J.E.H. Farmboy

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Nov 17, 2019
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Now building a second Farmboy. My first runs great but dies after heating up. Usually 30 - 45 minutes. I put antifreeze in the water hopper, but doesn't seem to have any effect. After cooling it starts right back up for another 30 - 45 minutes. Wonder if I'm the only one with this cooling problem?
Something doesn't sound right. I never put anything in the water hopper and after a half hour it's barely warm. Is something tight causing friction?
I built SN# 003 in 2009 and have never put water in it and it runs 9+ hours straight, no stopping at the engine shows. I've never seen it any hotter than 96 degrees ( outside air temp). You may not be coasting between firings long enough. Listen to the cam "clicking" between firings and multiply X2. You should coast about 30 revolutions between firings. More is better. Did you use the "O" ring or a cast iron ring? Iron rings will run hotter.
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I used an "O" ring. I never counted the revolutions. Not sure what the can clicking is. With the piston out it's free from any friction.
Sorry, can't type for **&*(. Meant to cay CAM. If you listen when the engine fires you can hear the cam hitting the roller on the exhaust rod. Count the clicks till it fires again. Since the cam turns at 1/2 the crank speed multiply by 2 to get revolutions coasting.

Just cause ti turns free it still maybe firing way to often and running like a regular engine that never misses.

Jim G
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Three questions that come to mind, to sort out where @Engine maker is going: when it's running does it fire every other revolution? Does it go like a bat out of hell (fragments of flywheel stuck in the wall is an indication that the answer is "yes")? Are you running it under load (generator, pump, whatever) or just letting it free run?
It runs very well at a slow consistent speed. Much nicer than all my other engines. I just wonder why it gets warm. I shouldn't complain, at least it runs.
I'm just guessing from general engine technology here, but if it's just getting warm enough to gently simmer the water in the hopper, then it shouldn't be seizing up.

(Come to think of it, antifreeze would make things worse, by raising the water's boiling temperature. Hmm.)

I don't know what materials the Farmboy parts are made of, but if it has a normal aluminum* piston and a cast iron or steel cylinder, then maybe the piston's too big? It needs to be small enough to still have clearance after it and the cylinder have both warmed up. At a guess, if it's making anything close to a nice seal in the cylinder without the ring, then it's probably too big.

* Normal aluminum meaning anything that's not hypereutectic. I.e., 6061, 2024, etc., are all normal. If it's a PITA to machine and you either cast it yourself from a car piston, or you went to great lengths to obtain a block of high-silicon aluminum for casting -- then maybe it's hypereutectic.
Now building a second Farmboy. My first runs great but dies after heating up. Usually 30 - 45 minutes. I put antifreeze in the water hopper, but doesn't seem to have any effect. After cooling it starts right back up for another 30 - 45 minutes. Wonder if I'm the only one with this cooling problem?
I would inclined to check the ignition system as what you are offering is either an open circuit HT coil and or crook capacitor
It sounds to me like you have no air vent in your fuel tank, - or it is blocked. As you suck fuel out of the tank it creates a vacuum and starves the carburettor
Hi. I've just made Farm Boy #800, my very first hit and miss engine. I am not very experienced but I will give you my thoughts.
The aluminium piston ring will expand greater than the cast iron cylinder sleeve when heated. I would double check three things.
1. Ensure that the piston diameter is 0.002" less than the cylinder diameter.
2. Make sure the piston ring groove is within spec for the o-ring. JEH recommends the groove width to be 0.011" greater than the o-ring section. The depth of the groove should be 0.016" greater than the o-ring section.
3. The od of the o-ring should match the id of the cylinder sleeve.
I hope you get it sorted.
Agree it shouldn't get hot at all. Remember when its in hit&miss mode the exhaust valve is held open and the engine sucks cool air in and out on every stroke.

Is it firing and coasting? Maybe adjust governor for longest coast possible and lowest speed.
Is the exhaust wide open, no restrictions?
A lean mixture, premature spark, high compression, wouldn't help but should not be a factor on our small engines
Too tight a piston fit could be the problem but if you have .002" shouldn't be a problem. If crank pin, crank, rod bearings and wrist pin are not all in alignment considerable side forces can be imparted to piston flanks. Does the piston show signs of uneven wear?

Look forward to your eventual diagnosis. My hit and miss engines run for a long time, without water, and only get warm, not to say they don't have problems sometimes.
My only suggestion would be to check the sealing of inlet manifold joints. My modified 25cc brushcutter motor would die after about 10-15 minutes running, which was just enough time for the inlet manifold to heat up from the cooling air coming off the cylinder which in turn relaxed the mounting bracket which in turn allowed a small air leak through the manifold gasket. After tightening the manifold bolts I did not experience this problem again. I had other problems but they don't relate to this thread :cool:
Still quits after about 30 minutes after trying helpful suggestions. It is not hot to the touch. The liquid in the hopper never gets warm. Noticed an odd sucking sound near the carburetor. Maybe that is normal. (Don't normally wear hearing aids in the shop.)