1980 Evinrude 70HP 3 Cylinder 2 Stroke Help please

by Scott Bennett
(Croydon Victoria, Australia)

I have got a problem that has started recently and is getting progressively worse. Motor was serviced three months ago, new plugs, compression good, carbies cleaned out, new fuel tank, primer. Starts first time, idles well in and out of the water. Once warmed up will make way at 10kph (10kph limit) out of the river to open bay, but when you put the hammer down it is slow to pick up over a period of 1 minutes your finally up to about 25kph then another minute we are at 4000rpm and 35kph. The throttle the whole time is in the WOT position, then after another 4-5 mins at 4000rpm and 38-40kph, like someone flicks a switch, instant full power 6000rpm 55kph we are flying, like it used to, for as long as we want or until we slow to only a complete stop, shut it down,fish for a while(1 hour) and try to take off again. Once the problem has cleared we can troll at slow speed, move to another location at speed, troll again, take off again all without incident. Boat is a 4.5m fibreglass open cab runabout and up until recently would shoot out of the hole up on the plane and accelerate to 55-60kph @5500rpm in under a 30 seconds, if my fishing buddy was still standing I would have lost him overboard when I went to WOT, previously, before this issue it was one instant movement of the throttle from 1/4 to WOT and away it would go.
I dont believe its carbies because it will run all day at 5000-6000rpm with plenty of power once it gets there. Im thinking power pack, once it gets hot everything is fine, maybe heat expansion? I cant test it on the muffs as it runs great.
Open to any advice, just dont know where to start, my boat mechanic has offered a test powerpack as this is his thoughts or maybe a stator.
Sorry for the long description but its a weird problem that I can find a similar one on any forum. Most issues are the reverse of mine, heaps of power first up then dying off. The motor never lets me down always starts and will always give me full power after 6-7 minutes of frustration. Happy to hear anyones thoughts! Help Please!

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