Mower Engine-RPM?-Drive Disconnect

[OnesNZeros]

Is there a relatively easy way (short of a timing light I guess) to determine the RPM of my lawn mower's engine? (6.75 HP Briggs & Stratton, in a Craftsman model 917.377593). I tried comparing the tone of the engine with a known note on my guitar and came up with either 750 or 1500, but I don't know how accurate that could be.)

ALSO, speaking of that model mower, is there a way to fully disconnect the power drive from the drive wheels? I don't mean to just shut the power drive off, because with the power drive off there is quite a bit of braking force on the drive wheels, making it tough to push the thing without the power drive engaged. The power drive creeps along at barely over 2 mph and I'd just like to see how I can fare with the thing rolling free.)

Happy 4th,

1s & 0s

[donald girod]

I have observed that newer mowers run awfully slow. These engines are made to run at about 3600 rpm top speed, at which point your mower would produce

6.75 hp., and you could walk briskly behind it. However, it is probably running at about 2000, and producing more like 3 hp, which is all a mower ever used to need. I think the manufacturers are doing that for "safety" and maybe for emissions as well. My mother-in-law's mower runs too slow to walk comfortably behind. Most likely you can get in there and screw with the governor to jump up the rpm's to a more reasonable speed. I offered to do that for my mother-in-law but she didn't mind walking slower (she's 81). This would harm nothing (maybe make a bit more noise).

You don't really need a tach, just listen to somebody with an older mower and see what it sounds like, and make yours sound the same. It would be easier than comparing it to a guitar. (Incidentally, 3600 rpm would be a 30 cycle/second note, 1800 explosions/minute, pretty low for a guitar fundamental.)

[meirman]

In alt.home.repair on Fri, 4 Jul 2003 20:26:54 -0400 "donald girod" posted:

ROTFLOL.

She's sounds like a goof-off to me. Set the speed up and tell her to put some effort into it.

Meirman

If emailing, please let me know whether or not you are posting the same letter.

Change domain to erols.com, if necessary.

[OnesNZeros]

Well the thing rang with a solid G, so I looked up the freq. for a G and found it to be close to 3000, but I thought (incorrectly it seems) 3K would be too high, so I bumped my guess down an octave or two. If 3K is not an unreasonable rpm then that makes some sense, although the question of "plug firings per rotation" is something I hadn't considered.

And on my model engine speed is preset, fyi.

Either way, thanks a ton for your time folks!

1s & 0s

[donald girod]

Plug-firings per revolution is a bizarre concept. Engines are either

4-stroke or 2-stroke and have either one explosion every other rev. (4-stroke) or one every rev. Your engine is assuredly 4-stroke. The plug could fire as much as it wants, you dont' hear the spark plug, you hear the exhaust. If it fired every rev. (uselessly half the time) it would affect a tach reading.

Anyhow, when you say the engine speed is "factory set", does that mean some kind of electronic governor? Briggs engines always used to have a governor which used a lever sitting in the air stream from the fan, attached to a spring. All you do is shorten the spring a bit, and bingo! it runs faster. Maybe this doesn't work any more. But if you already have 3000 rpm (I sort of doubt it), you don't want much more (and can't get a lot more speed walking anyway, 2.4mph instead of 2mph.)

I'm not sure I understand the music part. 3600 rpm is 30 cycles; the fourth harmonic would be 480, above standard A. The 4th harmonic for 3000 rpm is the fourth harmonic for 25 cycles, or 400 hertz, below standard A (maybe G, I don't know). Is this what you did? If so, then maybe you do have 3000 rpm.

Disconnecting the drive probably involves taking off a chain buried in the works.

[OnesNZeros]

Nah, just means that I have no control over the engine speed except to start it or stop it. There's no "faster" or "slower" lever as there has been on most other mowers I've used.

This fixed speed has not been a problem. The thing is locked at full throttle I assume, which is just fine for the blade, AND (interestingly) the wheel drive

*never slows*, even if the blade is momentarily choked by thick lawn.