I have a 2000 70 HP on a pontoon doing the same thing. did you get your problem figured out...???? Thanks

If you believe the guy about having the carbs rebuilt and that it ran 30mph, then I'd start by seeing if the carbs are getting enough gas.

Disconnect the fuel line at the motor connector and squeeze the bulb to make sure the line and pickup are clear and letting a good quantity of gas flow to the engine. Aftermarket fuel lines and primer bulbs (gray in color) frequently give trouble in this regard.

Next, get a buddy and run the boat. One of you operate the boat and the other one continuously squeeze the primer bulb to see if that gets it performing. If it does, then fuel pump could be weak. Or, there could be a crack somewhere in a fuel line or on the fuel pump allowing the pump to suck air as well as gas. Or, there could be a partial fuel blockage.

Next, with the motor shut off remove the air silencer cover and look into the throats of all three carbs. With your hand, open the throttle linkage and look to see if all three throttle plates are in sync with each other and opening fully into the horizontal (wide open) position. I would not do this over water as bolts and nuts have a tendency to jump into the nearest deep water as if by magic.

If all the above checks out okay, then go to basics: compression check and spark test. Then, I'd say do a sync & link, and if all that checks out, take another look at the carbs.

A 70 Hp motor on a pontoon doing 30 mph not going to happen.

I know. That is exactly why I am suspicious of the honesty of the seller of the boat.

Still, there seems to be something wrong with the motor if it will only go 8 mph, even though pontoons are absolutely hell on outboard motors. I have never met an experienced mariner who owned - or even wished to own - a pontoon. Personally, I'd rather sit in a washtub with an electric trolling motor attached. Deck boats were a giant step in the right direction.......

Also, I realize this is an old thread. But, what the heck - somebody might benefit from a good discussion from it. Might as well contribute what I can before I burn out from answering questions on this forum.