Very low RPM

[BrianO]

I have a 1990 VIP fiberglass 179 fish and ski fitted with a 1990 Johnson GT 150. Enigine height looks correct. Prop is 14 1/4 x 21, aluminium. Plugs all look good. Throttle position is ok. I can only turn up to about 4300 RPM WOT even lightly loaded with little fuel. I can trim up until prop looses water and boat porpises then back off a bit but still can't gain past 4300. Bow is low and boat is pushing water. any suggestions?

 

[wilde1j]

Re: Very low RPM

Brian, did your boat/motor combination ever turn more RPM WOT? Where is the cavitation plate relative to the bottom of the hull? I would expect you motor to be in the range of normal if it was turning ~ 5500, but some of your info doesn't seem to make sense (i.e. "bow is low, pushing water" ... a boat/motor trimmed properly should not be bow low, but on the verge of porpoising. You suggest you trimmed it that way, then backed off (bow down a little). How come it's bow low then?

 

[BrianO]

Re: Very low RPM

Spent the afternoon playing with this. Found one plug wire with a small crack in the insulation. Ignition was shorting on this cyl. Now have 5300 Rpm. Still can't get bow up. Motor revs-up before boat porpoises or prop sprays. Cavation plate is about 1" below the lowest point on the hull.