I got this 84 35hp Mercury last spring and it ran like a top all summer (normally I wouldn't buy an old Mercury but I got a hell of a deal on it) then back in November I was cruising down the lake and the motor just lost power and bogged down. After that it just wouldn't accelerate when giving it gas. So I had the carb and fuel pump fully rebuilt in February. Took it to the river and it was still doing the same thing. A friend told me his boat was doing the same thing and it was his spark coils. So I replaced both of mine and found one of them was cracked. I ran it on the muffs and it seemed pretty good just didn't wanna idle just right. So Today I took it to the lake to test run it couldn't even get it off the trailer because it kept dieing when putting it in gear. A new motor is not an option for me atm and I'm missing prime time of the year. Ive checked all the basic things like compression and spark plugs and all that is good.

Sometimes just taking to a mechanic can be cheaper and definitely faster than trying to diagnose and just changing out parts yourself. I had a similar problem and it was a coil. My mechanic had it narrowed down to 2 things, I don't remember what the other was. It would isle ok but had no power to get on plane.

Take a look at the reeds. Had one that did that and one of them had a big hole in it and it would not run.... just a shot in the dark.

How long did it set between carb rebuild and running it?

Did you run the same gas that was in tank? Was tank cleaned?

Did carb job include replacing old fuel lines, primer bulb, etc?

Take a fuel sample in a clear container and get back to us with what that looks like. Squeeze primer bulb to get a little in a jar.

A proper fuel system job when you have dirty carbs is drain tank and make sure its clean, new fuel lines if you are unsure they are alcohol resistant, all new filters, complete tear down, clean and replace of parts in carbs as needed.

Exports may be clogged. Sounds like a carb problem to me though