Re: Jack shaft drive with smaller 4 cycle engine
I believe you're on the right path, similar to one of my builds. The 4-stroke Robin/Subaru 1.4hp engine or Honda XH35 are two excellent choices. Key to an easy install is its 76mm centrifugal clutch and the pocket bike 5:1 transmission. Tranny bolts directly to these engines as well as many others sharing the same size clutch. With your skills, a simple steel flat plate can be bolted under the engine, then welded or bolted onto the bike frame. The tranny adds 3.5" to engine width. If you center the engine, it will position far right and need a jackshaft, your bike will need three chains. When positioned more to the left, a 206mm wide bottom bracket from Sick Bike Products (SBP) will align tranny sprocket with the outside chainring perfectly. You'll then need only two chains, engine drive and pedal chain. HOWEVER, the inner chainring needs to be positioned 1.75" inward. Positioning the engine more to the left allows you to use your local bike shop's bottom bracket. The outer chainring will move closer towards the inner chainring and frame.
You might consider a 2-stroke engine with 76mm clutch. Its advantage is that you can position it 360 degrees, even upside down. I believe you can't do that with a 4-stroker, but I may be wrong. It's much easier to fit an engine in the triangle if you can hang it in any position.
Gearing? A super low First gear allows you to climb very steep hills, while at the same time Eighth gear keeps your engine from screaming its life out in high gear. I use T8F pocket bike chain and sprockets for engine drive. It makes for lower gearing while keeping the chainring about the same size as a stock bicycle one. Using 11 tooth sprocket on the tranny and a 72-tooth chainring spring allows for 32.725:1. Use a 24-tooth inner chainring. Depending on what your hub sprocket and ratios are will determine how low your overall gearing will be.
This information should help you plan what you need.
Good luck, Rodger
Last edited by 5-7HEAVEN; 06-01-2013 at 12:13 PM.