plug wire and show 4x on the coil wire.
or
2) show 1/2 speed on a plug wire and 2x on the coil wire.

It depends on whether the tach is intended for an engine that throws away a spark at the end of each exhaust stroke.

So, in the event of case #2, if you wrapped the wire around say, both #1 and #2 plug wires, where they come out of the distributor, it may in fact work correctly, if the induction pickup wire is long enough.

Just an idea......

[Jim Becker]

It will either:

1) work correctly on a plug wire and show 4x on the coil wire.
or
2) show 1/2 speed on a plug wire and 2x on the coil wire.

It depends on whether the tach is intended for an engine that throws away a spark at the end of each exhaust stroke.

So, in the event of case #2, if you wrapped the wire around say, both #1 and #2 plug wires, where they come out of the distributor, it may in fact work correctly, if the induction pickup wire is long enough.

Just an idea......

That would be worth a try. I would try using 1, 4 or 2, 3. That way the pulses to the tach are evenly spaced. Also, if wrapping 2 plug wires together causes an inductive cross fire, the induced spark won't cause a problem (other than the energy it bleeds off the plug that is supposed to be firing).

[doug v]

thanks for everyone's wisdom, i'll try it this weekend and post results- thanks

[John *.?-!.* cub owner]

I have an older version of that, and it was a total waste of money, readings were very inconsitant. hope your wokrs better.