can't get above 4000 RPM

[astampfel]

I have a 1990 V4 115 Hp Johnson. I took the boat out yesterday at it ran at 5000 RPM (36 mph). Today when I took it out, the boat wouldn't go above 4000 RPM (26 mph). It was more sluggish also and took a while to plane out. I wasn't able to look at the butterfly valves under load, but when I got back to the dock, I cycled the throttle cables and the butterfly valves are full open at WOT. I just bought the engine (used) last week and this is the second time I've taken it out. Any idea on what it could be??? Could one cylinder not be firing? What's the best way to check besides yanking off each spark plug wire while the engine is idling and see if it bogs down (I hate getting shocked).

 

[metriccrescentwrench]

Re: can't get above 4000 RPM

Use an inductive timing light, that'll at least tell you if each plug is firing or not. BUT... even if a cylinder is firing that doesn't mean that it's contributing it's proper share of power, pulling the plug wires will tell you that.

Warm the engine up on muffs and then do a compression check, all cylinders should be within approximately 10% of each other.

BTW, are you saying at wide open throttle you saw 5000 rpm? If so thats too low, should be more like 5600 to 5800 and after you get the engine straightened out you may need to consider a lower pitched prop.

 

[steelespike]

Re: can't get above 4000 RPM

You could run the motor and CAREFULLY feel each head see if you can tell if one is cooler than others.heads should be too hot to hold your hand on but not so hot it burns you.You could pull each plug and note its condition.you could run the motor and see if tickling the choke helps.