Doodlebug of Doom: Saving a Superpowered Minibike
If you’re already planning to skip this article because it includes the word “Doodlebug” in the title, then I implore you to read on. What we stumbled onto is no kid’s toy: some deranged individual took a cheap, Walmart bike and then strapped an actual go-kart racing engine to it, lost interest, and put it on Craigslist. And then we picked it up because if it’s one thing a man needs in his life, it’s an interesting story to have told at his funeral.
Let’s start this story off with what exactly a Doodlebug is. There are plenty of recipes out there for mini bikes, but they all share a few common traits: Most are powered by generator type engines (usually under 100cc). Chain drive with an open centrifugal clutch powers the rear wheel. Many have no suspension. Most have only a rear brake, drums in the olden days, or disc today. They are basically cheap engines with two wheels and a seat bolted to them. They are intended for kids, or light adults to do some basic riding; nothing more than bombing around the backyard or maybe the woods if you have one nearby. But as with many toys today, what should be for kids has been mutated into a weird sub-culture by adults with all sorts of odd trends arising. See it turns out while the 97cc engine is adequate for kids, adults find the 4.5 hp to be far too little. The easy engine swap is a 212cc Honda GX200 motor which makes 6.5hp. Harbor Freight sells their own knock off version called the Predator 212 for $120. Dropping one of these engines in isn’t even an afternoon’s work as all one has to do is remove the old engine, bolt in the new one, and hook up the clutch, chain, and throttle cable. That’s it! Best of all, the 212cc engines are used in go-kart racing and have a bunch of aftermarket support. It’s possible to more than triple the 6.5 hp output given the right wallet.
A good friend of mine has a couple of these and we’ve had a blast riding around the dirt trails near his house with them. I decided I needed one of my own and while stumbling around on Craigslist one afternoon, we landed on the motherload: A neglected Doodlebug with an 18hp kart engine that no longer ran. The price alone was less than most stock bikes were selling for used. Even if the motor was a dud, the parts alone would pay for a Predator replacement. Best of all, I am very familiar with these engines: I spent a summer as a go-kart mechanic for a public track and all of the cars used these exact 6.5hp Honda motors. Hell, the first ever 4-stroke engine I rebuilt was a Honda GX200, so I know my way around these things. There was no way we were passing on this insane toy.
The bike appeared in the back of a truck and looked pretty much there. The engine was out of the bike, but it’s only held in by 4 bolts: child’s play popping it back in (working on the self contained engine outside the bike is actually a lot easier than working around frame bits, so this was quite convenient). Along with the bike and engine, we also got a custom made, large capacity gas tank (the stock tank might as well be a thimble with this engine) and a few leftover engine parts from the engine’s first build thrown in with our purchase. We bought the whole package for under $300 and brought it to our garage.
The engine itself is actually Harbor Freight's previous knockoff engine. According to the seller, he and his friends would buy a bunch of them, dyno the engines, and return the motors that made less power. Inside this engine is a forged connecting rod, high compression piston, upgraded camshaft, and new valves and valve springs. A custom exhaust header and larger carbeuretor round out this engine package. While we have not opened this motor up to verify these claims, a peek under the valve cover did indeed show upgraded valves, springs, and retainers. I wish I had more details on this engine, but it was built a number of years ago and the seller didn't remember all of what went into it. The bottom end alone would cost as much as we paid for the whole bike, so we're still quite pleased with it.
Once we were home, we jumped right into coroner mode, trying to find out why the motor didn’t run. Tugging on the pull starter revealed compression was there. The oil was brand new and looked like it had never been run on. The gas oozing from the carb smelled recent. Pulling the plug revealed we were not getting spark. Well that’s an easy fix! See in these GX200 motors there are three electrical components: the spark plug, the power switch, and the ignition coil. Some motors also have a low oil light, but ours did not. Our plug looked pretty good and the power switch had continuity. Obviously, we had a bad coil. We picked up a brand new coil from ARC Racing which stocks all sorts of go kart engine parts.