Neighbour pegged his mower at WOT

[jrustles]

How high can these things safely rev?
The gentleman next door was having some carb problems with his mower yesterday, so his solution was to peg the throttle open and finish his lawn that way. Well, I was working in the driveway and heard this mower just screaming! 7-8-9K? Whatever the valves began floating at? Who knows. I dropped my tools and went into the backyard, a good safe distance from flying debris.

Was that an over-reaction? Anyone see a small engine grenade?

FWIW- the mower operated like that for about 5minutes while he finished up, and I think it still runs

 

[Nick R]

I think redline on those flatheads is only like 5,500 or something like that, it's pretty low. They do scream, but I don't think it's so much the engine you have to worry about as the blade itself flying apart. (for danger level anyway, I'm sure if you let the engine run long enough at that speed something will break)

 

[IndyIan]

I don't think a regular 4 stroke mower can rev very high.
Maybe 4k rpm?
My buddy did the same thing to get his lawn done as well, nothing dramatic happened. His made far more noise than power up there and a brisk walk kept the mower near normal rpms.

 

[Drew99GT]

Apparently they can handle the abuse. The company that mows the lawn across the street from me has a mower that I think has a broken governor. It runs like it just floats the valves. I heard it the day it broke because the guys kept starting ti and stopping it because it was obviously over revving. They gave up and let it be. It's still running after a month!

 

[racin4ds]

Our kart racing flat heads would go to 7-7200 max with very good valve springs so I doubt that stock thing will go much over 5k, either way the rod will break in it before anything, that was the weak point in almost all briggs motors.

 

[CBR.worm]

What is the weak link on the old honda GX motors?

 

[racin4ds]

The Honda GX motors were tough!! Very hard to kill them, can't say I ever built one though so I'm not sure where the weakest link is.

 

[Danno]

I run WOT on mine all the time.

I have a powered walking mower so the lawn gets done in a shorter time. These things are goverened so if there is any kind of a load on it such as cutting grass, I think they could run "forever" that way - given regular maintenance.

 

[itguy08]

Hit up Youtube and pop in Blow Briggs or Blow Lawnmower. They will run almost forever with a governor stuck at WOT.

I'd be curious what the Max RPM was though - my guess is 6k or so.

 

[bepperb]

That is awesome.

My small engines experience in High School (mid 90's) has led me to believe that the valves float and do keep a Briggs from revving itself to complete failure immediately. My CraigsList wanderings, though, have seen quite a few with a rod through the case so I imagine that's the eventual mode of failure. For what it's worth my teacher at that time would stroll over and shut them off, not dive under a desk waiting for an explosion.

And please keep in mind governed WOT is probably 3300~3600. Bypassing the governor sounds like a lot higher than that. My mower at 4000 rpm still sounds "normal" so I'd suspect in the 5000-6000 range as well.

 

[JHZR2]

Ive pulled the spring on the carb a bit to help the engine run smoother without as much rpm oscillation (even under idle) on flathead engines.

Does not take much though. Id be afraid to push them too fast...

 

[The_Jeff]

I've done this, both accidentally and on purpose.

Once, had a mouse nest get in the governor/air vane area and both lock it WOT and blocked some necessary cooling. Mower dieseled, e.g., ran without spark. Only possible because the deadman brake shoe was long since missing.

PS when it cooled down and was cleaned out it still ran fine.

 

[Cardenio327]

According to the owner's manual for an old Honda rider I rescued out of a field it said (verbatim): "The tractor can be operated at intermediate throttle settings if desired, but always maintain sufficient engine speed for good pulling power and efficient blade operation. Use the shift lever to select the desired driving speed range." I take that to mean it can run at a lesser speed but there is no mechanical reason to do so.

I was tired of buying parts for it after sinking about $300 into it so I drilled a hole through the plate at the carburetor end of the cable and put a bolt through it at maximum governed speed. I cut half an acre once a week for three mowing seasons with the accelerator pinned until I sold the house. The mower ran fine that way, started easy cold & hot, and I sold it in running condition for the same price I payed for it.

