Topic: 1981 Hoda cb650 Cafe build

[konradnp]

So here's the gist, I have 1981 Honda CB 650 and trying to mod it to a cafe build but already having issues with the engine.

150,000 Miles (Pretty sure engine was never rebuilt) However the bike generally runs great.

Symptoms:
After riding for about 40min or more, and getting the engine nice and hot, I get white/blue smoke coming out of the exhaust and tailpipes. If I continue riding the bike after the engine is hot, smoke gets more and more prevalent. After about an hour of riding I will start to loose power at low RPMs, so much so that when not giving the bike throttle the bike will want to die completely.

Things to note: I have good throttle response through all gears and all RPMs

I can answer more questions that will help to diagnose. Throw em at me.

[Syscrush]

I don't think there's much to diagnose. Given the symptoms and the mileage, that engine almost certainly needs to be rebored, with new pistons and rings.

Have you done compression and leakdown tests?

[SOHC4 Cafe Racer Fan]

At minimum, if the mileage was not so high, you would likely need new valve seals and a valves lapped.  With such a high mileage bike, an overbore is likely in your future. 

NOTE: you can likely purchase a running low miles CB650 1979-1982 (1981-82 preferred because the head is different) cheap and swap it in.