1978 Johnson 150 wont go full throttle.

Hello i bought a 1978 Johnson 150 not long ago recently took it to the lake and whenever i give it full throttle it sounds like it just stays at half throttle. rpm's do not increase it will only go about 10 - 15 mph. i have cleaned the carbs aswell. fresh gas. not sure what else to check as this is my first boat. thanks!

Compression reading on all cylinders?

With plugs removed, does the spark jump a 7/16" gap with a strong blue lightning like flame... a real SNAP!?

If the above two area are as they should be, did you manually clean the two brass high speed jets in each carburetor located horizontally in the bottom center portion of the float chambers with a piece of single strand steel wire? If not, do so as solvent just doesn't do that job properly.

Clogged HS jets will cause the problem you mention... also a sticking timer base under the flywheel (retarded timing)... OR... having the throttle butterflies starting to open too soon.

The throttle butterflies should just start to open when the scribe mark on the metal cam is aligned dead center with the throttle roller.