Use digital contact tach to test RPM?

[isaksp00]

I can borrow a CenTech (Harbor Freight) digital contact tachometer. This has a mode where you put a wheel against the side of a rotating shaft and it then measures feet/minute.

The flywheel on my Honda BF8A is mostly exposed, and it is easy to touch the side of it. I measured the circumference with a piece of plastic cable tie, and it is 21.8125 inches (21 13/16). My idea is to measure linear speed of the flywheel edge and convert to RPM, where (using the nominal idle RPM of 1200), it would be 1200 revs/minute, or 1200 X (21.8125/12) feet/minute at the edge (or about 2181 ft/min). I would guess the circumference I measured has to be within about 0.5% accurate.

Questions:
Has anyone ever done it this way and did it work OK?
I assume the RPM spec is for is crankshaft revs, which is the same as flywheel revs - is that correct? E.g., it's not camshaft revs (probably a dumb question...)
Thanks, Peter

 

[pvanv]

Re: Use digital contact tach to test RPM?

While possible to do that, it is the most difficult and inaccurate method. Yes, you want to see crank RPM, which is double cam RPM. Use a decent shop tach on a plug wire instead.

 

[isaksp00]

Re: Use digital contact tach to test RPM?

Thanks, Paul!
I guess I can buy something like one of those TinyTachs (or do you know another affordable model?). Do you happen to know how many sparks per rev on the Honda 8HP BF8AM motor (seems to be 1997 model based on serial num), 2 cyl 4 stroke (or how I can figure it out from the shop manuals)?

 

[pvanv]

Re: Use digital contact tach to test RPM?

4-strokes must fire every-other revolution. (The 4 strokes of suck, squeeze, bang, blow, as stated in Formula-1 terms). That said, many smaller OB's have a siamese coil, so the plug has a "wasted" additional spark on the exhaust stroke. So you are firing either every, or every-other revolution.