Author Topic: Pondering using an 18hp oppy and mower gearbox in a gokart build, have Q's  (Read 2023 times)

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Offline JennyC6

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I'm planning a somewhat unorthodox gokart build that will be using an 18hp vertical crank Briggs opposed twin and a 6forward 1reverse shift-on-the-fly gearbox. Engine'll be mounted up front under the driver's feet, trans in the back, with swingarm rear suspension to make it all work.

Aside from the obvious things(Making sure the CL Special runs properly, getting the pulley ratios right, etc), what mods would be best to do to the engine to make it work?

* I'm expecting the kart to be fairly hefty, somewhere around 150-200 pounds without rider and nearly 500 with my big backside seated in it, and it's going to be used in all manner of terrain up to and including sticky mud, tall grass, snow, et-al. I might even use it to pull trailers from time to time. As such, low end torque is priority the first. I'm not looking to boost the revs much, if at all, so I'm not concerned with it flying apart on me or anything. I'll get my top speed out of the transmission(Between the two belts that'll be in play and the final chain drives going to the drive tires I have all the wiggle room I need for this) rather than engine RPM.

* Cooling. These things aren't designed to go very fast and I expect this kart to be able to run 30-45MPH on the top end. In doing so, however, itt'l be screaming along against the governor under pretty hefty load(And possibly for extended periods of time if I decide to take it out on 5-mile-each-way trips to the nearest gas station for snacks). I don't want to overheat it, and the cooling ductwork is designed for effectively stationary operation. Will it be ok on stock ducting or should I look into making a ram air scoop to force air into the cooling ducts at speed?

* Oil mods are a given in this application, yah? Even with oil mods, will I face oil starvation issues during hard cornering? Tilting? Kart will have four wheel suspension and a fair bit of body roll, after all.

* Is the fuel pump on this engine good to pull fuel from a tank that's mounted six to eight feet away from the carb, even when the tank's downhill from the engine? Currently I'm planning on putting a 2-3 gallon tank right up front, with the engine just behind it, to minimize stress on the fuel pump but if they can reliably feed these things with relatively remote mountings I'll relocate that to the rear and double its size.

* Exhaust wise I'm lookin' to make it sound good more than anything else(Though if I can buff the torque with the exhaust I will). Gonna run pipes out the back or dump just in front of the rear swingarm mounts, so plenty of piping to play with. Ideas? I live far enough out in the sticks that dB level isn't a concern and am currently leaning towards more or less just straightpipes with a crossover underneath the floorpan to help with scavenging. Found an oppy on YT that had been straightpiped simply by removing the factory muffler and it sounded pretty sweet.

* If I do replace the flywheel, I want more or less the same inertia that the stock one has. Stronger would be nice, though I don't plan on revving high enough to risk exploding the stock unit so it's not a necessity(And there'd be a pretty sturdy steel floorpan between the engine and the driver so even if it did go bang I'd be fine), but the possibility of tweaking the timing does mandate an aftermarket flywheel as these things are fixed from factory. If I were to go this route, what flywheels would work best?

* Charging system up to the task of running a pair of sealed beam headlights, pair of LED taillights, and a basic car radio? I don't care much for road legality...out where I live cops rarely patrol and even when they do they tend to not care if the oncoming vehicle is road legal as long as it's not visibly a hazard...but I do plan on running at night so having the ability to see/be seen is important, and I love my tunes besides.

Thanks for your time!