After pulling the diesel engine out of the bike, I tried positioning a couple of other NOS engines in the frame to see how they might work. First was a Tecumseh OH195SA. It would fit great except that the carb is situated a fair distance away from the rear of the engine, and I’d have to raise the engine up about 2 inches or so for the float bowl to clear the jackshaft. That meant that I had to remove the muffler. Regardless of how it might fit, it is not a very attractive engine. This one was ultimately a “no.”
Next up, I tried my new Honda GS190 engine, which I recently purchased without a firmly defined use in mind. I just knew that the vertical cylinder would make it easier to swap into an existing motorcycle frame, and look better, too.
For those who are not familiar with it, the GS190 (mfg ~2008/09 to ~2018) is similar to the cheaper GC engine architecture, but was upgraded with a cast-iron cylinder liner, 3-piece oil control ring, and all-ball-bearing crankshaft, plus cast alloy valve cover. It retained the same nylon cam and toothed rubber timing belt, so this is strictly a run-as-stock engine. But it’s lighter, smoother, and quieter than a GX clone/Predator, and I really like that it has a vertical cylinder. It’s also somewhat unusual, which makes it a little more interesting. This one is NOS from very early in production. The serial number is #1795, and it has the “6.0” on the decal, which they did away with fairly quickly, because it is actually a 5.2 HP engine by ANSI test methods.
I removed the rear-mounted fuel tank and airbox, then set it in place so the crankshaft was the same distance from the jackshaft sprocket as the diesel had been. Not gonna lie, I don’t dislike this. It doesn’t have the seemingly “made to fit” aesthetic that made the 173F look so cool in this frame, but this combination will probably be a much more practical and useful end result.
I have not made any firm decision at this point. I’ll sleep on it for a few nights before I decide how I want to move forward. Several people have expressed deep disappointment I’m not sticking with the diesel, and I feel as though I’m letting down everyone who has expressed enthusiasm for this build. I suppose I will need to mess with the diesel engine again at some point, even if just to sell it off to another tinkerer.