I was very happy w an etec 150

Look at a Honda 115 or 135 if you want to see the heavyweight. Right at Yamaha 200 weight.

20' max motor weight
 

My first post! Re power for a new to me 1973 Seafari. After reading what
others have done and not wanting to overload my boat I am thinking about
the new 2014 90hp e tec pontoon model to get the gear reduction to
turn a larger prop (maybe four blade).. My fishing is mostly trolling at
2 1/2 mph most of the day and hopefully being able to plane at something
above 20knots going in and out. Only have the 30 gal tank and may wish
to try a few overnights............

That should work on a 20 footer. Rumor has it 70 is the minimum. I have 100 hp and like it.

20' max motor weight
 

Also in my thoughts was the 2014 yamaha four stoke 70hp at 256lbs....

Primetime, welcome aboard. The etec 90 will be more satisfying. 335 lbs on a Seafari is the best weight to power ratio your going to find and youll wonder if yiur fuel gauge is even working. I think maybe Bilgerat had an old 70 on his Seafari, but I think the 90 will prove a better choice as that 20-25 extra ponies will prove significant in that range.

I have a 367 lb F100 Yamaha. Jetted for non ethanol gas, but otherwise great. I think top speed with a F70 might be underwhelming. Or even a 2 stroke 70, if it exists.

PrimeTimeBob, I have a stripped down Bowrider with the 90 HP, 20" (2 to 1 gear) E-tec Triple. I have the boat down to maybe 1400#'s because I almost always have a load of 'Stuff" in it, plus 3 people that pushes me back up to almost 3000#'s when I'm the water. I have recently settled on at Power Tech 13" X 14" wheel and the following occurs - planes at 3000 RPM, makes 18.9 knots at 3400 RPM, Spools up to 5300 making 29.8 knots. It is unbelievable on fuel at 1500 RPM or less. I ran idling between 1000-2000 in two Christmas Boat Parades last year and logged 46 miles on the plotter. I topped the tank off with 5-1/2 gallons. If you "mash" on my name it should take you to my home page where it has a "Dredge Marshtackie" Album. (It's the only boat on this site with rust streaks running down the hull) BTW - the 25" E-tec 90 triple has the larger and lower ratio gear-case on it that is the same as the 150 - I believe. I hope this adds to you assessment of all data.

Very useful information....thxs...bob