I have a 21" Craftsman push lawn mower with a Briggs and Stratton 7 hp engine that is about 6 years old. I've got it cutting on the highest setting, and it still has issues cutting the grass without loosing rpms in the thicker areas of the lawn. I've done 0 maintence to it besides putting some oil in it this year because it looked low, and actually getting the blades sharpened (that helped immensely), but I still feel like it should be powering through the gas more than it does. What are some steps I can take to improve the power / cutting ability of my lawn mower?
Changing the Spark Plug and Air Filter are good ideas, and are general maintenance items. The carb cleaner could also be sprayed into the carb through the intake.
If those items dont restore power, there is a good chance your motor's compression is low - and you need to rebuild it. That could be anything from new gaskets to needing machine work done.
I wrote this article on my forum (yes, selfless plug) about two years ago. Its title is called Hot Rodding Your Lawn Mower. I believe it is pertinent and will explain after I post the article:
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.
The reason why this is important is this: I discovered on my own lawnmower that I had inadvertently bumped the piece which holds the spring, thus shortening the spring's holding distance, allowing the mower to run at a lower rpm and not building enough speed or HP to power through the tough grass. A lawn mower with a 7hp engine should have no problems powering through even deep grass. There is more going on, I'd think, than just tuning.
I shaved .060" off the head gasket surface using a 8" wide belt sander at work. This increases the compression and REALLY provides more torque when the grass gets thick. I use 93 octane gas and have no heating problems. It really works great! I even installed this modified head on the brand new lawnmower I bought. Of course a clean filter and sharp blade are a MUST on any lawn mower.
Get a motocycle turbo kit from a junk yard increas the gas intake and install a platnum spark plug add some cooling a computer liquide cooling kit would be perfect for a push mower have the pump and fan run off an old cordless drill battery or a few AAAs. When you install the turbo start with low pressure you dont want to blow up the engine.
Estimated boost 2-6 hp estimated cost 100-300 usd
Go to lawnmower racing sites. Lots of info. I had a 6.5 honda clone I put aftermarket racing exhaust with very low restriction muffler an a K an N style air filter then bumped the main jet up 2 numbers. Very surprising results! You can put a billet flywheel on as well an the engine will spool up faster but might loose torque. Stay with the stock wheel an go from their.
7hp is a strong engine for a small mower. I have a 6.5hp and it is no problem cutting thick tall grass. You definitely have a problem. I'm guessing the engine is starving for fuel because it needs a good cleaning. Spraying down the throat of the carb or putting cleaners in your fuel WILL NOT HELP. Take the carb off and clean all jets and passageways. You can do it very easy. Check out youtube if necessary. Cheers!
Check the fly wheel shear key on the rotor is in correct alignment with the crank hitting a small object sometimes moves the crank just off or can shear off the key giving a incorrect timing stroke..Mower will still run but lack power. Had this happen a few times with mine.