Mods for Propane

Hello all! I'm new to this forum. I decided to join after seeing all the awesome ideas you all have.

I have a briggs 3.5 HP flat head vertical shaft engine that I converted to run on propane. It runs great as is, but since propane contains less energy than gas, it doesn't make quite as much HP in the current configuration.

I've already milled the head down, and I noticed a good gain from that.. (With propane, you can run a higher compression with less detonation)

To get more out of it, I know I'll have to at least advance the ignition timing using an offset key or by moving the coil... but how many degrees should it be advanced?

I also heard about how you can grind down the exhaust lifter by about .010 to close the exhaust valve earlier and reduce overlap. Has anyone heard about this? Does it help?

Are there any other modifications you recommend without buying any aftermarket parts besides a flywheel key and maybe a thinner head gasket?



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Where would you put the propane tank on a minibike? I'm interested in how you went about converting to propane. Used a small demand regulator?
After gas leaking out of my tank twice (I forgot to close the fuel shutoff) I started researching this same idea. I haven't seen a good way to control throttle like you need when using the engine for riding a minibike. Might just have to start tinkering and figure it out with trial and error (we're playing with highly flammable gas, maybe I shouldn't use the term "error" :doah:)


Well-Known Member
After gas leaking out of my tank twice (I forgot to close the fuel shutoff) I started researching this same idea. I haven't seen a good way to control throttle like you need when using the engine for riding a minibike. Might just have to start tinkering and figure it out with trial and error (we're playing with highly flammable gas, maybe I shouldn't use the term "error" :doah:)
How does your system work? Do you use a demand regulator?


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The parts shown on the first site would work for a small engine. A safe system for a stationary engine and blessed by every department in the Obama government. The problem is that the expense would be the price of several new engines. The thing that they call a "fuel controller" is simply a demand regulator and is really all that is needed to supply propane gas to a small engine. When the piston in the engine goes down on the intake stroke there is a slight vaccum on the gas supply line attached to the carb. This slight vaccum acts upon a diaphram that normally holds a needle valve closed. The pressure on the needle valve is removed and allows some propane to flow to the carb. I have attached two pictures. The first is a simple drawing of a demand regulator I make for small model engines. It uses a couple of Tecumseh parts. A diaphram out of a gasolene carb and a needle and seat, also a stock part out of a gasolene carb. They are simple and work every time. The second picture shows a tiny 1/4 scale model of a 1912 Gade hit and miss motor. The propane bottle has a step-down regulator that came off a single burner camp stove. It supplies propane gas, at a greatly reduced pressure, to the demand regulator, the little square thing. Another line runs to a needle on the "carb". But this system is sized for a very small engine. If you are really interested do some research to find a larger diameter diaphram and patch one together. The other method would be to purchase only the "fuel controller", as they call it, and supply propane to it by a regulator on your propane bottle. Probably about 10 inches of water column. Is it dangerous? If you used a simple system like this on an indoor stationary engine and not out in the open like a minibike, gas could leak and be a big danger.
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Thanks for all the great insight! You all have some neat ideas for throttle control and regulating the propane.

I have a bit of a confession to make. I don't have a mini bike. You're gonna laugh, but I have a crappy lawn mower that I bought at a pawn shop for $30 bucks. But the engine is pretty much the same as what's in the mini bikes. The reason I posted on this forum is because you guys have a lot better ideas, and your forum has tech that's actually useful.

About the propane conversion: It worked perfectly without making any modifications to the engine whatsoever. Funny story, when I bought the thing, I couldn't even get it to start on gasoline, but when I converted it to propane and emptied the gasoline tank, it started on the first pull.

I used a propane blow torch for the regulator. I just cut the end off of it to remove the restriction in gas flow, and so I can connect tubing to it. I ran the tubing right down to the carburetor. As luck would have it, there was already an existing nipple where the crankcase vent tube connected to. I just removed that connection and replaced it with the tube from the propane cylinder. To keep crap from getting into the engine through the PCV tube, I ran it up into the air filter, so it's basically connecting to the same area.

As for throttle control: Being that it's a mower, it doesn't need the type of throttle control that you'd need in a mini bike. This mower doesn't have much of a throttle besides a spring-loaded throttle blade. However, I can control the throttle very precisely with the knob on the blow torch tip. It's almost as responsive as using a regular gas throttle!! As for the fuel/air mixture, it seems to adjust that automatically. For you mini bike guys, you could probably use a throttle cable to control the knob from the blow torch tip like you would a normal carburetor.

As for modifications, propane contains less energy than gas, but you have the potential to make way more horsepower because of its resistance to detonation, using the same principles as you would with E-85.

In order to make the engine more efficient on the propane, I've done a few things. I shaved the head .020" and installed a thinner head gasket (.020" vs. .045"). I also installed an offset flywheel key to advance the ignition timing by 9 degrees BTDC. (propane and E-85 motors can take much more compression and timing, thus more horsepower). I also installed one of those "E3 Diamondfire plugs", and I filled the crankcase with Royal Purple Synthetic oil.

I noticed it had MUCH better throttle response when I bumped the timing and increased the compression. I also used less propane. On a side note, after going through about 5 or 6 propane canisters worth of gas (6 months worth) , I pulled the plug and it still looked almost as clean as the day I installed it.

You can see the pictures of where I mounted the propane cylinder, routed the hoses, and hooked it up to the carb. I could possibly delete the carb altogether and just run an open pipe connected to an air filter with the propane running into it, but I wanted to keep the carb and gas tank to give it a stock appearance and so I could run it on gas if I ever needed to.

Don't call me crazy for modding a mower! It cost me $30 bucks, and I've only put $20 in parts into it... It's just kinda fun to mess with these simple and bulletproof motors.

