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How to read RPM's without a tachometer

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Andrew H

06-07-2005 17:43:42

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I have an old Ford 550 backhoe. Tachometer is inoperative. Is there a simple way to get a reading on the engine speed so I can adjust the idle speed/ Please let me know. Thanks

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Caveman

06-08-2005 22:59:41

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Mechanical tach ME Lookin for 1-Lungers in reply to Andrew H, 06-07-2005 17:43:42

I WISH I knew where to find one, but my Brother-in-law had a small hand held gauge that has a 2" long rubber tipped shaft coming out of the 1" diameter guage. He'd touch it to the flywheel shaft of his various stationary/"one-lunger" engines for 10 seconds and then read the number of revs. Then he'd just multiply the reading by 6 to set his governor speeds. I'm sure that this little manual tach was picked up at an antique engine show. Possibly a full sized mechanical drive tach could be used the same way???

P.S.
(I'm real poor, and on SSI Disability, but if anyone out there has a one-lunger engine that is rough but restorable and close to Perry FL,{N.W. FL} I'd love to know about it)

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Tim B from MA

06-08-2005 09:19:37

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Re: How to read RPM's without a tachometer in reply to Andrew H, 06-07-2005 17:43:42

Andrew,

Go to the bottom of this web page:

Link

See the vibratory tachometer ($14.95) about the third item up from the bottom. Works on the vibration of the engine so it works for diesel and gasers. It is supposed to be plenty accurate for most applications. I have not tried one (I went with a digital TinyTach) but you can go to the Tech Forum on this web site and see lots of folks thing these things are great.

http://www.perr.com/tecstore.html

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Davis In SC

06-07-2005 21:08:18

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Re: How to read RPM's without a tachometer in reply to Andrew H, 06-07-2005 17:43:42

Digital Tachs are common these days. All you do is put a piece of reflective tape on a shaft, & point meter at it. With a little searching, you should be able to find someone that has one. Seems you could adjust idle by ear to somewhere close to where it should be..... .....

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Kmiller

06-07-2005 20:48:27

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Re: How to read RPM's without a tachometer in reply to Andrew H, 06-07-2005 17:43:42

Not sure what kind of idle speed you want, but I heard of a trick once to put a piece of tape on a belt pully on a tractor to find where the PTO speed was. Using a flourescent light, the tape will seem to stop at 600 rpm. Then you can guess at where 540 would be.

I suppose a similar thing would work if you put a mark on a pulley or harmonic balancer. If it will stop at 600 rpm, maybe you can find another multiple of 60Hz that will get you close to your desired idle RPM.

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Andrew H

06-07-2005 20:54:50

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To: Kmiller: Thanks in reply to Kmiller, 06-07-2005 20:48:27

Hello Kmiller!
Thanks for this tip. 600 rpm is the exact idle speed I was looking for. I'll try your trick with the flourescent light. This is the exact inexpensive trick I was looking for. Thanks!!!!!

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buickanddeere

06-08-2005 08:17:01

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yes but................. Re: To: Kmiller: Thanks in reply to Andrew H, 06-07-2005 20:54:50

You have to know what you are doing. A single piece of tape will show up "once per flash" at 3600,1800,1200,900,600,450,300 etc rpms. The area has to be fairly dim so the fluorescent light is the primary lighting. Careful not to get the cord wrapped up in the shaft or fan etc. Only the old reactor core ballasts that flash 120 times per second will work. The electronic ballasts flash much faster.

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Rod F.

06-07-2005 18:40:42

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Re: How to read RPM's without a tachometer in reply to Andrew H, 06-07-2005 17:43:42

There are devices that clamp to a fuel line for these kinds of situations. I think they are available through NAPA. I've never used one, so I can't speak to it's use, but I think it was in conjunction with a multimeter. I wouldn't be overly worried about idle speed anyway, unless you have a specific purpose for setting it. Adjust it up until it idles smoothly when warm. It may idle a little rough when cold. Good luck.

Rod

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Jerry/MT

06-07-2005 17:52:18

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Re: How to read RPM's without a tachometer in reply to Andrew H, 06-07-2005 17:43:42

I presume it's a diesel engine. If so it's pretty tuff to do unles you have a strobe light. I believe Harbor Freight Tools has a two devices( at touch type tach and an optical one) for about $70 that could be used. Maybe some diesel mechanic has a trick or two up his sleeve to help you but your best bet is to fix the tach.

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Andrew H

06-07-2005 17:57:15

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Thanks! Jerry/MT!! in reply to Jerry/MT, 06-07-2005 17:52:18

Thanks Jerry!
Can iy be done with a strobe light though?
If it can be donw with a stroble light, I'll buy one. I'll also look into that company you mentioned. Thanks!
Can it be done with a strobe light?
Please let me know. Thanks

Andrew

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Jerry/MT

06-08-2005 20:42:12

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Re: Thanks! Jerry/MT!! in reply to Andrew H, 06-07-2005 17:57:15

The strobe light that I'm refering to has an adjustment so thet as you turn the dial the flashes get faster and faster. There is a meter that tells you how many flashes per unit time. You place a mark on the crankshaft and start the engine. You turn the dial on the strobe light until the mark on the crankshaft stops. Then you record how many flashes per unit time and that's the revolutions per unit time. You then convert that to rpm by whatever factor is needed (example: X rev/sec x 60 sec/min = rpm). We've used these in laboratory situations and I think they are pretty expensive. You don't get them at NAPA.

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06-07-2005 19:21:10

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strobe tachs in reply to Andrew H, 06-07-2005 17:57:15

Uh, since there is no ignition, I don't know how an automotive strobe light would help. If it is a gas engine, a cheapie ignition analyzer will tell you rpm.

However, there are strobe lights that you vary the flash rate manually until a rotating shaft appears to stop. Then, if you know how many flashes per second it is putting out, you can determine rpm.

Of course, this is presuming that you have a rotating shaft you can see... and I bet that for the price of such a strobe, you could just get your tach fixed.

All this to say, don't expect an automotive tach to do you any good if you have a diesel. The kind of strobe I am talking about is an industrial instrument, like this one for $289 (http://www.kerncoinstr.com/461825.htm)

Google "strobe tachometer" and you'll see a bunch of hits.

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