 

[Rat407]

I was always under he impression that you run an air cooled mower at WOT, in other words as fast as it is meant to be run. Reason being that it is the most efficient at keeping it cooler for the most airflow around the engine. I remember reading that somewhere in an owners manual.

 

[UNC017]

I don't know about the new OHV engines, but the old flathead Briggs mowers could always tolerate revving significantly higher than the governors were set. The governors tended to be set to minimize danger from rocks and debris flung by the blade, not by the engine's limitations.

When I was a kid we had a 3-horse Briggs that had a weak or broken air-vane governor, and it would scream far higher than it should have. Ran that way for YEARS. Not the safest thing in the world, but the engine never had a problem. The tiny little valves and flathead layout were somewhat self-limiting, too. After a certain speed, the engine couldn't breathe in enough air to spin faster, and the load from the fan action of the blade helped limit the speed as well.

 

[bchannell]

Originally Posted By: Rat407
I was always under he impression that you run an air cooled mower at WOT, in other words as fast as it is meant to be run. Reason being that it is the most efficient at keeping it cooler for the most airflow around the engine. I remember reading that somewhere in an owners manual.


What they mean, is to run at WOT on the dash or handlebar throttle control, but that only means that it is running at "top governed speed", which is where your mower or tractor is designed to be run.

 

[Errtt]

Went fishing with a neighbor who just got a old kicker outboard running (he says he got it running real good). So off we went and launched. He fired it up (took many rope pulls) and once it fired it screamed like WOT or more.
I was standing against the dashboard and next thing I know, something came flying at me so fast, hit me in the forehead and landed on deck between him & I. I was a bit stunned. I looked down and a piston and part of the connecting rod was laying there. Good thing the engine cowling was on which had to slow the velocity of the piston coming at me. Still seem so fast I didn't have time to react to something coming at me, but wasn't expecting to get a piston to the face.
Something hit him in the mouth and split his lip open.
A breeze pushed us away from shore. After we got ourselves together, that's when we found the inboard engine wouldn't start. No oars either. I laid on the front of the boat reaching down and paddled my arms off trying to paddle us to shore. He ended up jumping out and with hands on the back of the boat, kicked with his feet to help. Took what seem forever but we made it. Struggled as my arms were about to fall off and his legs turned to rubber.
Told him I'll fish from shore next time, never again from his boat.
Well on the way home, he lost the transmission in his truck. Dropped the boat and got a tow from a fella. Once at his home, found out his wife's Buick Rivera was broke down, on the driveway. She just made it home from work.
Get my truck to get his boat. Found someone stole the gear in the boat, radio and main prop.
I've had better fishing trips, where I actually got to fish.

 

[steve20]

I believe top governed speed is 3400.
B&S flatheads max hp is usually measured at 3600
IMO they run about 4500 max--and not for very long (25-30 hours)

 

[bsmith717]

Mower motors power (especially torque) drop off pretty quick after the peak. Running one high speed to curb stalling is understandable but if anyone's looking for more power, not so much. If anyone's thinking about it!

 

[meep]

I think the newer OHV engines will probably rev higher than the flatheads. I'd handbuilt a few tecumsehs a long time ago from parts and recall running one at WOT on a test stand (stupid). They'd top out sooner than you'd think. 4500 rpm? maybe 5k?

Remember -- they don't have variable timing.

Newer OHV--- I suspect would rev higher--- though I don't mean that would be any safer.

I've been feet from spinning prop that flew apart, and inches from a sheave that got obstructed and flung apart beneath a 3.5HP at 3600 rpm. in both cases, the damage was quick and violent. Fortunately I wasn't hit either time, and I learned more than ever, that I don't ever want to be.

I'd follow the OP and walk away from a blade spinning at runaway RPM. It's not the blade... it's not the crank... it's the blade /mount/ that fails...