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Antique Gas Engine Discussion

measuring rpm on sparkplug engine with mag


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  #1  
Old 05-25-2005, 01:01:30 PM
CGBusch CGBusch is offline
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Lightbulb measuring rpm on sparkplug engine with mag

If I bought this off of ebay, would it work to measure the rpm on my fmz ?

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marine and small engine. CE Conformity. Meets IEC 1010 Safety Standards

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Old 05-25-2005, 02:43:03 PM
Donald Kuhl Donald Kuhl is offline
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Default Re: measuring rpm on sparkplug engine with mag

I suspect this will not help you much. Multi-meters of this sort are meant to measure aspects of electricity that are being continuously fed. The current/voltage that is created by the mechanical means of a magneto is not continuous. It is created and grounded imediately via the spark plug or the igniter points. There is also the matter that the current/voltage created by your engine may be outside the readable range of this multi-meter.

Two ideas that might help..
1) You might think of using a tachometer in contact with the crackshaft. Generally there is a turning/grinding center on the end that will work well for a contact type of tachometer.
2) A frequency variable strobe light against a mark on the flywheel or crankshaft.

The strobe light is the less expensive method.

Best Regards,

Donald
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Old 05-25-2005, 04:14:15 PM
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oldironcollector oldironcollector is offline
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Default Re: measuring rpm on sparkplug engine with mag

I don't think what you have there will help you out much, If the engine were throttle governed it would be closer in accuracy than a hit and miss style governing. With the hit and miss style and an EK mag, the mag is normally only tripped when the exhaust valve functions so this would be a poor representation of engine rpm. What I use is a combination contact and photo tachometer from Snap on a little pricey at over the $300 mark but extremely accurate. The reading can be taken off the end of the crankshaft via contact or a small piece of reflective tape can be placed on an object and the reading can be taken by the infra red photo feature of the tach. I am also a diesel engine mechanic so the tach does double duty for me.

Denny
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Old 05-25-2005, 04:17:16 PM
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Default Re: measuring rpm on sparkplug engine with mag

Oops, sorry I did not see your first line about the engine being a FM Z, depending on if the Z is spark plug or ignitor what you got there may give you a reading, if you can select to read a single cylinder engine.

Denny
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Old 05-25-2005, 04:27:02 PM
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Elden DuRand Elden DuRand is offline
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Default Re: measuring rpm on sparkplug engine with mag

If it's a throttler, all you gotta do is count the number of bangs in a minute and multiply by two.

You could also count the number of times the exhaust rocker moves (both full and short strokes on a hit-n-miss) and multiply by two.

These engines run slow enough that you can do that. Don't try it with your chainsaw!

Take care - Elden
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Old 05-25-2005, 06:00:43 PM
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Andre Blanchard Andre Blanchard is offline
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Default Re: measuring rpm on sparkplug engine with mag

"If it's a throttler, all you gotta do is count the number of bangs in a minute and multiply by two."

"These engines run slow enough that you can do that. Don't try it with your chainsaw!"

It wasn't a chainsaw but I have seen it done on pulling tractors, some of them at 5000 RPM or better. A guy in the stands with a laptop computer and a microphone. He just had to input the number of cylinders and whether it was a 2 or 4 stroke. Diesel or gas did not matter you could watch the RPM from start to finish.
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Andre' B. Clear Lake, Wi.
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Old 05-26-2005, 12:39:00 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: measuring rpm on sparkplug engine with mag

Check EBay for hand held Tachometers - one was on yesterday - a Stuart, and it was only about $15 You want a mechanical tach, not electronic. Starrett made them, so did Stewart-Warner I have both types. The Stewart- Warner is good up to 2500 RPM, and gives you actual RPM as the engine is running. The Stuart and the Starrett are timed types - you hold them against the crank for a given amount of time and then multiply the read-out to get your RPM ( 1 minute = RPM, 30 seconds X 2 = RPM, 15 seconds X 4 = RPM, 20 seconds X 5 = RPM) I have about 5 different Starretts, all work well, and they can be had for between $5 and $20 at flea markets, and on EBay
Andrew
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Old 05-26-2005, 12:45:13 AM
CGBusch CGBusch is offline
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Default Re: measuring rpm on sparkplug engine with mag

Quote:
Originally Posted by Andre' Blanchard
A guy in the stands with a laptop computer and a microphone. He just had to input the number of cylinders and whether it was a 2 or 4 stroke.
That's how they do it for F1 racing and I suspect NASCAR as well for the in-car tach views.
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