Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
5/28/20 7:56 p.m.

Now that I have a new mower, I want to sell the old one.  I always thought it ran at a lower RPM than it should.  It cut great, but just seemed like it was at half throttle.

How do I test it?  I have an old-school diagnostic tach with alligator clips on it, but that was designed for the low voltage side, as in... it goes on the points side of a coil and it is calibrated for 6 or 8 cylinder models, but it also says that you can take the 8 cyl reading and double it for a 4 cylinder.

I'm also not entirely sure it's accurate since the needle seems to rest at 2000 rpms when it's not connected.

How would you check it?  I thought about taking a 10 second slow-mo video and counting the firing pulses and then multiplying by 2 (for 4 stroke) and then 6 (to convert 10 seconds to a minute).  I figure from there I can tweak the spring on the governor to change it if needed.

Any other less-intensive ideas?

RevRico GRM+ Memberand PowerDork
5/28/20 8:00 p.m.

I'll be seeing Mike tomorrow, he has a little digital thing that wraps around the plug wire to read RPMs on the mowers he fixes. Knowing him it was cheaper than dirt too. I'll find out where he got it/exactly what it's called

Patientzero HalfDork
5/28/20 8:00 p.m.
barefootskater SuperDork
5/28/20 8:08 p.m.

I thought this would be about finding redline on an old gocart or something. 
The cheapies that clamp around the plug wire like a timing light seem to work pretty well from the reviews. I've thought about getting one for my dirt bike. 

No Time
No Time Dork
5/28/20 8:16 p.m.

The tool for that is a sirometer:

Sirometer info

RXBeetle Reader
5/28/20 8:30 p.m.

Somewhere I have a sirometer. I had to try it out just for the novelty of it. Physics actually works, who would have guessed?!

There are phone apps that will do the frequency analysis off the mic and spit out an rpm.

Mr_Asa GRM+ Memberand Dork
5/28/20 8:37 p.m.
TJL (Forum Supporter)
TJL (Forum Supporter) HalfDork
5/28/20 8:45 p.m.

In reply to Mr_Asa :

Yup, knockoffs of the (maybe)original "tinytach" tachs. Ive been using them for equipment at work. I bought a pile of the bootleg ones on ebay for non important small motor stuff. They seemed to work fine.  

Turboeric GRM+ Memberand Reader
5/28/20 9:12 p.m.

Hmmm, for small motors my diagnostics were usually "does it have rpm?"  Yes - keep on using it, No - figure out why!

DrBoost MegaDork
5/29/20 8:04 a.m.

In reply to No Time :

This is what I came in here to say. It's the right tool for the job. I've had one for YEARS!

SVreX (Forum Supporter)
SVreX (Forum Supporter) MegaDork
5/29/20 8:18 a.m.

Mechanical RPM gauge

Mechanical RPM gauge

It's got a little spinny thing on the back.  You hold it against the shaft.  Perhaps they can be borrowed through the auto parts stores?

Some of us are old enough to remember tools without any wires.  wink

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
5/29/20 8:24 a.m.

So many fun tools to buy.  Thanks!

NermalSnert (Forum Supporter)
NermalSnert (Forum Supporter) Reader
5/29/20 8:39 a.m.

I've had a 2 or 3 year old push mower with a fixed throttle B&S engine that sounded like it was turned down. Are they set lower than 3600 rpm now, I wonder?

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
5/29/20 8:55 a.m.

Not sure.  I'm off to google rpm specs for B&S

1988RedT2 MegaDork
5/29/20 9:56 a.m.
Curtis73 (Forum Supporter) said:

Not sure.  I'm off to google rpm specs for B&S

3600 RPM sticks in my head for some reason.  I want to say it's written right on the engine somewhere.

Patientzero HalfDork
5/29/20 3:41 p.m.
1988RedT2 said:

3600 RPM sticks in my head for some reason.  I want to say it's written right on the engine somewhere.

I think that's the RPM they are typically rated at.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
5/29/20 5:34 p.m.

Makes sense since generators would need to run around that RPM.

I ordered one of those Sirometers.  $19 and it will be here tomorrow.  Looks handy.

On a sidenote of irony... the two lawnmowers I was considering were an AWD Husqvarna and a Honda HRX217.  I bought the Husqy yesterday.  Today I made a LetGo ad for my old Cub Cadet.  I guy reached out to me and wanted to trade.... for a Honda HRX217 with a bad cable.  He had the cable, but wasn't mechanically inclined.

So now I have both of my dream mowers; an AWD Husqy and a Honda HRX.

So the progression of mowers goes $9 auction sale Honda from the 80s.  Gave that to my nephew when my neighbor gave me his Toro for free.  I sold that for $75 when I found the Cub Cadet in the trash and I spent $6 on blade sharpening.  Traded the Cadet on a Honda worth about $200, and bought a Husqy for $200.  If I sell the Honda, I will have owned about $600 worth of the cream of the crop mowers and pocketed $60 in the process.

Now if I could only find a free Ferarri in the trash, I'd be set for life.

93gsxturbo SuperDork
5/30/20 9:02 a.m.

Fluke 88 has a nice inductive clamp that goes on the plug wire for reading RPM - use it all the time on small engines.  Just set it for 2 stroke or 4 stroke and away you go.  

3500 or so is your go to RPM on most two strokes.  A good guy can set them by ear.  

noddaz GRM+ Memberand UltraDork
5/30/20 9:38 a.m.

There are laser tachs also.  You put a shiny piece of tape on what is spinning and shoot it with the laser.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
5/31/20 6:52 a.m.

Oh Em Gee.... That Sirometer is the most brilliant and simple analog steampunk thing ever.  I got it in the mail yesterday and I was amazed.

I gotta make a video.

RPMs are at 2800.  I checked everything there was to check on RPM spec, but can't find much.  The only thing I found was that peak torque happens at 3060, and the rated hp is measured at 2600.  I think I'll set it to around 3000-3100 just because.  That way it's not lugging, but definitely splashing enough oil.  That's my wild guess.

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