Need help with motor issue with my pontoon!!!!!!!!

shaneohio21

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Joined
May 12, 2015
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3
Ok this is my first post and i have read thru 100 or so post on this problem so i thought i would ask and narrow it to my boat and motor specific. I recently bought a 24 ft 1996 sea nymph pontoon and it has a 90hp johnson 1986 V4 we have been out on the lake a few times and the previous owner said something about it cavitates since researching i have looked at all the likely problems and started to self diagnose so far here is what i have done and the results/symptoms. First when on the lake at about 3/4 throttle motor stands right up and takes off the last 1/4 the rpms spike around 6k and motor winds out really loud and i get no extra speed when i back the throttle back down it picks up and pushes on so first i tried to move the trim and i am unable to even slightly move it without this rpm spike and no more speed so i keep it all the way down. after this i saw i had an extra mounting hole left to drop the motor a little lower so i did now the stingray thing on cavitation plate sits about even with bottom of the toons . looking at the boat in the water it seems to sit right also this problem occured with one other person as well as 5 others in the back of the boat, so after this all happened the little bit it lowred it still does it the prop is 14x13p alum it does show signs of where on blades and the edges are chewed up some so after the research today i ordered a 14x11p prop. not sure its the cure but seems the cheapest route first sorry this is so long just something that is keeping me awake at night worrying whats wrong.when i do put it full throttle u can definatley tell its reving and not pushing water. any help is greatley appreciated the old owner mentioned a guy told him a jack plate but to me it seems low enough.
 

Sea Rider

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Sep 20, 2008
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12,345
Welcome to Iboats

Have you checked speed with a GPS at 3/4 and full tthrottle to know there's no extra speed increase ? So engine is reving 6 K, is that while being lightly loaded ? Is 6 K the max wot rpm factory stated for that engine. If having a hull that has too much water drag, assume pontoon boats have, there will be no significant speed increase from 3/4 to wot. At least not that wow speed difference...

Pontoon boat should be evenly weight distributed on deck and engine trimmed for cav plate and hull to ride paralell to water level when on plane.

Happy Boating
 
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shaneohio21

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May 12, 2015
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i have not checked the speed to be honest the only thing i can say is that when i have throttle left right before it seems to rev there seems to be plenty of throttle to pick up some speed its really hard to put in words but its almost like im cruising at a great speed then i try and increase it and it seems like the prop comes out gets louder and rpms jump up the max says 45-5500 op range on my book wot thank you
 

Sea Rider

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Sep 20, 2008
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12,345
If factory wot numbers for that engine is 4500-5500 you must check your current rpm numbers to check if inside safe working parameters for that engine. If engine jumps rpm abruptly with no speed increase you have a spun hub or on its way to become one. Go for a wot spin lightly loaded with a GPS and good working order tach and report back your findings.

Happy Boating
 
Last edited:

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
i hadn't thought of the Hiub.
I agree, could be a spun hub.Your new prop should solve that.
Wonder why the change to an 11"?You didn't have an actual wot rpm.
 

shaneohio21

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May 12, 2015
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3
The only reason for the pitch change was after all the research i was readingseems like alot of people with my size motor and boat used a 11p prop
 
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