'71 Buick I/O (with OMC outdrive and exhaust) has high RPM power problem

[mcclan9]

I have a 1971 Fiberform boat with a Buick I/O that has OMC stern-drive unit and exhaust. It has been stored on the coast for the past 2 years. When I'm accelerating from idle the boat gets up and planes slowly and cruises at 3000 RPM at full throttle for 2-3 minutes and the speed gets up to 23 mph by the GPS. Previous to this problem, the boat has cruised at 41 mph (2 years ago). Also, after that, the boat slowly begins to lose speed and when it gets down to about 17 mph it starts to back-fire. It can then continue to drive at about 1000 RPM for a few minutes (3-5) and after that I can accelerate and the whole process starts over. When I?m going 23 mph, if I back off the throttle to ? throttle and drive at 21 mph (still 3000 RPM) the boat will maintain that speed indefinitely. I have changed the coil and ignition resistor, changed points, condenser, distributer cap and rotor cap, rebuilt the carburetor, and changed tachometer. The distributer doesn?t have a vacuum advance on it. The fuel tank and filters are all new. Temperature gauge stays at 160 degrees. Exhaust port and 2 pee-holes are streaming good amounts of water. I have also checked the alternator voltage while the boat is at full speed and when the problem starts there is no change in voltage (14.2 volts). Does anyone have any suggestions why the loss of power and inability to go over 21 mph? Please help!

 

[Howard Sterndrive]

Re: '71 Buick I/O (with OMC outdrive and exhaust) has high RPM power problem

I was scanning your list of parts serviced for fuel pump... kind of sounds like a fuel delivery problem but don't just throw another part at it - check engine compression, check spark plugs for any signs of moisture (failing exhaust or riser gasket), check fuel pressure....
also, if there's a clear/yellow line from your fuel pump to the carb- check to see if it has any gas flowing through it - definite indication of ruptured fuel pump diaphragm

but really sounds like fuel pump...when you slow down, you give the fuel pump time to refill the bowl on the carb again.... once you empty the carb....the weak pump can't refill fast enough
it's backfiring because air-fuel mix is leaning right out as it runs out of fuel

you put a new fuel tank? are the fittings and lines sized big enough for the fuel requirements? 1/4" outboard stuff won't cut it...is there an anti-siphon valve on your boat - could be clogged
is the tank vent open? - wasps will clog it with mud

How clean is the flame arrester - engine room insulation likes to rot and make a dust that will clog it on old boats like that

good idea you had to watch voltage - sounds more like a fuel problem, but you could check voltage right at the + post of the coil - should see 9v or so