Hi Biscuit,
We have a few of these motors at our club, and all of the new ones that belong to new members had this revving or nothing problem. I can't speak from first hand ownership of one of them, but have been using marine IC's since 1963.
The SC engine is very tight when new at the top of the stroke, which they mention in the instructions.
I have heard of people actually skimming the piston sides to remove some of the binding, which is part of the design of an ABC engine. I have an SC21, which feels so tight when turned over by hand with no plug in as if it would tear itself to pieces when running. I don't know how the con rod takes the strain.
I have an OS 32 ABC engine that does not bind at all (which would suggest clapped out), with the plug removed, so no compression to misslead, and that starts first time every time winter or summer with no problems at any rev range. I would have thought that to have lost enough compression that it is noticeable, the motor would have to be almost minus the piston. It's also strange the the motor would not throttle down. If the air intake is closed, the motor should stop no matter what the mixture setting is. I have a few motors with marks on the piston and bore, and they run with no problems. They may be down on power, I don't know, and they are only used for sport not racing boats, so a slight loss of power does not matter any way. It is also normal now days, to loosen the glow plug about half a turn, and then re tighten it as soon as the engine starts.
I would put it back in the boat, and give it another try. Whats to loose?
Regards Bob
The post above was added while I was typing my reply.