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McEwen Bros half breed ignition -- need help


Need some advice. I am working on this McEwen Bros. half breed ---- running on a mag and seems to prematurely "double fire" (after 30 seconds of running near perfectly) and then stalls to a stop.

So I replumbed it completely to now include accumulator tank and Garretson KN demand regulator. Yes, the accumulator is between the gas grill regulator and demand regulator. Yes, I am using a 100 lb propane tank. Diamond valve is located after demand regulator (at intake). Question -- what is the cause of the additional ignition? I think it occurs BTDC, but unsure. Could a faulty intake valve result in this? Wrong spark plug? too rich? Currently, the mag ignites ATDC. Terribly confused. I notice the RPM's increase gradually before this "double fire" event happens. I can send pic and/or video if needed. Tony Skowronek, Southern :bonk::bonk:Ohio.

Bill Hazzard

Last Subscription Date
The ignition should take place before TDC, 5 or 10 degrees advanced should be fine for running slow.

What is happening with your engine is that as the engine speeds up the fuel mixture will get lean. When it does that, the burning fuel in the cylinder will burn so slowly that it will still be burning when the when the air and fuel is transferred from the scavenging side of the cylinder. That lights off the charge that is in the scavenging side of the cylinder which is what you are experiencing.

It is common for two cycle engines running on propane to experience this. Running the engine richer helps as does throttling the air down so it makes less power. A load on the engine helps as well.

Brian Lynch

Last Subscription Date
McEwen Bros. engines run better on a hot tube. That is how they were designed to run. They will run OK on a mag too. If you have the hot tube set up, try a 6" tube first. Work your way down to 4" to see how it reacts. Long tubes fire earlier, shorter tubes fire later. Where you heat them makes a difference too. Heating one at the bottom will fire sooner than one heated at the top end
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