81 228 mercruizer,,need help deciding fuel or ignition problem

this is an 81 228 merc..305 with quadrajet,standard points and coil..
the boat is running strong for about 15 minutes,then u can tell is a little low on get up and go..them bam amout 1500 rpm is all she will do but will stay there all the way back to the dock...seems i can let it cool for an hour and it will repeat the same thing
exactly..while running it runs fine,, comes out good,four barrel opens smooth ,no pops or sputters....seems almost to be a problem with something heat soaking,,,
so far i have done a full tank to carb inspect,,changed points and condenser,,,
i am not sure if it could be the coil breaking down or high fuel temp???
i keep thinking lean fuel would give me a backfire or something at full throttle..
i did run with the hood off today and it seemed to run about another 5 minutes or so..
any help would be great guys

Re: 81 228 mercruizer,,need help deciding fuel or ignition problem

look at the coil. if it's buldging a little at the bottom and feels a little too hot try replacing it.