When you are saying it doesn't engage, do you mean the starter won't crank the engine, or the engine won't fire?
I assume you know that you need to work the foot stomp linkage to push the starter gear into engagement with the ring gear. I also assume that you know that you need to hook up the ignition coil for it to spark. The starter switches can go bad. You should at least get a clunk with the jumper cables, but they can be too light, especially with a 6 volt battery. However, I was able to crank over my 302 with heavy jumper cables just fine.
