How to increase lawnmower engine RPM?

Hi all, I’m working on a “krazy project”, and I need a bit of advice.

I’m working on a motorized surfboard. Why? Why not. I’m using the lower unit of an old 3.5 hp Yamaha outboard engine. The original engine would rev up to 5.500 rpm max. I’m going to be using a 3.5 hp OHC 4-stroke lawnmower engine. It’s a no-name brand, but similar engines usually rev up to between 3.000 and 3.600 rpm. So there will be a big top-speed gap. Gears aren’t an option, don’t have the tools nor the budget to set something up for this near-free project.

I removed all the governor stuffs from the outside of the engine. I’ll add cable throttle control, where I’ll make sure the carb valve opens up to it’s max. (stock max would be maybe half way?) I’ll most likely am going to make a submerged (~10 cm of water column) exhaust. But I’m ok with adding a ball-valve to make it into a straight-pipe exhaust.

What other cheap and affordable options do I have to increase engine RPM? Getting close to 5k RPM would be really nice, considering I’d be getting closer to the intended max RPM of the propshaft. (could cut up a fully functional 5 hp outboard engine, but that’s kinda sad. These are the parts I have)


2 random pics of the project so far. They are slightly recent.

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Be careful with removing the governor, sometimes it’s also the oil slinger in vertical shaft engines like a lawn mower. 5500 is a lot of RPM for one of these to begin with but you can do that with a new carburetor. Leave the inside governor parts to sling oil.

Beware of the flywheel, cheaper cast flywheels can grenade from constant high RPM.

I would test it at the factory set RPM before anything else.


I’m not ready to open up the engine itself. Something with tools, “don’t touch if it’s not broken” and I kinda don’t want to be replacing bearings/gaskets. So the internal governor parts are still all there. Just removed all the external governor/throttle controls. I have heard of “porting the exhaust”, but I haven’t even removed the current exhaust.

Are valve springs something I’d need to look into?

(am considering adding a RPM gauge, but I want to keep things very cheap. Because, well… Lawnmower engine on a surfboard. How serious can such a project be… :stuck_out_tongue: )


If you are planning on riding this on the water, I would build a protection ring tube around the prop. One flip of the board and you are cut badly, life threatening. Might as well be biten by a big shark.


Dutchlandian freshwater lakes don’t have sharks. Do plan on mounting “some” kind of a cage around the prop. Should be decently easy, with the very wide mounting plates.

Any ideas on increasing engine RPM, to increase top speed? I’ll usually be cruising at lower speeds, but it would be fun to be able to open her up a bit. (and get solid “planing” speeds)


Also, don’t mind the very oil-y engine. It ran perfect as a lawnmower engine. Would cough a bit @ full open throttle. But in it’s defense, I’ve had the engine upside down a few times. Major upside down when trying to weld the driveshaft connector to the crankshaft. (engine oil would solidly drip trough the breather tube, in the air filter, which I didn’t initially notice… So with an up-to-date picture, the engine looks just horrible… But the engine has LOADS of compression, and should be very easy to clean up)

Had the engine covered in non-stop wet towels, while welding. To prevent any kind of damage to the crankshaft bearings and it’s seals.


You could use a gear reduction to increase speed without altering engine RPM. Your valves will float at 4000 with those lawn mowers. I doubt it will fit any of the go kart performance parts.

Just use a sprocket that is say, 20 teeth on the engine and 10 teeth on the prop shaft gearbox. This will double the output speed.

That would be a 1:2 ratio

Or go for a 1:1.5 ratio. 15 engine 10 propshaft.


Don’t have the funds or tools, to make a quality gearbox. I MIGHT be able to install an prop from a 5 hp/4800 rpm outboard engine, but that’s about it.

When I came up with this project, I accepted the not-optimal top speed, although I am wondering how I can easily increase engine RPM to get some more top speeds.

Currently looking at an Aliexpress Tachometer Gauge, which should be able to display RPM’s. Will keep your 4k RPM warning in mind.

Also, how easy/expensive would it be to upgrade valve springs?

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The fact you’re using a lawnmower engine is kind of risky enough. Water will get mixed with the oil and then you’ll have to open up the case to clean it out. It’s not like a 2 stroke outboard engine that doesn’t retain oil

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I’m kinda using a stand-off, to raise the engine block from the surfboard. This does increase the “top heavy” effect, but it should reduce the negative effects of a random small wave.

