I have a 1960 Glasspar G3 powered by a 1992 Mercury 75 HP 3 Cylinder. The engine has been running well since I got her out this Spring, but in the last week started running rough at low speed. Then, I took it out last Tuesday (August 6). It idled roughly but smoothed out as I accelerated. However, I could not get it run faster than 20 mph at full throttle. Normal full throttle speed would be between 35 and 40 mph. When I came in to land, it idled so poorly I had trouble landing and it finally cut out. It finally got to the point where it would not idle slow enough to land. The boat had set idle about 4 days prior to my August 6th outing and I surmised that some fuel had gummed up one of the carbs even though I have been careful to add an enzyme fuel conditioner to mitigate gumming caused by ethanol additives.
I discarded the fuel remaining in the fuel tank and replaced it with "recreational fuel" which here in Michigan means no ethanol additive. I also added Sea Foam at 1 ounce/gallon to stabilize the fuel and help clean the carbs out. Anticipating that I would need to shoot carb cleaner into the carbs to flush out gumminess, I removed the engine cover and started the engine. It fired right up but I was surprised to see fuel squirting out of the bottom carb vent jet in a strong steady stream. My question is what would cause this? Would I be correct to interpret the fuel squirting as indicating that the carb float is either stuck or fuel soaked and not shutting off the fuel flow when the carb bowl is full? I will appreciate any help/suggestions readers may have. This is the first large Merc I've owned and am a bit intimidated by the amount of disassembly required to get to the bottom carb. Thanks.
Yup, sounds like a stuck/leaky float needle to me. At least the clamshell engine cover comes all the way off, so you'll have decent access to the bottom carb. Just a matter of pulling everything out of the way to gain access. You'll probably have to remove the starter unless it's not in the way of the bottom carb.
This diagram is close to your engine's year range:
Likely you'll only need the inexpensive carb gasket kit and a new float needle. Floats are available separately if you need one. The "repair kit" is quite a bit more expensive and has a lot of replacement parts that you won't really need. These are simple carbs, with typically uncomplicated problems.
It could be something as simple as a piece of "trash" stuck in the float needle/seat. Be sure to check your fuel lines (inside and out) for deterioration, as your engine is plenty old enough to have that issue (especially running ethanol fuels). Change the fuel filter while you're at it.