I have a 1986 Mercury 50hp tiller on a aluminum Duracraft 1860. The motor sounds great on the muffs. But under a load, its a dog. It's barely doing 10mph.
We have checked the compression with all cylinders hitting at 125. Spark test came back all sparking. Took the carbs off and did a cleaning, very little varnish if any at all. Blew out all the jets, and put back together. Fuel sample did not show anything water, or trash. So, we started looking at the prop. The motor had 10 x 15pitch 3 blade on it when I bought the motor and boat. So, they had me put on a 10 x 13 pitch. The boat doubled its speed, and almost planed out. Had a little bit of hesitation at WOT, but it was finally stretching its legs.
So, they ordered me a 12.1 x 9 pitch prop, said it would make this rig sing! Got it put on Friday, ran it saturday morning, and its worse than the original!!!! At WOT it wants to bog down. I am at my breaking point now. Went back to the dealer/mechanic, and he said its in the motor, just bring it in. He said the 9 pitch should have jumped it out of the water, and humming. May even have to back off the throttle even.
Went by two other repair places today, stated everything above. One said to go with a 10.25" x 14pitch prop, and the other said to go with a 10.50-11" x 11pitch prop. Both said the 9 pitch prop was a waste, and that a prop like that is for pontoon style boats.
Where do I go from here??????