Broke Auto - Hot Rodding Your Lawnmower?
  • Hot Rodding Your Lawnmower?

    Well, maybe not in the literal sense, but I wanted to discuss how you can go about getting a little bit more out of your lawnmower.

    I, like many of you, have a newer self-propelled lawnmower which has a set engine speed. You grab the safety handle, pull the starter cord, the thing goes vroom, you release the cable and it shuts off. You do not have any direct way to change engine speed. Either it is on and running, or off and not. The other day I had an issue with the engine where it didn't seem to be running up at its normal speed. I started looking at how the carb and all was put together to see if there was something wrong. What I figured out turned out to be an easy problem to fix.

    Looking at how most lawnmower engine speed is controlled, it comes down to two things. First, there is a small vane type lever which is connected to the throttle on the carburetor. The cooling fan which is on top of the engine, blows air on this vane as the engine speeds up, actually causing the butterfly in the carb to close, slowing down the engine. What causes the engine to speed up is the second part of the equation, which is a small spring, which pulls against the vane. It ends up being a basic tug of war between the spring and the vane, which allows the engine to operate at a consistent speed.

    Basic output of any engine is computed by using torque output and engine speed and running it through an equation to come up with a horsepower figure (NOTE: See below). There are two ways to increase the horsepower: increase the torque at any given engine speed or by increasing the engine speed itself (the second works to a point where the torque value decreases to a point where there are diminishing returns ... but that is for another discussion). On a small engine as I describe above, the easiest way to increase horsepower is by speeding up the engine. You can speed up the engine of this type by either putting a stiffer/shorter spring on to pull against the vane OR you can lengthen the distance the current spring has to pull on the vane. The opposite is what had happened to my lawnmower. Specifically, the small metal mount which the spring hangs off of had been bent towards the carburetor just a little bit. This was caused by something hitting it during mowing. I took a screwdriver (any long prying tool will work) and gently bent the metal mount away from the carb. This allowed the engine to speed back up to its normal running speed. Thinking if a little was good, more would be better! Letting the "Tim The Toolman Taylor" in me come out, I bent it just a wee bit more. This made the engine run a little bit faster than it was designed. As described above, this increases the horsepower output of the engine and allows me to run through deeper grass more easily.

    While this may not seem like something we'd be looking at on this site, to me its a basic form of "Hot Rodding" or making our mechanical stuff work better/harder/faster. This to me is the heart of what this site is about. I hope you will use prudence when using this information.

    (Please note, if you over-stress your lawnmower engine and blow it up, Broke Auto and its management/owners will not be held liable. This article is just for information purposes only. Use it at your own risk.)

    ** NOTE: Horsepower is computed by the following equation:

    HP= (torque X engine speed)/5252
    torque = 300 ft lb
    speed = 3000rpm

    HP = (300 X 3000) / 5252
    HP = 900000 / 5252
    HP = 171.36
    Comments 1 Comment
    1. Tony's Avatar
      Tony -
      Lmao, I was actually going to do something like this because my governor on my rider is sticking cause it's old. Great article. I'm also going to be doing some other things to my rider as well, but you guys will just have to wait and see what that is. hehe
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