[Putershark]

Low Octane / low boiling point fuels (meaning the actual level of the sample fuel) can start to boil in the fuel lines under extreme heat (mostly around the engine block area) and cause a "Vapor Lock" type of condition, thus starving the engine of fuel when under a load.

Now we have not seen much of this type of problem since the early to mid 80's and then the major cause was the reduction of lead in gas. I also saw that other posting about the alcohol blended fuels out west which can also cause such problems (due to the lower boiling point of fuels) and had a Coast Guard "flashback" when I fixed quite a few stranded boats over the radio that acted just like yours, by telling them to taking the fuel cap off to allow the system to vent some.

Now back then Mercury’s had the issue more than others due to being (not very logical) painted black of all colors. That color did not help the matter any and created a little reactor temp wise under the cowl of their engines and they really showed the problem in the summer months.

So I would also start looking for a vacuum leak of some kind around the different engine gaskets. Fire the engine up, let it idle (if possible) and try lightly spraying some “Throttle Body” cleaner around (be careful) the different mating surfaces (intake, injector /carb bases, etc.) and see if the engine starts to either increase in speed or stumble some from it “sniffing” the cleaner past the gaskets or a cracked vacuum line.

Trust me, it does not take much of a vacuum leak to cause major problem which usually only gets worse as the engine heats up and causes the gaskets or vacuum line to expand and take in air past the main air/fuel metering device, be it either the Throttle Body for injected engines or the base of the Carb.

You could very well have an electrical break down (ECU, Loose Ground wire, etc.) that can act the same way. Check all your ground connections and make sure your main start / running battery has at least 11.5 volts for anything under 10.5 VDC can cause a ECU to go “Brain Dead” and force the engine to run in what is called a “Limp In” mode.

Hope this helps ?

Dave