Small Engines (Lawn Mowers, etc.): I have an older B&S Fast twin..., carbon buildup, hp motor

Question
I have an older B&S Fast twin 18 HP motor in a boat. The engine has a rattling noise similiar to what I have heard when a pull start engine starter is not releasing.
If I remove the sparkplugs, and rotate the engine, the engine does not seem to bind in any way, but I get a "click-clunk" noise on each revolution. I can't spin it fast enough to get the rattle
I am wondering if the electric starter is not disengaging, and is causing the  noise?
Can you advise me on how to remove the starter so I can inspect it?
Do you have any other advice?

Answer
It is a common problem for these engines to get a carbon buildup inside the cylinder heads and this will produce the noise you describe.  If the starter was staying engaged, I suspect you'd hear a grinding or screeching sound.  You should be able to start the engine and take a look at the starter gear to see if it's still engaged.  Hope this helps.