This rear view sketch shows the basic arrangement of the engine, primary reduction pulley system, and secondary reduction cog and chain system. The big circle at the top represents the engine, with the small v belt pulley on the end of its axle. The big, 14" pulley sits beneath, with a small cog on its axle. Below the cog is a freewheel with another cog on its axle, which connects to the rear wheel left hand side drive cog.
Here's the Honda GX31. Again, it's a 31cc, 1.5hp, 4-stroke, low noise, low emission engine. It weighs about 7.5 pounds dry. The centrifugal clutch engages at about 3,500 rpm, it's peak torque rpm is 4,500, and its maximum rpm is around 8,000 rpm. Complete specs can be found here. I bought mine at Roy Padgett's Small Engine Warehouse for only $180. That was by far cheaper than anywhere else I saw.
Here's a closeup of the centrifugal clutch.
I bought some extra parts from Tusla Engine Warehouse including a clutch drum, shaft, housing and bearings. Here's the drum.
The clutch housing is a thing of beauty, nicely machined and anodized red. These pictures show the bearings pressed in place, the shaft in place and the 1.5" v belt pulley mounted on the shaft. For these particular parts at Tulsa Engine Warehouse click here.
Here's the 14" pulley I bough from Grainger Industrial Supply. It has a 5/8" bore, that's the smallest available. The part number is 1L847. The second picture in this series shows the machined surface where the belt rides.
Little by little I've been reusing almost every part from a Quickie wheelchair I bought years ago at the local Goodwill store for $15. I plan to use the hub flanges and sealed bearings for the bearing mounts for my shafts. They're light and of high quality. I drilled 4 holes in each flange for mounting to the sheets of aluminum I'll use for a frame.
While my longer term goal is to mount the power assist on my full suspension recumbent bike, I plan to start out by mounting it on this mountain bike I pulled out of the trash at our nearby transfer station (the place where homeowners dump junk and yard waste). There was a good sized pile of bikes, and I can't believe someone would throw this bike away.
Here's a closeup of the steel hub that I brazed a piece of a coaster brake hub onto so I can attach a driving cog on the left side of the hub. No cog is on the hub in this picture, so you can see the groove where the wire retainer ring sits to hold the cog on.