78 70hp Evinrude no power past half throttle

[Jimmy01]

Hi,
im having issues with a old 78 model 70hp evinrude. I took it out for the first time since purchasing and I can’t get any power past half throttle. Boat idles and start well and runs perfect on muffs but as soon as I’m in the water and try to get up on the plane it just putts along slowly. Rpm doesn’t change past half throttle so I’m assuming it isn’t a spun prop, carby is extremely clean and must’ve been rebuilt previously. All cylinders have compression and are firing, Coil packs are reading the correct resistance with an ohm meter. I’m starting to run out of things to check so any help with diagnosing this would be greatly appreciated.
cheers

 

[boobie]

Did you do a cylinder drop test on it ??

 

[racerone]

They will sound good on a hose running on 2 of 3 cylinders.-----Testing on a hose means nothing.-----Could be as simple as a sheared flywheel key.----Or an exhaust cover water leak.------Or a leaking diaphragm in the fuel pump.----Answer this.-----Does spark jump a gap of 3/8" or more , yes or no ?

 

[oldboat1]

Measure with one of these (cheap), not an inline tester: