A recent project was figuring out how to adapting a YZ-125 motorbike engine to a go-kart frame.
A kart usually has two parallel tubes to mount an engine to sometimes a flat plate that is welded into the frame. A motorbike on the other hand has a frame that encircles the engine or even uses the engine as a stressed member forming part of the chassis. The kart for this engine had a flat steel plate with a few slotted holes to bolt the engine in place and was situated behind the axle close to the centerline.
What was required was an engine cradle that would bolt through the lower horizontal engine mounting holes on the engine and adapt that to the horizontal mounting plate of the kart but still had to allow forward/aft adjustment to set the chain tension. Here is what I came up with for fabrication out of ~10mm plate aluminium;
The two vertical plates pick up on the rear and lower YZ-125 engine mounts. The plates could be extended further forward to pick up another engine mount but shouldn’t be required for this installation. The lower plate has a number of tapped holes so bolts can come up through the kart into and thread into the cradle. The vertical plates are offset to the left to allow clearance for the chain drive.
An extended sprocket will be needed to extend the drive line beyond the magneto cover to allow it to come forward to the axle.
This could be fabricated by machining the teeth off an old sprocket, to use the splines. Then weld a tube on to extend the drive-line. A new sprocket would be bored out to match the tube and welded on the end taking care to keep the whole assembly concentric.
Other items to consider in a conversion like this are:
- fuel supply – a fuel tank needs to be above the carburetor or use a diaphragm pump from a marine outboard motor. The pump also needs a pulse line from the crankcase. A hole could be drilled and tapped in the crankcase where the reed block bolts in, this would allow a rag to be stuffed down the intake to prevent swarf falling inside the engine, then a hose fitting could be screwed in.
- gear selector – the original gear lever could be cut down and a linkage added, either cable or control-rod, to a lever beside the driver.
- clutch – a clutch lever can be added to the control column or if the gear-lever is the correct size, a motorbike clutch lever can be added to the end. This would mean the driver has to take one hand of the wheel when using the clutch but since a motorbike has a constant mesh dog engagement gearbox the clutch would only be needed when launching or coming to a complete stop.
- exhaust – this needs to be re-routed to clear the frame and exit at the back of the kart. A muffler is good for your hearing and keeps neighbours happy. Now two stroke exhausts are a simple piece of pipe… but their dimensions have an incredible influence on the performance of the engine. An small change can have a large effect on the power delivery. Try to make minimal changes to the exhaust. The best option is to cut the pipe perpendicular to the exhaust flow, rotate the cut and re-weld without changing the length. If you want to experiment with building your own tuned exhaust I highly recommend “Two-Stroke Performance Tuning” by A. G. Bell.
- The electronics – most dirt-bike engines have a magneto ignition so don’t require a battery just an ignition coil and kill switch.
The two stroke motorcycle engines have an incredible power to weight ratio, great part availability and are fairly simple to rebuild (if it’s a top-end only rebuild). If you visit youtube you can find numerous videos of karts with large road-bike engines of up to and over 1000cc in them. This is a waste of time unless all you want to do is smoke tyres. A 250cc two-stroke dirt-bike engine has more than enough power in a wide usable power band, can still easily smoke the tyres and is a much lighter package which gives great acceleration & handling. Riding in a kart equipped with one of these engines when the powerband comes on can be an incredible (exciting & terrifying) experience.