Need help with speed/rpm problem on 115 hp merc

[boaterntexas]

Hey guys, took the ole 1976 115 hp merc out, the same one with the speed problem. this is the 17.5" fiberglass bass boat that wheighs around 1000 lbs unloaded. I talked to a guy and he told me that it was okay to run it at 6000 rpms, on this older motor. I'm able to get to 5800 rpms but my spedo only reads 35 mph. I had a hydrofiol stabalizer on it, when I took it off the boat planed out a little better and was able to reach a higher rpms. befor I took the wing off the cavatation plate was about 1" to 2" under the water, when at full speed, with the wing off, the cav plate is actually on the surface when at speed. since I removed the wing I noticed better hole shots, and easier planing, also I can reach 6000 rpms and be going 38 (on my spedo) mph I give it more throttle the rpms go up past 6000, but the spedo really doesn't. I my prop is in bad shape, one blade is nicked up way out of pitch compared to the other blades, I plan to get the prop pitched up, I belive the will relieve some of the rpms when at wot <br /> My question is was the wing the problem or is this just a figment of my imagination? when the rpm change but my spedo doesn't where should I check first? and is it truely okay to run the motor at 6000 rpms. If I leave the stabalizer of, should I fill the drilled holes on the cavplate or is it not a factor.?<br /><br /> Thanks for you help guys. here are some pics, of the rig b4 I took the hydrofiol stabalizer off.<br /><br /> http://www.geocities.com/tlcnnetex/cavplatjpg.jpg <br /><br /> http://www.geocities.com/tlcnnetex/draw.jpg <br /><br />Thomas

 

[mellowyellow]

Re: Need help with speed/rpm problem on 115 hp merc

pics don't come up tex... gonna ask you again;<br />"is the cavitation plate even with the bottom<br />of the boat?"<br />also, speedo's are not very accurate.<br />a bad prop can damage lower unit.<br />6000 rpms is too high.<br />good luck,<br />M.Y.

 

[boaterntexas]

Re: Need help with speed/rpm problem on 115 hp merc

thanks for the reply mellow. the cavataion plate is about 2" higher than the lowest part of the boat. what would be a good solid rpm range on this motor? 1976 115 hp merc with thunderbolt?<br /><br /> thanks<br /> Thomas

 

[mellowyellow]

Re: Need help with speed/rpm problem on 115 hp merc

5500-5800rpm is reccomended. I run a 19p 3 blade<br />on my '70 115hp at 5500rpm's WOT.<br />motor is 1-2" higher than should be, but not sure<br />if that's the cause of yur woes???<br />go to mercs website and input your serial# and<br />they'll give you necc. prop info. since it's due<br />for replacement anyways, I'd try new prop first.<br />good luck,<br />M.Y.<br />ps. my 1150 orig. came with a 2 blade.<br />cavitation will cause excessive rpm's I think