Minibike Rebuild

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The Story

I found this minibike on craigslist for a price that was too good to pass up. It was mostly together when I got it home and took a look, but definitely needed some TLC. The carburetor was off the engine and the air filter was missing. It had no key and something was wrong with the electronics. It wasn't in working order but at the very least was decent rolling chassis. So I set out to fix it up. A big thanks to the people at MyTractorForum.com and DiyGoKarts.com who helped me to figure some of this stuff out.

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MiniBikeRebuild.jpg
Minibike Rebuild

Engine Upgrade

Since the engine was small for my liking, the carburetor was hanging loose from it, and the air filter was missing entirely I decided to reuse a 5HP Briggs engine that came off my thunderkart rebuild.

5hp Briggs Missing Shroud


The motor was in pretty decent shape. The only problem was it was missing the engine shroud with recoil starter. A big thanks to the guys over at MyTractorForum.com for helping me to figure out exactly what size this engine is and search for a replacement blower housing. Without the shroud the engine would overheat and damage itself after running a while.

5hp Briggs


I finally found a shroud locally, but it had the wrong kind of recoil starter on it. So I've removed the recoil for now and have been starting it with a drill.

Raising the Seat

The new engine fit inside the frame with the air filter removed, but just barely and the seat support blocked the carburetor input.

5hpBriggsOnBike.jpg
CarbBlock.jpg
5hp Briggs on the Minibike Frame Blocking Carb Intake


The only logical choice, aside from buying a new engine, was to raise the seat. I used a cutoff wheel to cut through the weld points, made some extension stands out of some scrap 1" square tube , and welded everything back together.

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New Seat Stand in Place It Fits!


Once I got the engine mounted and hooked everything up I took this thing for a test ride. Now, I've put on a few pounds over the years, but not enough that I should be bottoming out a 450 lb suspension spring. So out goes the spring to be replaced by a solid square tube.

Solid Bar Suspension


Engine Mount

The frame had 2 plates on the bottom. One was a little thicker and had some pieces sticking up for the original motor. It is mounted to the top side of the lower frame bars and didn't leave enough room for the new engine even with the seat raised. The second plate is slightly thinner, but mounted to the bottom of the lower frame leaving just enough room for the engine to fit in.

Engine Mounting Plates

The problem with the lower plate is, it's further forward and not wide enough for the engine mounting bolts. The engine bolts actually straddle the plate. So I got a 3/16 inch thick flat bar, 2 inches wide and cut 2 pieces long enough to fit the engine bolt pattern and drilled slots to give the engine some wiggle room when mounting. This setup allows the to rattle a little more than I like, but it holds and the tensioner helps keep the chain from jumping off the gears. I plan to add a thicker plate at some point, but this works for now.

Engine Mounting Bars

Chain Tensioner

I'm not sure if it was the length of chain I had to use or the cheap engine mount I rigged up, but I could not get the chain tight, even though it was a real pain to get on. So I decided to add a tensioner to try and keep the chain from jumping off sprockets. Ideally I'd get an idler sprocket to build a tensioner, but I had some bearings on hand. I cut a small plate to make a swing arm and drilled 3 holes in it. One to mount it to the frame, one to hold the bearing roller assembly, and one to attach a tension spring.

ChainTensioner.jpg
ChainRoller.jpg
Chain Tensioner Chain Roller

Because of the position of everything on this minibike I couldn't install the tensioner the way it should be. A tensioner should be on the side of the chain that returns from the engine sprocket to the axle sprocket. With my setup I would need 2 points of chain redirection to work around the frame. Even then I'm not sure I could get the chain tight without rubbing the frame.

Proper Tensioner Design

Sprockets

After hooking everything up and checking chain and sprocket alignment I couldn't figure out why it kept throwing the chain. Finally I noticed that the bolts holding the sprocket on the back wheel hub weren't doing much holding.

Loose Sprocket


The threads where stripped on the aluminum hub, so I drilled the holes and re-tapped them for 1/4-20 screws. They held for a little bit, but then stripped out again. I drilled 1/4 inch holes all the way through the hub and used about 4 inch long bolts with lock nuts to hold the sprocket on.

Long Bolts Through the Hub


It worked like a charm. The only problem was the bolts and lock nuts interfered with the coupler that fit between both back wheels to lock them together. Once I modified those everything went back together more or less as designed.

Coupler Modification


With everything back together, I could actually ride this thing now. The next problem was it didn't want to take off and go very fast at all. The clutch had a 10 tooth sprocket and the back wheel sprocket only had 26 teeth. Based on what I've seen you really want these go karts and minibikes in the 5 to 10 ratio of wheel teeth to clutch teeth. Since my back tires are 10 inches the biggest sprocket I was comfortable putting on was a 36 tooth with a diameter of about 6 inches for #40/#41 chain. Unfortunately I couldn't find one locally that matched the hub pattern, so I had to drill mounting holes in the new sprocket. I used a hole saw to drill a piece 2x4 so that half of it fit snugly into the new sprocket center hole and the other half fit the old sprocket.

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Sprocket Aligner Sprocket


I put both sprockets on my home made alignment tool and drilled mounting holes. I mounted the new sprocket to the old sprocket and then mounted the whole assembly onto the hub.

New Sprocket Mounted

Electronics

The bike came with a nice little set of meters and electronics. Unfortunately none of them were working and I haven't had a chance to test/troubleshoot them yet.

Minibike Meters

I found this wiring diagram on line, but I'm not sure if it's for my bike or not.

Wiring Diagram


More to Come!

Enjoy and feel free to report any problems or suggestions to longet@spider.dnsdojo.net.