1963 Merc 650 slow acceleration and speed

[Jaw20]

Hello, i have a 1963 mercury 650 on a 1970 15.5 foot glasstron. When i run it under load the engine does not have much power, but when I run it in a bucket it seems to work great. When i take it out the the lake it takes a long time to accelerate and only gets up to about 10-15 mph. I have good compression, 120 psi, i have rebuilt the carbs and have spark in all plugs. Rebuilding the carbs gave me an extra 5 mph and better idle, but it still isn't running like it should be. I have also checked the fuel pumps and they are good. Not sure what i'm doing wrong but everything seems to be good. Thanks to anyone who helps.

[boats.net]

Has it had this problem the entire time you have had boat and motor or did it start instantly? Did you properly link, sync and adjust carburetors? Check timing and timing advance to make sure working properly.

[Jaw20]

Has it had this problem the entire time you have had boat and motor or did it start instantly? Did you properly link, sync and adjust carburetors? Check timing and timing advance to make sure working properly.

I recently bought the boat from a friend knowing it will need some work, so as far as i know it could have been like this for a while. The carbs are all adjusted by a boat shop so i would expect them to be set up right. The timing and timing advance was also done by them. The only thing they said is that the prop looked a little big on it, other than that they are not sure why it would do this.

[boats.net]

What size propeller is on it? May try dropping two pitch sizes.

[Jaw20]

The propeller is 14x17. I think that is way to big so i will try to find another one that is smaller and try that out.