Johnson 70hp outboard lacking power

I have a 15' 1974 Seaswirl tri-hull with a 1975 Johnson 70ESL74 motor. We are the third owners and it has been in the family since new.
In the past this boat did everything including towing a skier with 3 people in the boat. It would get up to about 37mph without the skier.
It sat for about 10 years and was only ran once about 8 years ago. At that point it had full power but developed a issue with the coils. Father in law had them replaced and then it just sat until this year.
Runs fine, but has lost power since it sat. Compression is 98-100 in all 3 cylinders. Mixed info on line about whats right, but one source said 100lbs is correct. I would think if compression were the problem it would not be even on all 3.
I replaced the plugs and removed and cleaned the carbs as best I could and changed the lower unit oil. This helped a little but it still wont pass 30mph with 2 adults and a child. With this load it also takes about 20-30 seconds to get out of the water and plane. With 4 adults it simply wont do it.
Fuel filter screen is clean and all throttle linkages are in correct time. I have no tach so I cant tell you what RPM I get, but it seems low.
As best I can determine it has a 13.25 dia x 16.75 pitch prop, and this is the same prop it has had for a long time. It has also had a whole can of seafoam run through it over several tanks.
Suggestions as to why it runs weak? Things to look for? I don't have a book on this motor. If the motor is just tired (from sitting?) could going to a 15 pitch prop help it? Seems logical, would get the rpm's up again and should get on plane faster. Help!!

make sure the spark will jump 7/16 inch with a tester...they are available at any good auto parts store cheap..then make sure the timing is advancing all the way...keep in mind that you probably dont know if the motor performed like this after the coils were replaced if it was only run once...so dont assume that it has the correct coils..i also would remove the carbs and clean them and put kits in them..

Well I did disassemble the carbs and clean them, well as good as I could do without removing anything that could require adjusting. I suppose its possible a jet is partially plugged as I did not remove them. Not sure how to check the timing and advance as it is hard to run here at the house at anything more than an idle.

you can see the timimg advance mech at home...with the boat not running advance the throttle in forward slowly as far as it rill go..you will see it advance....do not put it in reverse though without the motor running...

i would completely rebuild the carbs for two reasons..you have a 39 year old motor thats been setting up....and on that particular vintage of 3 cylinder motor burning the top piston was a problem if they are run lean...