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Checking RPM's

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Andrew Z

05-07-2005 07:02:45

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I want to check the RPMs on my A and I am unable to find anyhting that will test it. What devices work to check the RPM's? It is still set up using a Magneto. If that matters. Thanks for the help Andrew

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Bob M

05-07-2005 10:42:13

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Re: Checking RPM's in reply to Andrew Z , 05-07-2005 07:02:45

A couple ways:

1 - Some (but unfortunately not all...) tach/dwell meters will read off the mag"s kill switch terminal as dr sportster suggests. Read RPM directly from the "4 cyl" scale, or from the "8 cyl" scale and divide the reading by 2.

2 - Get yourself a "Tinytach". This reads ignition pulses inductively from an antenna wire wrapped several turns around a plug wire and displays RPM"s on a digital display.

Tinytach

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Andrew Z

05-07-2005 10:55:52

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Re: Checking RPM's in reply to Bob M, 05-07-2005 10:42:13

Thanks Bob M thats exactly what I'm looking for. One question what one is the right one. I see a varity of options what one do you use. Thanks Andrew

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Bob M

05-07-2005 11:28:06

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Re: Checking RPM's in reply to Andrew Z , 05-07-2005 10:55:52

Andrew - You"ve got a couple of options:

1 - Get the tach that"s calibrated for a pulse every 1/2 revolution. Wrap the antenna wire around the high voltage wire that runs between the coil and the center of the magneto cap, or...

2 - Get one calibrated for 1 pulse every 2 revolutions - wrap the antenna around any one of the spark plug wires.

Bob

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Andrew Z

05-07-2005 15:31:22

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Re: Checking RPM's in reply to Bob M, 05-07-2005 11:28:06

Thanks everybody!!
Andrew

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RayP(MI)

05-07-2005 09:01:24

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Re: Checking RPM's in reply to Andrew Z , 05-07-2005 07:02:45

I have a "passive" rpm meter - uses a wire sticking out of a plastic housing about the size of a chewing tobacco container. As the wire is extended, it vibrates with the motor in question - and the calibrated dial reads out the rpm based on the amount of wire extended. Suprisingly accurate. Checked it once against a calibrated electronic frequency counter - if I remember right, I probably made an inductive pickup coil out of a couple turns of wire around one of the plug wires. These devices are usually used on single cylinder lawnmower type engines, but if you could find the right place on a multi-cylinder it should also work.

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dr.sportster

05-07-2005 08:36:49

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Re: Checking RPM's in reply to Andrew Z , 05-07-2005 07:02:45

I am going to eperiment a little and get back to you now Im rethinking this one.

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dr.sportster

05-07-2005 08:01:03

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Re: Checking RPM's in reply to Andrew Z , 05-07-2005 07:02:45

I beleive you can get a reading off the mag kill switch terminal with a tach and dwell meter.You might have to divide that reading by four but Im not sure .The meter instructions should say.

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