Let me know what you think!

On a side note, the blue propane bottle looks like a nitrous bottle!!! These propane tanks cost about $2.30 at Home Depot, and I can mow my lawn about 4 times on one bottle (about 2-3 hours of run time per bottle).
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I think that's the simplest and cheapest way to do it. I saw another guy with the propane tube going directly into the intake without a carb. All I gotta do is attach the propane bottle clamp. I like the look of the bottle. Then figure out a way to attach that knob to the throttle.


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Good work. I like your ideas, and of course it makes no difference what the motor is on. What do the neighbors think about a propane powered mower? Probably confuses them I'm guessing. I bet that if you robbed a small air adjustable regulator off an old compressor and put it in the line to the carb the mixture could be set even closer for throttling by gas volume. Today I continued to convert a 15KV Onan 4-cylinder gas powered generator to propane so I been playing with this stuff. Went to a local propane outfit and he sold me a special shut-of that shuts off the high pressure propane if the engine does not have any vaccum [stops running]. We don't need that for our purposes because our engines will be out doors. Also sold me a regulator/vaporizer/demand regulator. One piece that does all those things. The demand regulator portion is, as described earlier, necessary for throttle control by the stock throttle plate in the carb. The Onan has a governor that works the throttle plate so all that stuff must be retained to keep a steady 1800 RPM to make 60 HZ current. The vaporizer portion is simply a heat exchanger that has radiator water circulating through it so that the liquid propane is coverted to gas and not quickly freeze up into a big block of frost. That also is not necessary for our purposes because we would use propane vapor from the propane bottle, not liquid propane. On systems that use a lot of propane the valve on top the propane bottle has a tube that goes down to the bottom so that liquid is supplied. That is the system for a big engine. The small propane bottles, such as a Bar-B-Que set-up and the small bottles we use, the valve is supplying only propane vapor [unless the bottle was turned up-side-down]. So all that is needed is what you supplied: A stepdown in pressure and a hose to the intake of the carb. Again, if you wish to have the engine work with it's regular throttle [useing the little bottles] an adjustable regulator [to get the vapor pressure down to about 8 or 12 inches of water column] and a demand regulator will be necessary. Higher pressure will make a demand regulator unworkable because they like low working pressure. Went back to the propane place this affternoon and asked the tech there if he had ever seen a Briggs lawnmower sized engine operate on propane. He said 'Sure', and proceeded to show me a 5 HP Briggs that was hooked up to a pump. The stock looking engine had a demand regulator beneath the stock looking carb. I'm going to stop by tomorrow and take a picture of it. It was a manufactured artical, not a home made deal. A demand regulator diaphram for my model engines are about 1-1/2 or 2 inches in diameter. The one on the Briggs looked to be about 3 or 4 inches.

Thanks for sharing your ideas. Glad you like the setup. I think that's a great idea to use an adjustable regulator from an air compressor. Those are a bit more precise. The mower isn't too loud, but the neighbors sometimes look at me like I'm crazy, especially if they are out "trimming the bushes" or whatever they have to do to look busy while watching me. My next door neighbor asked about the "NAWZ", and I told him about the propane. He thought it was the coolest thing since sliced bread...

Thanks for sharing the information about the generator. Coincidentally, I have a Generac 13.5KW generator that I'd like to convert to propane at some point. For the time being, I haven't messed with it, because the reliability of my generator is a bit more important than the reliability of my pawn shop lawn mower.

My generator has a spring loaded magnetic governor or regulator that keeps it at a steady RPM depending on vacuum, engine load, etc. I'm glad you mentioned that regulator that shuts off the propane if there is no vacuum. I'll likely need some sort of automatic regulator to keep the generator at a steady RPM without having to mess with it.

What size propane bottle are you using for the generator? 20#? 40#? 100#? You mentioned the "frost". That occurs when the bottle starts to run empty and the low pressure is compounded by the cold temps in addition to being less propane in the bottle. What are you doing to keep the pressure up and the propane flowing when the bottle gets cold? Does it have a serious effect on runtime, even on the bottles that have a tube that goes to the bottom to supply liquid propane?



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A propane tank that does not have a tube from the valve on top to the bottom, so that it supplies just gas and not liquid, will get very cold because the liquid is turning to gas on the surface of the liquid propane inside the tank. You will be able to run a motor, turning a big 13.5 KV generator, for only a short time before the surface of the propane would get so cold it would not continue to vaporize. A bar-b-que or something that does not have a high demand for vapor would not have this problem. That is why it will be necessary for you to use a system that utilizes a tank, with the tube to the bottom, that supplies liquid propane to a 'vaporizer'. The vaporizer is designed to have radiator water warming it so that it does not freeze up. It can then supply a much greater amount of propane vapor to the engine. The tank does not get cold.

As far as tank size for my eldest son's generator: I found a guy that has an old El Camino that is on propane and he is willing to sell me the tank. Probably about 30 gallons or so, I'm not sure, but will be O.K. for our purposes. We have a land speed car that uses an onboard 30 gallon water tank to cool the engine [no radiator]. We need to warm the tank to about 180 degrees before the 5 mile run so the engine will be up to temperature. A 6500 watt heater element is necessary to get the water temperature up in less that an hour [about the time spent in line before a run] so a big power source is necessary but will run an hour on something as small as a forklift propane tank.

If your big generator is to be used for back-up when the power fails at your home, and you have natural gas piped in from the gas company, convertion to that gas is a piece of cake. No expensive high pressure safty cut off needed. Also no vaporizer. Only a cheaper low pressure safety shut off and a demand regulator. I have one of them because the Onan I'm working with was set up for natural gas. I took off them parts and put on the high pressure propane parts.