1977 Mercury outboard, 850hp, lose of power

i have a 1977 Mercury 850 hp, I have been having some problems since I bought the boat about 4months ago. The problem is the engine cuts off when I give it some throttle. The trolley speed works ok, its very slow. Now when I am trolling threw the water and gun it to full throttle it runs at full very well, and I can pull back on the gas to mid throttle and it will do fine. But if I gradually take it from trolley speed to full, it will cut out and die. I have put new gas lines, new inline filer and pump; I have rebuilt the fuel pump and cleaned out the gas tank, put new gas, and rebuilt the crabs also put a smaller prop. Still not working, can anybody help thank you very much

Re: 1977 Mercury outboard, 850hp, lose of power

You start with compression check to see if you have compression on all cylinders it should be with in 5lb -10lb top to bottom.
Next you need to check spark on all cylinders it need to jump 3/8- 7/16 gap bright blue. if all this comes back good then you go back to the carbs I would test by closing off one carb at a time by holding your hand over the mouth of the carb to see if there is any change and your not dragging any cylinders along. If you find you are then clean the high speed jets again or you try spray primix into the carbs to see if it picks up speed at wot
I hope this helps, books are worth a mint. If you don't have I would get one .

hamit
bt1 usn ret

Re: 1977 Mercury outboard, 850hp, lose of power

Thanks ill try that this weekend