"portable" tach?

[Merc90HPnewbie]

Dont know if this is the right forum, I am trying to verify the reading on my dash tachometer. Is there a temporary tachometer I could hook up to check what my RPMs really are? I Think I have seen some for working on single cyl 4 stroke lawnmower engines, etc. Where could I find one that doesnt require much installation but would work on my 1985 Mercury 90 HP outboard? It's a Straight 6 cyl (L-head). If it's as simple as getting a two stroke tach from an autoparts store or Home Depot, clipping a lead onto a sparkplug wire, and then multiplying the reading by 6 that would be great. Can anyone recommend a model or brand? Or post a link to one?<br />Thanks !!!

 

[Merc90HPnewbie]

Re: "portable" tach?

I havent had any luck websearching for one. I was thinking I could inductively couple an oscilloscope lead to a plug wire, or maybe a frequency counter, but if I wanted to test it at higher RPMs I'd need something battery powered so I could use it out on the water...<br /><br />Would I be right in assuming that I take the number of times I get a voltage pulse on a spark plug wire in one minute and multiply that by 6 (it's a 6 cyl engine) to get true RPMs??<br /><br />Thanks

 

[ZmOz]

Re: "portable" tach?

I would try one of these: http://www.tinytach.com/origgas.html <br />They have several different versions depending on the number of cylinders/type of engine. I actually have one of these on my I6 Jeep Cherokee.

 

[Merc90HPnewbie]

Re: "portable" tach?

Awesome! Thanks!<br />These are the ones I read about in a lawnmower manual, but they didn't mention the brand name. Now I know. I wonder if it's better to get the 1 spark per rev model and just do the mental math....<br />This makes me think I could take a handheld multimeter and measure the spark firing rate with the frequency meter option...<br /><br />2 strokes fire once per revolution right?

 

[airman]

Re: "portable" tach?

A good timing light with an inductive pickup can do it.