Go Kart Project¶
My friend and I aquired a YerfDog 3203 gokart from someone on Facebook whose son outgrew it. It came with a 8hp engine which drove a CVT to a jackshaft with a sprocket and chain running to the live axle driveshaft. When we got it it had no seat, but it was driving. We drove it around for the day using foam floor tiles as a seat, and the brake band would fall off of the drum when you hit the brake pedal. We decided to replace the engine with a Predator 420cc 14hp engine. With our plan of almost doubling the horsepower, we decided to start taking it apart to put in the new engine. For the next year and a half, the go kart did not drive again.
In taking the go kart apart, we learned that the driveshaft was badly rust welded to all the parts including the sprocket, the keys, set screws, and brake drum. The only thing we could do was cut the shaft out and order a replacement. When test fitting the engine we leared that the engine did not fit in the frame, the driver pulley of the CVT did not fit on the crankshaft of the engine, and jackshaft for the driven pulley and sprocket. We purchased a new driver pulley and then the go kart sat for six months.
Coming back as freshman enginering students who had completed their first semester in college, we decided that we needed this thing to drive over this winter break. We worked in my garage every day for two weeks straight, from at 8am and staying until 1am. It was freezing so we had setup two electric heaters in our work area to keep warm. We cut parts of the frame to fit the engine and made modifications to the steel plate which the engine sits on. We didn't have a proper way to make precisely spaced holes to mount the engine so we used a hand drill, angle grinder, and dremel to mount the engine onto the plate straight. We tried to remove the driven pulley of the CVT and the sprocket from the jackshaft, but it was not coming off and we didn't have proper tools to take it off. We decided to modify the steel bracket that the jackshaft mounted to and weld it onto the engine mount plate. Getting the new driveshaft on was a pain and took forever to get the sprocket and brake drum on it and then putting the shaft through the two bearings. We then decided to remove the governer from the engine, no we did not just remove the governer springs, we decided to open the crankcase and remove the governer flyweight assembly from the inside. Removing it was easy, but the camshaft also fell out, and we put everything back together.
We spent a while hooking up the throttle cable, hooking up an old motorcycle battery for electric start, and wiring up the kill switch to the driver seat. At this point we didn't have breaks either. We took it outside, at 11pm, and I tied a rope around a tree to the go kart because I wasn't sure where it would go. The engine wouldn't start and the battery died, we started pulling the pullstart. We heard a loud pop noise and saw a firey burst a foot long shoot out from the muffler. We took apart the crankcase many times, emptying and refilling oil each time. We made changes and tried to diagnose the issue. When taking the camshaft in and out, the connecting rods would come out, so we had to remove the overhead valve cover to put them back. We stripped the threads in the engine for the bolt that holds the overhead valve cover on. We dealt with the issues for three days. Oil was everywhere and I stepped in it and was covered in oil. We messaged a friend who can rebuild engines in his sleep. We researched online and figured out that we didn't time the engine correctly when putting the camshaft back in. We tried to fix the timing and after many tries we learned that there was a marking that showed us how to time it properly. As we finished that up, we saw that our friend messaged us back and told us exactly what we did wrong. What took three days of us messing with the engine, scouring online, and youtube - he knew right away. We put it together, put new oil in it, tired it to the tree, and started it again. It was a good thing we tied it to the tree because it turned out that we had hooked up the throttle backwards so it went full throttle on startup.
The go kart now drove. It sometimes had electric start, no brakes, a plywood seat, and oil leaking from the overhead valve cover. It had some issues, but it drove. We drove it a lot and we dealt with a few issues but we had fun driving it.
We came back for summer break and worked on making it work reliably and make it less jank. We drilled out the threads for the overhead valve cover to M8 because that was the only tap and tap drill we had that would work. We used an adjustable wrench as a tap handle and had to vaccumm the metal shavings we had made. We learned that the driver pulley of the CVT was mounted backwards which is why we kept breaking belts, we flipped it around and as a result we moved the driver pulley's jackshaft to make sure the belt was aligned. We cut the jackshaft mount bracket out and welded it where it needed to go with the proper tension in the chain and belt. We put a new brake band on, since the previous one had no pad material left, and hooked it up. We did some upholstery with some walmart fabric, foam, spray foam, and our plywood seat and we made ourselves a comfortable professional looking seat.
Our go kart worked reliably. We drove it for a while by my house, we took it to my friend's house, and to a friend's horse track. We had a lot of fun driving it, we climbed some very steep slopes and the go kart had no issue. It never bogged down and climbed steep slopes with ease. With two people sitting in it, weighing approximately 350lbs combined, I was surprised how fast it could go and how well it could climb steep slopes. With our calculations, we estimated that we reached ~55 miles per hour with a 100lbs driver on a loose gravel horse track.
On the last day of driving the go kart, I was driving towards the garage and noticed that my foot was all the way on the throttle and it seemed like it lost power. We parked it and tried to restart it and the engine was not turning over properly. We did not have time to diagnose and fix the issue as we moved to other projects. We got our fun out of this project, and next we will repair the engine and then sell the go kart for the next person.