[GB SISSON]

Vacuum leak? Try plugging the outlet to the wipers and spray carb cleaner or unlit propane torch around leak prone areas. That, combined with the low compression from sitting. At idle, drizzle some marvel mystery oil down the carb. While you are at it, add marvel to the fuel and oil. I have had two engines recently brought out from many years storage, where the compression went from poor to good with this treatment. After a few hundred miles of hard driving compression went to excellent. One more thought is something I tried on my H flathead six with very sticky intake valves. (stored 30 years). At idle, fog some WD 40 down the carb until it almost dies. repeat, repeat. If you have a vacuum gauge, have it hooked up while doing all this. I have found that seeing the results of your efforts on a gauge instead of 'thinking' there was some improvement is a far better way to go while troubleshooting. Keep us posted. Good luck and post up a pic or two of this merc. Edit: Do the compression test on a warmed up engine with throttle fully open.