What happens when you insert a Honda CB350 quad engine into a monkey?

Have you ever thought how cool it would be to put a Honda CB350 Four engine in a 1973 Honda Z50 Mini Trail bike? Even if the answer is ‘no’, don’t worry: long-time Honda collector, builder, racer and enthusiast Bob Guynes took it upon himself to create just this combination. At Mecum Las Vegas 2022, a number of bikes from Guynes’ collection crossed the auction block without reserve and sold, including this not-so-mini mini-bike.

From the factory, the Z50 churned out a blistering 2.5 horsepower from its tiny single-cylinder engine. Guynes loved building bikes and decided that just wasn’t enough. Could it fit Honda’s smallest inline four-cylinder engine in this frame? Apparently he made it work—adding extra downtubes, custom motor mounts, and making that motor a stressed member for that heck of a minibike. We can’t tell you how it rides, but it at least looks pretty fantastic.

It’s also unclear what kind of power this particular custom Z50 produces. A stock CB350F produced a claimed 34 horsepower at 10,000 rpm at the rear wheel, but Guynes was well known for tuning every bike he got his hands on, so the true horsepower is unknown. This bike is listed as having been sold at Mecum Las Vegas 2022, so we can hope that the new owner will dyno it and post those numbers somewhere (or, you know, email us about it). We can always live in hope, can’t we?

Other customizations include a twin-carburetor setup to keep things a bit simpler, as well as a four-into-two exhaust setup. The stock parts of the Z50 include that adorable little peanut of a fuel tank, as well as the saddle and wheels. Tommaselli Cross handlebars give the front end a more serious look, and there’s an adjustable center stand to make your life easier. According to the listing, it “must be sorted”, although it’s not entirely clear what that means. The details suggest it could be made road legal with the installation of things like turn signals and lights, but it’s unclear if any additional work needs to be done just to make this bike usable in general. Here’s hoping it’s as fun to drive as it is crazy to watch!

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