Before I put the carb back on, I took one last look and found that the throttle butterfly was different from the 7.5 to the 9.8 also. The three small holes in it that let air through at idle are slightly larger on the 9.8, so I swapped that part too – easy – it took about a minute. That’s why its minimum idle speed was faster with the 9.8 part in place and wouldn’t shut off with the throttle.
I put the motor on a boat and it runs great at all speeds above idle. I adjusted the low speed needle for best running and with everything I’ve done, the idle is much improved, but still not the best at slow trolling speed, but I can idle it down much more before it starts missing occasionally, and I think it will be fine for my neighbor to duck hunt with. It idles slightly better without standard plugs instead of surface gap.
It has a starting quirk. When it’s warm, it doesn’t want to start easy unless the low speed jet is adjusted richer than where it will idle fairly good, but choking the warm engine starts it right up. Given that, as you may expect, if I stop it with the choke instead of turning the throttle way low, it starts right away without the choke on again, because it pulled in a shot of gas as it shut down. It just seems to need to have a bit more gas in the system to start. The stop switch is disintegrated so I disconnected it.
I’m thinking the really low idle issue and the starting quirk is either reeds or crankcase seals, and I have no desire to pull and split the engine block to get at these parts. I did that several years ago to change a crankshaft bearing and a piston on a similarly constructed Mercury and don’t want to do that again, so I’ll explain the situation to my neighbor, and if his nephew has trouble living with it, I’ll loan him a motor for this year’s duck season.
I put more time into this than I wanted to, but I did learn a lot about the differences in the 7.5 and 9.8 carbs.
Thanks for all the suggestions!
Dave
