1969 18hp Fastwin gear jumping issues

[alaskanick]

Am having issues with a 1969 18 refusing to stay in gear under a load. Both I and a professional mechanic(whom I've since fired for wasting my time and money, and then getting snide) were sure it was the gearcase. BUT It's apparently NOT in the gearcase--same symptom after a gearcase rebuild and then after subbing in another complete lower unit in excellent condition. And it's NOT the shift linkage. No matter how you cheat the adjustment, it won't stay in forward gear. It WILL stay in gear with no load (with a hose hookup) but soon as it's on the boat, jumps out above idle. Could it be the drive shaft bearing? I'm looking at the exploded parts diagram and that's all I can come up with. I am currently away from the motor but trying to solve and and find appropriate parts. She's a good motor otherwise with perfect compression and one-pull starting. I'm not being cheap. I just like keeping old guys like this running. ANY IDEAS?

Alaska Nick

 

[flyingscott]

Does it bang out of gear or just suddenly rev with no warning. If it is fhe second one look at the prop hub slipping.

 

[alaskanick]

Thanks, Scott, but it does it the same with any prop (I've got four, and think I know a bad hub anyhow) and it's the first scenario. But that was a good thought. Sure appreciate your input.