With water ingress obvious - the carb tops have almost rusted through, the exhaust pipes have rotted out, and the engine is severely corroded, I wasn't expecting too much from the little twin. The inlet rubbers had come away from the alloy plates that mount to the head, which made it a little hard to get it running.
But today I decided to check the compression to get some idea on the internals, it wasn't pretty. 50psi was about as good as it got. That's why, in my opinion, it's better to get an engine running before strip down, but each to their own. Once it's running, it'll clear out a lot of the crap hanging round inside (and there was a lot of rubbish in the ports!). It puts pressure on the prison rings, which start to scrape the bores properly. Fuel/air mixture going through helps to clean valve stems and seats. All of this should help the low compression and, if it runs, gives you some idea on the bottom end. If you pull apart an engine without having heard it, you'll probably end up replacing every component inside for peace of mind.
With a battery hooked up, the ignition switch bypassed and a squirt of carb cleaner down the inlet ports, I spun it over on the starter. She tried to fire. That's encouraging!
It was time to attempt a repair job on the inlets rubbers. I cleaned up the badly corroded, alloy plates which bolt to the engine with a wire brush, and wet & dry. Then I made sure the inlet rubbers, still mounted to the carbs, could turn on the carb flanges and cleaned them up too. Using two-pack body panel glue, I stuck them back into the right position and placed some heavy steel on top of the carbs while it set.
I'm not saying this is a permanent fix by any means, but it should be ok to try the engine. After four hours, the glue had set solid (although it takes 24hrs to set completely). I whipped the float bowls off to check the internals, surprisingly they were clean, and refitted them without removing jets etc. For now I just want to see if the engine will run, cleaning can take place later.
With the inlet rubbers able to turn on the carb flanges, I refitted the O-rings (still supple) with grease and bolted them back to the engine.
With throttle cable back in place, I hooked up a big syringe full of fuel and filled it. The float bowls filled up with no flooding, and it was time to try it. The choke lever is currently seized, so more carb clean sprayed in served as the extra fuel needed. Before long she was running, and sounded sweet! No knocks or rattles - I'm stunned!
After running it for a while, I decided to check the compression again. This time it registered 150psi on no.1, and 85psi on no.2. I did run an upper cylinder cleaner through the right cylinder to see if it could be increased, but no improvement. That's ok, it runs, it idles and sounds great. A top-end rebuild will probably sort the mechanical side of this little beauty out.