1999 Merc 115 HP, 2 stroke, WOT Question

[Mscanon99]

Hi All,

Spring is almost here in the Northeast so I am doing my thing as a newer (since last Fall) boat owner and reading the boards...asking questions.

As mentioned, I purchased the boat in Oct of 07 and put 5 hours on it. All is good, however when I push it all the way the best WOT based on the dash tach is about 4800 rpms. In reading different posts, this motor should get up to about 5200 at WOT. I am not sure about the prop, however what is on it is almost certainly what it came with. The prop has some dings and looks like its been repaired (small chips), would this throw my WOT off? I have an extra prop that came with the boat, so I could try switching it, however I wanted to ask about other possible causes or if this is an issue at all before doing this.

The motor has about 175 hours on it, the first owner was an older guy who used the boat about 2 weeks a year, so he did not do much to it.....I called him to find out about the impeller and he said with such little use, why bother...I changed it promptly. Could my WOT be reduced becuase of carbon buildup? I was thinking it might be a good idea to decarb the motor in any event.

Thanks for any/all advice.

Regards,

Mike

 

[rndn]

Re: 1999 Merc 115 HP, 2 stroke, WOT Question

It can never hurt to decarb a 2 stroke outboard. The WOT reading could be and probably is affected by the prop that has been hit and chipped. Decarb and put the other prop onto the motor making sure it's the same diameter, pitch, and that it is made for your outboard. If you did not change the lower unti oil last fall you should do so now and for future use you should change the oil in the fall when you winterize the unit.

 

[Laddies]

Re: 1999 Merc 115 HP, 2 stroke, WOT Question

Usually if the blades aren't sprung the RPM is not effected much, so if your 4800 in a lite condition you might be able to go down a inch of pitch is hard to get all boat and motors to the exact max. If the 4800 is in a heavy condition then leave it alone. You could take the prop to a good prop shop and tell them what you want and when they build the prop they can tweak it for RPM

 

[j_martin]

Re: 1999 Merc 115 HP, 2 stroke, WOT Question

Dinged prop usually increases rpm while decreasing speed. It might be overpropped, or the boat might be wet. (heavy)

Almost any stainless prop will give better performance than an aluminum prop, just because the blades can be made thinner and don't flex.

hope it helps
John

 

[hkeiner]

Re: 1999 Merc 115 HP, 2 stroke, WOT Question

I would suggest first doing a compression test and a spark test. Poor compression or a missing/weak spark on one or more cylinders could cause lower RPMs at WOT. If both of these tests show no problem, then a set-up issue (such as a prop with too much pitch or incorrect timing) could be the problem. Of course, peforming a full link and sync should not be ruled out either.

You do not mention if the last owner experienced the same low RPMs at WOT all the time while he had the boat or if this is a recent change. This might give a clue as to possible causes.