Engine Bogs when throttled up

I have a 4.3 liter mercruser 1989 Sear Ray cuddy cab. I bought it used and had a new/used motor put in it. I have problems when throttling it up and it bogging down.

Symptoms: It struggles to get on the plane. When full throttle or even 3/4 it putters and almost dies. I have also noticed that when using the trim it then does the same thing. Last weekend I messed with the timing and it started running great full throttle did not bog it down and I was able to use the trim without any problems. I took it out again yesterday and the same thing as before started happening. I have read that it may be an electrical issue also maybe a carb or timing issue. Any suggestions as far as how to narrow it down to one or the other would be greatly appreciated. Since it is a used motor I am not sure exactly what it is.

Thanks all responses in advance.

I would start by cleaning up your battery terminals and associated wiring including block ground and starter connections. Sand terminals shiny.