I'm 73 years old and have seen a bit of transition with outboard systems. If this is a carbureted engine I would tell you to run some carb cleaner through it because in this age of ethanol based fuels water can clog a idle system where flow is not as brisk. Check fuel filter and gas tank for...
You never stated where you set the high seed jet, if in fact it is adjustable. Mercurys of that time period had very sloppy chokes. They even had to put holes in the lower cowling to drain the copious fuel.