Wasted spark in 2 cylinder engine

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xander janssen

Well-Known Member
Jan 5, 2012
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Hi all,

I have a question regarding a wasted spark scheme i.e. two spark plugs firing at the same time in a two cylinder engine.

In the book of Doug Kelley, there is the Titan, a 2 cylinder 360 degree engine. Here I understand the wasted spark idea: one plug fires at the end of the compression stroke, while the wasted fires at the same time, which is at the end of the exhaust stroke. This is exactly how Wikipedia explains it.

However, in the same book there is the Nash engine, which is a 180 degree configuration. This is also stated to be a wastes-spark design. Here I do not understand the wasted park idea. As the second cylinder is fired at the end of the compression stroke, the wasted spark is fired in the first cylinder at the end of the intake stroke. Wouldn't this cause ignition of the mixture? I can imagine that the mixture, although not yet compressed, is still able to burn. Or is it just not burning as it is not yet compressed causing no flame propagation.

Or does it sometimes fire the uncompressed cylinder and by that have no mixture for the following power stroke? This could explain the irregular sound that all the Nash engines in YouTube produce?

Wasted spark.png


Last edited:
Hi Xander,

I have two engines that run with wasted spark. One is the Titan that fires the cylinders 360 degrees apart. The other engine is a Bob Shores Silver Bullet that I modified to fire the two cylinders 180 degrees apart, like the Nash. The Bullet fires into the intake stroke, but the pressure in the cylinder, at that point, is too low to ignite the fuel air mixture. Both engines run just fine with wasted spark.

The engines firing 180 apart have a distinctive and somewhat uneven sound as the cylinders fire close together then there is a delay before they fire again. Pop-pop.................pop-pop............... The two cylinder John Deere engines fire and sound like this.


I have a hit and miss with a mag. To get a hotter spark I'm turning the mag at twice the engine speed so I get sparks at some very odd times. It works fine but every once in a while running rich, I hear an unusual exhaust note that I think is an early ignition on the intake stroke.
The exhaust cadence of a 180 degree crank in a two cylinder engine is definitely different just as it is in an old JD two cylinder engine. Your question about the possibility of firing both cylinders simultaneously with a wasted spark ignition is valid. Just looking at the condition in your diagram I would have to say Yes, it is possible. Is it likely to happen? I would say No for the same reason Chuck has stated. There was one instance when it could have happened on my Nash. The engine had been running at 600 RPM for about 2 hours under a small load and 170F coolant temp. The engine made a "pop" sound and quit running. I think it was more likely a sticky intake valve since my valve guides were tight on that build. Real Nash engines had a drip oiler on each valve guide and so I normally have been careful to put a few drops of MMO in the carburetor occasionally when running. I had forgotten to use any MMO that day. That instance has not repeated.

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