Factory RPM's for lawnmower engines

[jeepman3071]

I usually set them at 3000-3200 RPM. Most small engines are rated to run at 3600, so 3000-3200 is still quite safe for the engine.

While the blade tip speed limit exists, I've seen many mowers, even brand new that are running at barely above idle speed, and die immediately when put into any kind of grass. I'm not sure if it is a lack of quality control at the factory or what.

 

[Cujet]

Possibly of interest. I have a 60V Toro mower, and it runs 2480 RPM under no load conditions. Gives a nice cut, is quiet and smooth. But when the grass gets really tall, the motor RPM increases considerably. Unfortunately, I have no way to measure it while mowing tall grass but would guess it peaks about 3300-3400 RPM.

My point is that the lower RPM works just fine with regard to cut quality.

My re-powered Snapper Commercial mower with Honda 190 engine runs a factory pre-set 3100 RPM, and I've found no need to change it. The previous Kawasaki engine was a full 3600RPM, and I cranked it up to 4000 when the engine started losing umph. That did seem to help at all, as the thing would scream and then stall out in the tall FL grass anyway.

 

[oil pan 4]

Every Bittogger should have one of these.

View attachment 229822

I have 4 of them and got them when they were under $10 each.

 

[oil pan 4]

Possibly of interest. I have a 60V Toro mower, and it runs 2480 RPM under no load conditions. Gives a nice cut, is quiet and smooth. But when the grass gets really tall, the motor RPM increases considerably. Unfortunately, I have no way to measure it while mowing tall grass but would guess it peaks about 3300-3400 RPM.

My point is that the lower RPM works just fine with regard to cut quality.

My re-powered Snapper Commercial mower with Honda 190 engine runs a factory pre-set 3100 RPM, and I've found no need to change it. The previous Kawasaki engine was a full 3600RPM, and I cranked it up to 4000 when the engine started losing umph. That did seem to help at all, as the thing would scream and then stall out in the tall FL grass anyway.

To get power out of the additional RPMs you need to advance ignition timing.

 

[Cruisin'n5thgear]

Generally, mower engines are designed to operate at a constant speed.

Even on mowers where the operator has an engine speed control, it's not actually directly connected to the throttle disk in the carburetor. Opening and closing the throttle is the job of the governor-under light load it keeps the disk nearly closed and as load increases it opens the throttle more and more(at least up until the point where you've overloaded the engine, at which point it will start to slow down). If you have a speed control, it's adjusting the governor to a lower speed.

On a lot of push mowers, the governor is an air vane tucked up under the flywheel/fan, and often actually directly attached to the throttle disk. Typically there will be a spring on the air vane that will hold the throttle at the full open position. Air from the engine fan pushes on the air vane to close it, so in operation the air vane will settle into a spot where air speed counteracts the spring pressure. If the engine bogs and starts to slow down, the spring will pull the throttle more open, and if the engine starts to speed up the spring closes it. The operator's speed control generally will change the spring tension, although at least on the older carbs I'm familiar with there's also a gross adjustment for spring tension that lets you change the maximum operating speed and the range over which the speed control works.

You're correct, and you've explained it better than I could. Thanks!

 

[bunnspecial]

To get power out of the additional RPMs you need to advance ignition timing.

How does one do that on an electronic magneto type ignition? At with the ones I've played with/used the electronics control advance(at least on the ancient 70s Lawn-Boy ones they are super retarded below 1K RPMs for easy starting, then advance to something more reasonable above 1K as the engine is running).

Presumably locating the coil pick-up differently with respect to the magnet would change the timing, but every small engine I've ever changed the coil on will let you adjust the coil-to-flywheel gap but there's normally little if any room to move it around the flywheel.

 

[Engineer]

Finally, I got that rpm meter and measured it, and it was 3030 rpm. Sounded about right. So previously it might have been around 2800 or less. I think I will leave it at that.

 

[oil pan 4]

How does one do that on an electronic magneto type ignition? At with the ones I've played with/used the electronics control advance(at least on the ancient 70s Lawn-Boy ones they are super retarded below 1K RPMs for easy starting, then advance to something more reasonable above 1K as the engine is running).

Presumably locating the coil pick-up differently with respect to the magnet would change the timing, but every small engine I've ever changed the coil on will let you adjust the coil-to-flywheel gap but there's normally little if any room to move it around the flywheel.

Timing advance key ways.
It's a go-kart thing.

 

[bunnspecial]

Timing advance key ways.
It's a go-kart thing.

Didn't even think of offset keys-thanks!

I've actually used them before-at one point I had one on the crank pulley in my MG to advance the cam timing a bit(I finally gave up and bought a Vernier sprocket to dial it in exactly).

Makes perfect sense that they would work to change ignition timing on the flywheel of a small engine with magneto ignition.