extremly high rpms @ WOT 6500 +

boaterntexas

Petty Officer 1st Class
Joined
Feb 4, 2003
Messages
254
my 17' fiberglass bass boat with a 115hp merc and 21p ss prop, can reach 6000, or 7000 rpms at WOT had the rig checked, and it checked okay, had carbs cleaned, and fixed the problem for a bit, now the high rpms are back, hole shot starts out normal, the when the boat starts to plane, the rpm will jump dramticly to almost 6500 unless I back off the throttle, then after boat planes out they will level out at 5900 or 6000, unless I trim down, and take it off of plane then the level out at 5600. I have noticed that at some times, if I take a sharp turn or jump a wake a bit the rpms will jump way up, and will not level out, unless I disengaged the prop come to a stop then take back off.<br /><br /> is this the prop slipping or is there and adjustement to the carbs that can bring this down.
 

Jack Shellac

Lieutenant Commander
Joined
Aug 7, 2002
Messages
1,661
Re: extremly high rpms @ WOT 6500 +

It sounds like your prop may be slipping on the hub. A 21" pitch prop on a 115 HP and a 17'fiberglas boat dosn't seem to be an excessive RPM setup. If the prop is OK, it may be cavitating under heavy acceleration. If it loses it's " bite" on the water and RPM's jump up but get's the "bite" back when you back off on the throttle, this could be it.
 

miloman

Lieutenant Junior Grade
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Nov 3, 2002
Messages
1,181
Re: extremly high rpms @ WOT 6500 +

jack is right seems the prop is slipping are you running a jack plate
 

boaterntexas

Petty Officer 1st Class
Joined
Feb 4, 2003
Messages
254
Re: extremly high rpms @ WOT 6500 +

No jake plate, do you think it is cavatation, or spun hub, I have noticed that one of the blades are our of pitch, and nicked up pretty bad, Also the boat is drinking an unuasaul amout of gas, 4 gallons it nothing to this 115 hp merc<br /><br /> Thomas
 

sparkroost

Petty Officer 1st Class
Joined
Mar 23, 2003
Messages
312
Re: extremly high rpms @ WOT 6500 +

definatly sounds like the prop is slipping. Mark the prop and the nut then run it and see if the hub slipped.<br /><br />Cavation plate even with hull?
 

boaterntexas

Petty Officer 1st Class
Joined
Feb 4, 2003
Messages
254
Re: extremly high rpms @ WOT 6500 +

The cav plate is a little high, my boat, is not a v-bottom it is more of a flat bottom hull , witha pad, the cav plate comes 1.5" form the bottom of the pad I had a tech look at it, and he siad it would not be the cuase of my problems, maybe I need a secound opinion?? I have marked the prop once, and noticed that it did slip, but I had done a really sharp u-turn, I thought it was common for the hubs to spin when you do sharp u turns. I guess I should mark it again and just be a normal bass boat driver. ;) If there is cavataion due to the hieght of my motor, is there any remedy with out having to buy a jack plate or modify my transome? also any one know if the rig shoul dbe shooting a rooster tail, it a old 1976 hydrasports fiberglass bass boat<br /><br /> Thanks<br /> THomas
 

boaterntexas

Petty Officer 1st Class
Joined
Feb 4, 2003
Messages
254
Re: extremly high rpms @ WOT 6500 +

Is ther nay way to test the tach? It could be off, sometiems during the hole shot the motor will begin to shake and rattle violently, so I'm sure it is reaching high rpms then, but during topend it really doesn'y sound what I would think 6500 rpms would sould like
 

Solittle

Fleet Admiral
Joined
Apr 28, 2002
Messages
7,518
Re: extremly high rpms @ WOT 6500 +

THomas - A prop should not slip during any maneuver.
 

miloman

Lieutenant Junior Grade
Joined
Nov 3, 2002
Messages
1,181
Re: extremly high rpms @ WOT 6500 +

from what you have saidI would bet the props slipping get that fixed and see if it fixes it I think it will but if not you will have 1 less problem to worry about
 
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