Also, I’m planning to use this thing on freshwater slow moving (if moving at all) Dutch canals/rivers, maybe a lake. During good sunny weather, due to getting wet. Will check if the air box is high enough, ok with making it into a snorkel set-up. (no air filter, because that’s not required in a marine setup)

TLDR: I’ll make sure water won’t be an issue for the lower unit/engine. And I do plan on changing engine oil every 10-ish hours, according to the operations manual for the stock lawnmower engine.

Closer up of the “spacer”, which should create a seal between the engine block, crackshaft bearing/seal and the driveshaft/lower unit. (should also increase the volume of the board, and thus have a bit more buoyancy)

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Minor update: still can’t find any sources where they mention increasing lawnmower engine RPM, past the point of governor tinkering. But in general, my 2 points of interest should be: valve springs and flywheel. Where a set of springs is more important, to know when my flywheel/crankshaft mods start to fail. Porting and larger carb don’t seem to be THAT important, to increase engine RPM. They “only” seem to increase engine HP/maybe torque, and 3.5 hp on a surfboard should be enough already. For higher speeds, I seem to need engine RPM, especially since a gearbox thingy won’t be an option-to-me.

Any insights are VERY welcome.

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These have lightweight aluminum connecting rods.
Overrevving past ??? the intended 3600 rpm, and this is what will break.

Simply disconnect the governor rod and crank open the throttle. Let is Rev. Then you tell us what is the breaking point. 4000? 4500? 5000?
Do not power yourself too far gone; farther than you are willing to paddle back.
Steve Unruh


Second what Steve said, I have killed a few Briggs and Stratton motors at about 4200rpm, but one lived pretty long at about 5500. Techumseh just wouldn’t rev out even with direct ether injection. Never did break a Honda, they would start to smoke and lose compression (gx series). The plastic timing gear ohv Honda gc line liked to snap the plastic timing gear in stock form, just on a pressure washer. Connecting rod was almost always point of failure except one Briggs that floated valves to hard and hung the valve out after destroying the valve seat. Think that was a bit of oil starvation too, hard to tell from the carnage. I went through a racing mower faze and killed a few of them :grin: last Briggs I killed I hit a rock with my riding mower hard enough to break the drive belt and connecting rod simultaneously that was fun


Guys, you are buissy with the “how” but didnt wonder about “is it possible at all?”. We are not talking idle rpm here… l seriously dubt that litle engine will be able to rev that high with a propeler engaged in water.

Im with Bob here too. Im not known to have safety as my number 1 priority but one mistake with that propeller will mess you up big time… be carefull


@ Kristijan Leitinger, I was already considering some kind of cage around the prop. TBH, mainly to protect the prop from rocks and the likes. (this windsurf board should be able to work in very shallow waters) And with all (small) boats, where there is a risk of you falling overboard, there is this prop milling around. I will also add a “deadman swith” to this engine, because the risk of my falling off at one point, are pretty high. So there won’t be ALL the force applied to the prop. Just inertia of all the moving components. (probably still enough to remove a few toes)

The lower unit (and the prop) came from an “egg” 3.5 hp Yamaha outboard engine. That stock engine would be able to rev up to 5.500 rpm, when pushing a small boat. (for example a tiny rubber boat) Granted, the stock engine was a 2-stroke engine, so each rotation of the engine, would have been powered. So there might be some torque figures. I however do expect that this lawnmower engine shouldn’t be too weak to power this surfboard. It’s probably less heavy, and it’s very flat/sturdy. So it will be faster ontop of the water, and thus needing less torque? (and maybe more HP in the higher rev range/straightpipe/no air filter)

fun fact I’ve had many smaller outboard engines in the past, and I’ve got 4 at this moment. None of them had airfilters in their airboxes. I had always assumed someone removed them at one point. Turns out, they never had an airfilter. The airbox is intended as a kind of muffler, to reduce engine sound. There kinda isn’t any risk of road debris/sand/dust/junk to be removed from the air, because you’re on water.

@ Marcus Norman: thnx for the insights. When I was tinkering with the engine, I had removed the lawnmower blade. And removed the governor and had manual throttle control. It’s idle performance was just perfect. Very smooth. But with zero load on this engine, it would act slightly odd when I would stab the throttle, and go for max RPM. I’ll be honest: it was slightly scary. And the engine would spit brown/black smoke at those max RPM’s. Had the engine at an 90 degree angle, to remove the lawnmower blade, so I had assumed the black smoke came from all the oil in the wrong places. (still can’t explain the brown in said smoke, had it produce brown-ish smoke for a bit, to rule out rust dust from the exhaust…)

Like you and Steve Unruh said: connecting rod is going to fail first. Very scared of such things, especially considering I didn’t plan on adding any kind of plate between my lower back and the engine. While the piston will be “shot” towards me when in operation.

