Evinrude 70hp 1979, won't take off under load

[GrantAgnelli]

Hi guys, have a 1979 70hp evinrude on my 14ft ski boat, starts and runs like a gem, but won't take off under load properly, takes a long time to wind up if at all, strangely enough goes better with the engine cover off,has 110psi across all 3 cylinders, mixing fuel at 50:1 has rebuilt carbys that are not blocked and are assembled correctly, new pump tank and lines all ok, tested and confirmed good coils/plugs and caps, new cdi module, good earths, water pump ok, reeds are good, no air leaks, new manifold gaskets, prop is a 17 which I am told is the perfect size, and no thermostat, primer bulb always firm, no airated fuel lines, spark is on time at idle advance lever moving,no sticky chokes and all butterfly's operating correctly and still having the same problem... any ideas or has anyone come across this? Im so close to burning it that it's not funny

 

[fireman57]

The clue is it does better with the cover off. Sounds like you have an exhaust leak. It can't burn air with no oxygen. Check the base gasket.