# Using old style Tach gauge to check RPM



## tpenfield (Feb 24, 2015)

I have an old 1980's vintage tach/dwell gauge and wanted to use it to check the Maximum RPM on my Troy-Bilt 2410 snowblower. It is the kind of tach that has a clip that surrounds the spark plug wire.

It did not seem to get a signal, when checking the RPM. . . very erratic.

Soooo, I took out my old Toro 521 and checked the tach gauge . . . it works !!! The Toro maxes out at 3800 RPM. Idles at about 1400 RPM.

I am wondering if there is something 'special' about the ignition/spark plug wire on the newer machines that shields the magnetic field on the wire? Is there a different way of checking the RPM?

TIA for any assistance.


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## classiccat (Mar 1, 2014)

tpenfield said:


> I have an old 1980's vintage tach/dwell gauge and wanted to use it to check the Maximum RPM on my Troy-Bilt 2410 snowblower. It is the kind of tach that has a clip that surrounds the spark plug wire.
> 
> It did not seem to get a signal, when checking the RPM. . . very erratic.
> 
> ...


I don't know enough about those old tachs to comment on why it's not working with the old machine...maybe the pulse width is too different on the newer machine? Is the old 521 a points ignition?

Regardless, maybe grab yourself one of these new inductive tachs and mount it permanently on your blower.

Then double-check the RPMs on that old 521...3800 is revving a bit high (_3600 Max; measure when warm_) if your old tach is accurate...1400 is on the low side but if it's idling steady, then leave good-enough alone. Most old tec idles are set around 2k.


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## whimsey (Oct 16, 2014)

I have an old Sear's Craftsman tach/dwell meter from 1972. It has settings for 4/6/8 cylinder engines. I think the snow blower being 1 a cylinder engine causes erroneous readings. Also it looks like the spark plug wire on the snow blower might be better shielded than older automotive spark plug wires causing inductive pick up problems for the tach. 

Whimsey


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## tpenfield (Feb 24, 2015)

Yea, this is a dwell/RPM gauge that is designed to go on the spark plug wire rather than the ignition wire to a distributor.

It has settings for '2 cycle' vs '4 cycle' and then a separate slide switch for 'low' rpm, 'high' rpm, and dwell. So, it should work regardless of the number of cylinders.

It works great on the *old* machine. . .

Does not work on the *new* machine. . .

So, maybe the ignition wire on the new is shielded and the newer gauges/meter are more sensitive.


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## Dave C (Jan 26, 2015)

tpenfield said:


> I am wondering if there is something 'special' about the ignition/spark plug wire on the newer machines that shields the magnetic field on the wire? Is there a different way of checking the RPM?
> 
> TIA for any assistance.


I would guess that modern ignitions with resistor plugs and/or wires might be suppressed enough that an older tach might not get enough signal. Older ignitions used solid wire and no resistor plugs.



whimsey said:


> I have an old Sear's Craftsman tach/dwell meter from 1972. It has settings for 4/6/8 cylinder engines. I think the snow blower being 1 a cylinder engine causes erroneous readings. Also it looks like the spark plug wire on the snow blower might be better shielded than older automotive spark plug wires causing inductive pick up problems for the tach.
> 
> Whimsey


I have one of those, same vintage. You need the coil signal at the points to make em work.

I use an optical non-contact tach. You put a piece of reflective tape on the output shaft and point the laser at it. I use it for my lathe and other tools too. Surprisingly cheap off Amazon.


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## tpenfield (Feb 24, 2015)

UPDATE: 

I got it to work finally . . .

I added a few more coil/windings to the inductive sensor clip on my dwell/RPM meter, and set it so the coil was pressed up tight to the spark plug wire. The current must be a lot less through the spark plug wire on the newer machines.

The meter is by "Sun Electric" may be as early as 1970's vintage, probably 1980's.

I got 3800 RPM out of the new machine (Troy Bilt 2410), which is a tad high vs. the engine spec is 3600, I believe, . . . so I tweaked the throttle setting back a bit.

The old Toro (521) was 3800 RPM, and it has been that way forever, so I left it alone


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## tpenfield (Feb 24, 2015)

FWIW - here is a picture of my Inductive dwell/tach meter. Click on image icon to see bigger view.









Came with a timing light as well. Probably have not used it in 20+ years.


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