# How do you guys measure RPMs?



## Stang (Mar 28, 2015)

Having re-assembled some linkages on my old Tecumseh HM80, I wanted to check that I'm getting a proper 3600 rpm at full throttle. Is there any backyard mechanic way of measuring rpms on an engine with the old magneto ignition?

I don't have a timing light, but I do have a good multimeter.


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## bad69cat (Nov 30, 2015)

https://www.amazon.com/Digital-Photo-Laser-Tachometer-Contact/dp/B001N4QY66/ref=sr_1_1?s=automotive&ie=UTF8&qid=1481640097&sr=1-1-spons&psc=1


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## Mike C. (Jan 28, 2016)

Remove sparkplug. Remove belt cover.Lock down the lever that engages the impeller/auger.

Note/mark the starting position of the auger and the engine pulley and count how many times the engine turns over for one full revolution of the auger.Note this ratio.

Note/mark starting position of auger.Safely start blower-get it up to full throttle.Engage auger for 1 minute(timed) and count revolutions.Do the math.

This would be much easier with two people.

A cheap digital tach would be even easier.


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## Mike C. (Jan 28, 2016)

Stang said:


> ......
> 
> I don't have a timing light, but I do have a good multimeter.


I was able to check the accuracy of my analog(needle type) tachometer recently by using my Fluke digital multimeter and a portable generator.

My generator is governed to run at 3600 RPMs so that the frequency of the ac produced will be 60 cycles-like house current.

The Fluke can measure ac frequency very accurately(.1%).So I started the generator and tweaked the governor to get the frequency exactly on 60 Hz.Then I measured the engine RPMs with the analog tachometer and found that it reads under by about 150 RPMs .I'll bet even a cheaper digital tach is more accurate.


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## db130 (Feb 16, 2013)

You can buy a Treysit sirometer on ebay... or there are several Android and Apple tachometer apps

Here's one:

https://play.google.com/store/apps/details?id=com.javiery.rpmgauge&hl=en


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## DriverRider (Nov 20, 2016)

I use a hand held mechanical tachometer which reads RPM of any rotating shaft.


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## Motor City (Jan 6, 2014)

Their are some cheap Inductive Tachs on Amazon.


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## Stang (Mar 28, 2015)

Great suggestions, guys. Thanks!


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