late 70's Johnson Seahorse V4 115 wont go over 20mph

[DylRod94]

My grandfather gave me this boat after the many years we spent on it and we just replaced the power pack because the top right cylinder was not firing. It has great spark now and getting fuel but when I open it up on the water it tops out around 20mph when it used to run 60mph no problem. While idling I can pull the plug wires and I get a rpm drop on all except the top right cylinder, but it has good spark and I can see the fuel mist being pushed out when cranking so idk why that cylinder wouldn't be running. Also it has good compression.

 

[interalian]

What do you consider good compression? Numbers help. If you hang a timing light on #1 (top right as viewed from behind the motor) do you see spark? Swapped plugs?

 

[DylRod94]

120ish compression don't remember exact number. Don't have a timing light, but Ive swapped plugs still no change in rpm when I pull wire but when I crank with plug out it sparks strong blue and can see the fuel mist getting blown out

 

[jakedaawg]

Instead of guessing and complaining lets test the systems...

What method tells you you are getting great spark? The proper test is an open air gap spark check set to 7/16". post results.

Post results of a compression test. These motors are famous for broke rings.

What RPM, not MPH are you getting at WOT?

Fuel sample into a clear jar. Report results.

Is the gas tank properly vented and is the vent functioning?

When was the last time said boat went 60MPH? what have you done since?

Is it possible the vessel has become waterlogged? Boats from the 70's had open cell floatation and many times this has become waterlogged. Just because it is kept in a garage or that no water comes out the drain hole does not mean its a dry boat.

Should you choose to answer these questions fairly it's quite possible someone here can help you. The members here have become quite adept at diagnosing and steering repairs to successful completion. The test need to be done in order and completely.

 

[interalian]

Yeah, those numbers are good. How do the spark plugs look in general, all the same? And a 'strong blue spark' needs to jump 7/16" - does yours on all 4?

 

[Tim Frank]

If your speedo used to read 60MPH....I'd have been suspicious of it then.
I'd still be suspicious.

If it has a tach, as someone suggested, read RPM at WOT and start there.

 

[DylRod94]

I'm just going based on what it used to read, I can clean the plug off and it comes out wet and black. The rpm gauge doesn't work