Engine rpms

welderskelter · July 20, 2021, 09:34:49 PM

Not sure where this post should be, but thought I wanted the expertise of the guys that know how to do things. So here goes, I am trying to find out how many rpms my engine is running so I can figure the feet per minute my blade is running. mI have a 3010 Equus tach tester. My problem is it only has readings for 4,6and 8 cylinders. Thats not the worst part. My engine is a Kohler KT17. And it fires both cylinders off the same coil. Beings it does that it is going to fire twice each complete cycle of this engine. After this my mind goes blank and I dont know how it can work. Need help. Harold.

Southside · July 20, 2021, 10:07:46 PM

If you have a smart phone you can download a number of tach apps that will strobe your flashlight. When the flywheel, or shaft, appear to stop moving you have a pretty close RPM. Not perfect, but it will get you close.

JoshNZ · July 21, 2021, 03:24:32 AM

You can also download apps that resolve RPM from the audio on your phone's mic.

Not sure how that tester works/how it senses, do you clip a sensor to one of the sparkplug leads? You know a steady idle is going to be in the vicinity of 800rpm can you work back from that?

welderskelter · July 21, 2021, 08:38:09 AM

Thanks guys. I am sure there was a lot of time wasted on this than was reported. I think I have it figured out. My tach hooks to the negative post on coil .and to ground. Beings the engine puts out a spark every time it turns over (twice each cycle) and my tach reads 4 times a cycle (twice a revolution) then the tach should be reading 1/2 of the right amount. Or is that twice the amount?

Time will tell. My flip phone isnt very smart and I have only mastered turning on the I phone. The wife runs it and she doesnt understand revs and cycles. She ran me off when I was only half ways thru explaining them to her.

JoshNZ · July 22, 2021, 03:32:48 PM

I'd say... A 4cyl engine coil will be firing 4 shots per 2 revs, 2 per 1 rev. 1000rpm would be 2000 shots. So Rpm = .5*spm (sparks per minute let's call it)

I have no idea what degree the Kohler crank is but I guess 180, so it will be firing what.. twice per rev, if it wastes a spark on the exhaust strokes too, is it points and condenser?

That means for 1000rpm it'll be doing 2000spm. Set your tach to 4cyl and it should read exactly right.
If it's a 360 degree crank a wasted spark isn't neccesary then double what your tach says.

I think it'll be obvious enough just working back from what is expected roughly. I'm not sure in my reasoning above but if you set it to 4cyl, any which way I think of it, it can only read accurately or incorrect by a factor of 2.

welderskelter · July 22, 2021, 06:48:57 PM

JoshNZ, I think you got it right. Which ever way it is off I think that will show up real good. Thanks for posting. I was thinking every body thought I was nuts. Maybe they still do. Yes , the phone might be better but I am a little literate where these things come in. Thanks Harold

dgdrls · July 22, 2021, 07:02:07 PM

Kohler specs that motor at 3600 RPM from the service manuals I found,
file:///tmp/mozilla_test0/Kohler-K-Series-Twin-Cylinder-Engine-Specifications-and-Tolerances.pdf
I also read those motors are susceptible to rod failure, so a little less that W.O. might be a good idea.

Get your pulley diameters and band wheel diameters and voila! you can compute your band speed to within a reasonable number.

D

sawguy21 · July 22, 2021, 07:11:51 PM

That is a 30+ year old motor, if it hasn't chucked a rod by now he ispretty safe.

jimbarry · July 23, 2021, 06:10:08 AM

I assumed most small engines maxed at about 3600 rpm. I bought a tach/hr meter online for $10 and wrapped it around the spark plug (13hp honda gx390). Works like a charm. I run it 3/4 speed when cutting knotty logs to avoid wavy cuts.

Curious to know why you want to know how many feet per minute you are running?

welderskelter · July 23, 2021, 08:33:19 AM

Jimbarry, I think there has been countless hours of sleep lost over this subject. I believe it is a thing to know when trying trouble shoot a problem. I doubt that there are many real sawyers out there that dont know the blade speed of there mill. Just trying to keep up with the real sawyers. Who ever they are.

moodnacreek · July 23, 2021, 09:42:37 PM

Quote from: welderskelter on July 23, 2021, 08:33:19 AM
Jimbarry, I think there has been countless hours of sleep lost over this subject. I believe it is a thing to know when trying trouble shoot a problem. I doubt that there are many real sawyers out there that dont know the blade speed of there mill. Just trying to keep up with the real sawyers. Who ever they are.

Every circle sawyer knows his saw or arbor rpm and by watching the saw marks you know the feed rate.