Interesting motor for sure.
I found Bob Grubbs on YouTube, and
it runs incredibly smooth.

https://youtu.be/acC8-Iz6qwI?si=iVd3_w6_w9u-uwGq

The distributor is driven off the cam shaft, so the rotor spins at one-half crankshaft speed. Thus the flywheel magneto supplies spark every revolution as per usual, and the half-speed rotor sends it to alternating sides of the engine. When it works, its OK. When it doesn’t, it can drive you around the bend figuring out what to repair.

As acquired, my magneto was stone cold dead, so I hooked up my buzz-box and was getting spark delivered to each plug as designed, but it wasn’t as consistent as I liked, so I did some rehab work to tighten things up in the distributor and eliminate what I considered to be excessive slop in the assembly. The housing is Bakelite and you have to be careful about applying pressure on it anywhere, so this is not a simple job, but it got completed and the rotor fit in the housing as well as the housing fit in the crankcase cover is much more stable than it was, so the rotor wobble is pretty much eliminated.

I can’t get my motor to run quite properly yet, but I think its a valve problem and not so much the ignition. I know that compression can be an issue in these motors, but I’m generating 55 lbs per cylinder so I don’t think lack of compression is a contributing factor. but I digress….

I haven’t figured out how the motor Harry is working on managed to be built and sold without the distributor assembly being on it, but that appears to be what has happened. Very odd. Would be interested to know what Brook Newcomb can tell you about it.

if worse comes to worse, you could hook up an Elto coil and try and run it like that. You’d be firing both plugs every revolution, but my Detroiters and Cross Seagulls run that was as well and since they are true four-stroke engines, it has no effect on how they run, as for every other revolution the plug merely fires at the end of the exhaust stroke and accomplishes nothing.

Hope this helps.

Best,
PM T2