"The solenoid part number is 8
"The solenoid part number is 862656A3 (4 BBL Weber carb) and 862656A2 (2 BBL Mercarb carb). The list price is $294.35 for either version. I would call around for price on this.
Again, look for the thread I mentioned before so you can do as much as possible to eliminate this. The higher octane and cooler plugs should help a lot. I have gone through the whole list in that bulletin on these motors and on quite a few, it worked- on a couple, I needed to install the solenoid. That took care of it every time.
I've never heard that there was a detonation issue, just run-on, either from running the boats with this condition myself or hearing it from others, including the Mercruiser technical dept. That doesn't mean it isn't happening but I don't know that there have been a load of piston failures. Still, better to play it safe."