Merc 115 inline 6 Help Needed

[Corvette]

I have a few problems that I could really use some help on.<br />First is that if i accelerate to fast, the motor will over rev, and seems to pop out of gear, although there is no real "popping", but when I drop down to neutral and then start to accelerate again (slower) all is usually fine. Is this normal, and if not, then what may be the cause of it?<br />Next is the year. I have done a ton of searching and calling dealers to try to find out what year this motor is, but have had no luck. The serial number is B197555. Hopefully someone can tell me.<br />Lastly, I am hoping that someone can tell me the low and high RPM's reccomended for this engine, so that I am sure that I have the correct prop on it, or can get the correct one.<br />Thank you for reading, and I really do appreciate any and all help that you can give me.<br />Thanks,<br />Russ

 

[Down South]

Re: Merc 115 inline 6 Help Needed

It's a 1987 model. your prop bushing may be spun. Make a mark on the prop nut and inside the prop hub with a perm. magic marker. The marks should be straight across from each other. Try the motor again and see if the marks are still straight across from each other. If they aren't. It's time to visit the prop shop. <br /><br />I'm not sure on the RPM. But somewhere around 5000 to 5300 with a normal load should be a good figure to shoot for.<br /><br />Good luck<br /><br />Good luck

 

[skier]

Re: Merc 115 inline 6 Help Needed

According to my information, you have a 1987 on your hands.<br /><br />RPM's should be around 6-700 in gear at idle and 5000-5600 at WOT. DON'T EVER go above 6000.<br /><br />Your first problem sounds to me like you are getting exhaust leeking infront of the prop and getting ventalating the prop. I'm not sure how you would correct this though. Also could be that the hub is spun. Someone will be on here soon to let you know.<br /><br />I hope this helps.<br /><br />Craig

 

[Corvette]

Re: Merc 115 inline 6 Help Needed

Thank you guys, I really do appreciate the help. Okay it is an 87, that's done. And I feel comfortable with the RPM's, however I forgot to mention that I just changed the prop and had the same proplem as before, so I guess that would rule out the spun hub (as I am assuming that it is part of the prop). Please tell me more about the exast posability that you mentioned Merc 140, and also please anybody with any ideas, please let me know. Thank you all I really do appreciate the help.

 

[steelespike]

Re: Merc 115 inline 6 Help Needed

Sounds like a simple problem with cavitation.<br />Check the cavitation plate.You may have to lower the motor so the cavitation plate is closer to or below the surface of the water.<br /> You may be able to trim the motor in a notch<br />and solve the problem.Though this will change <br />the boat trim at speed.

 

[Corvette]

Re: Merc 115 inline 6 Help Needed

Okay great, i am always happy to hear the wors "simple problem". <br />However I am new to boating (obviously) and do need further help. Please tell me what the cavitation plate is exactly, and also what height it is supposed to be set at.<br />Thanks again all.

 

[what200]

Re: Merc 115 inline 6 Help Needed

I think inlines are great motors, a lot of power for the weight. However, from what I have been told these motors should not be proped under 5500 rpm @WOT or you will risk burning a piston. I also would use 89 gas at minimum in this motor. The cavitation plate or more acurately the anti-ventilation plate is the flat surface just above the prop to keep air from being sucked into the prop and allowing it to slip excessively. Prop height is generally measured by projecting a straight edge from the bottom of the hull out and comparing the centerline of prop to that. For example your prop shaft may be set at 6 inchs below the bottom or pad. The best height for the prop depends on many different factors such as hull type, engine power, type of prop, motor setback, running weight, etc. Since there are so many different variables engine height is usually a trial and error process. Generally speaking, as you raise the motor, the top end speed of the boat will increase as there is less motor in the water to create drag, but you also increase the chance the prop will ventilate while doing accelerating or turning. On my boat I have a hydaulic jack plate to raise and lower the motor while running to help out on this compromise. Hope this helps

 

[Spidybot]

Re: Merc 115 inline 6 Help Needed

If you want to know about props, hull designs, motor mounting and a lot of related stuff, go here <br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=17;t=013479

 

[mbb]

Re: Merc 115 inline 6 Help Needed

Sure does sound like the hub is slipping. Does the prop have a large gap between the front edge and the lower unit housing, or is it like 1/16" or so? I have seen OMC props re-hubbed with merc hubs pass enough exhaust to cause some cavitation, but not too bad. None of the metal is broke out around the front of the prop?