How badly did your engines blow up due to a failing connecting rod? Were there pieces of the block just blown out, or was everything still contained in the engine. Would you recommend some kind of a plate between my back and the engine? If so, what kind? Multiplex/wood or steel? If so, how thick? (not concerned about a big fireball, water will be all around me)

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When i was younger i’ve played around with riding lawnmowers some (and even now) i once had a riding mower of really cheap/bad construction, it was very heavy “behind” the rear axle, at this one i’ve installed an old door-hinge, acting as a accelerator peddle, with a cable to directly bypassing the governor, this made it possible to do wheelies wich was fun, but ofcourse ive throwed the rod later.
On this one i’ve notice dents in the sheet metal surrounding the engine, noticed that on other blown-up mowers too.
This make me think some sheet metal, iron or aluminum, or even plywood, should be a good safety for thrown parts, shatter pieces, i suppose you don’t going to wear working boots, and heavy pants using this?
And what others said, a protection/ shielding around the prop is the most important thing in this case, see that as a priority :slightly_smiling_face:


I like how most of you are worried about the prop cage. Yes, I’m planning on adding one. But for ME, it’s my 3rd priority.

1: I had to shorten the driveshaft, and weld a connector to the crankshaft of the engine. My tools are limited, so that’s a serious concern. With my back close to that connection, it has my top priority. A spine is kinda important, and there also some major blood vessels in that area. Don’t want to bleed out. (hence the kinda sturdy tube around the connector/driveshaft)

2: Engine blowing up, with shrapnel hitting my back. Allot of energy will already be removed, due to having to exit the engine. But still scared of a spine injury/bleeding out quickly.

3: The prop. For obvious reasons. I seriously doubt that the prop can remove a limb. Especially with a dead-mans switch. Although it’s very likely capable of removing a few fingers and/or toes. Bad? Yes. Terminal: not likely. (and as mentioned, I’ll be adding a ring around the prop with an length-wise reinforcement bar/rod to keep it in place. Which will be extended, to also support the rudder)

Anyways, I think I’ll be looking for some sheet metal. An ancient computer case would have enough sturdy materials for such a protecting plate.

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As you said in point nr2, alot of the energy is released when the parts go through the crankcase, probably just crankcase pieces thrown longer? A shield of said computer casing should be sufficient.
Your going to ride this sitting or standing up?, it makes sense, when i talked about safety-boots and stuff.
Still worried about the prop, though, are you planning to use that kind of dead-mans switch that many mowers use, with a brake working at the flywheel, that immediately stops the engine when released?
I once built a jet-ski looking contraption, with a outboard engine, where i took away the part between engine and prop-gear, the more it got finished, the more the prop scared the ×× out of me, it never got tested… :smiley:

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I’ll be sitting down, on probably a bicycle saddle mounted ~5 cm above the board. (so water can flow under it/keeping my balls slightly dry-ish) Safety boots will only drag me down, when I fall off, zero benefits.

I’ll be using a “marine dead-mans switch”, where it only cuts spark. (it’s one of the VERY few mandatory things I’ll be required to have)

A prop is indeed a very scary thing. However, 99.9% of all boats in my area have one. (jet drives are kinda rare in my area) And like 95%+ don’t have any kind of prop cage. Even with barely-teenagers driving a tiny rubber boat. (which is illegal anyways) When people fall out their boat, it’s usually to the side or the back. The closer you are (inside the boat) to the prop, the safer. Because once you’ve hit the water, the boat has already moved a bit. Takes some time before your limbs might get chopped up a bit by the prop. While in the mean time, the boat also starts to move away from you. And as long as I won’t be run over by a boat, a prop should be safe towards me. And falling off this surfboard, on the front, so I’ll be ran over: VERY unlikely.

But I will add this ring I’ve mentioned before. Mainly to protect the prop from rocks or junk bicycles and the likes. (loads of bicycles end up in the canals here…) But it should also prevent large limbs from being carved up. Adding something like chicken wire mesh around the prop, is something I’m not willing to do. It would cause too much drag/no-one does that sort of thing.

Btw, if I get these results (cruising speeds/handling) from my surfboard project, I consider it a big win already. Although I’m obviously also looking forward to some higher speeds. Vid isn’t me, this guy ended up replacing this 3 hp outboard engine with a 9.9 hp outboard and has a decent amount of vids on his project. IMO, my surfboard project “should” be more stable, more relaxing, and even cheaper.