1986 merc 115 wot

[xmen3]

My 115 is on a 15' 1200 lb ski boat and is awesome out of the hole even with a skier or a triple tube right up til 5200 rpm @ 38mph ( not towing) where it goes no faster.Prop shop says prop ( 3 blade alum.)is a 19# in the middle but less on the tips as though it had been reworked.
I pulled motor off a 19' 2500 lb boat that also went 38 mph @ 5200 but a little slower holeshot.
My 1965 65hp got my ski boat going 33 mph @ 5000 and my Evinrude 50 got it going 30 mph @ 5000.
Any ideas as to why I can't get the 115 to rev up to 6000 as I have read it should?
I have read where people can get 60 mph out of 22 and 24 # props on similar small boats and the boat speed calculator at http://www.go-fast.com/boat_speed_predictions.htm says 77mph.
Is there a rev limiter on this model?
I have jetted the carbs down to what the book says are the stock .074's from the .076's that I found in there in an effort to save a little gas and because I was having the plugs fouled too often. Now it purrs like a kitten and the plugs are a nice brown.
I have followed clams canino's timing thread for inlines but my merc says it should be timed at 6ATDC at idle. Does anyone know why there is such a large discrepancy between this and the 1 0r 2 BTDC in clams' thread?
Any help is greatly appreciated.
PS: I have a navman 5500 gps with fuel monitor and this 115 runs @ 1.45km/ litre (3.52 mpg)at WOT and cruises most economically at 2.25 km/litre ( 5.46 mpg)@ 3000 rpm and 27 mph.
1971 evinrude lark 50 gets 2.75 km/litre (6.68 mpg)@ 4200 rpm and 22 mph and 2.05 km/litre (4.98 mpg)@ 5000 and 30 mph.

 

[denny4]

Re: 1986 merc 115 wot

my '81 115 on a 17' starcraft alum. fish'n'ski did 42mph on only 5 cyl. Sounds like u need a tune up. But what about the boat itself? is it taking on water under the floor that u don't know about,bilge pumps working?

 

[croSSed]

Re: 1986 merc 115 wot

I have an '87 Merc. 115 (same engine as yours, diff. year) and I have the same type of hole shot. It's absolutely an animal out of the hole. My boat is a 16' runabout that is not exactly a "high performance" hull. I'm running a 20" pitch Laser II. It turns 5600 rpm @ WOT. At 12% prop slip that's around 45 mph. You have some kind of problem. Have you checked the compression? If all 6 comp. is good start trying diff. props.

Re. the timing: You should set your timing at 27 deg. BTDC at CRANKING speed, NOT idle. A big difference. When you check the timing, open the throttle (I do it by raising the fast idle lever on my control box all the way, and then manually pushing the throttle linkage arm further opening the carb butterflys) and crank the engine with the starter - obviously, make sure you have things so the engine won't start. When you do it that way the timing is advanced as far as it will go. Timing at idle speed has the timing backed off as far as it will go.

Re. 6000 rpm. Don't run your motor that high very often. The factory red line is 5500. That is the speed you could reliably run it for an indefinite period of time. Also, you are giving up some top speed running the engine at WOT with that much rpm. That motor makes it's peak h.p. between 5000 and 5500. The h.p. has dropped off at that much rpm. Running a prop that allows it to rev that high will make it unreal out of the hole, however!

Back to your lack-of-speed issue. 5200 rpm @ 38 mph w/ a 19" pitch prop is mathematically all correct. When looking at prop slip calculators (I use the one at Bam; Google it) you must figure in prop slip. A high performance bass boat will give about 10% prop slip. On a pleasure boat such as yours figure in about 12% slip. That will give you your actually probably speed. When you are at WOT are you trimming your motor out? If not, you are giving up at least 5 mph. Good luck.

TG

 

[AMD Rules]

Re: 1986 merc 115 wot

How does that 27° BTDC at cranking speed relate to 21°max recommended at wot? Curious to know how that works. I set my motor to 21° without the motor running by moving the throttle to max wot and adjusting the spark stop for max 21° advance.

 

[croSSed]

Re: 1986 merc 115 wot

:^:^:^:^
EXCUSE ME!!! I AM WRONG!!!! You are supposed to set max. spark advance at cranking speed at 21 deg. BTDC, NOT 27!!!! AMD is right!

TG

 

[Chris1956]

Re: 1986 merc 115 wot

Xmen, Prop slippage based youn your stated speed and RPM is nearly 19%, which is a bit high for your setup. A 17" pitch prop may be a better prop for that motor, as it will raise your RPMS to about 5600, which may give you some greater powerband effect. Also, make sure the motor is trimmed properly

BTW, Dispite what the speed calculator says, you wont make 77MPH with that motor/prop.

 

[ricksrster]

Re: 1986 merc 115 wot

I have found that the speed calculator is WAY off!

 

[xmen3]

Re: 1986 merc 115 wot

Thanks to all who have replied.
Unfortunately this motor is now dead and in many little pieces as I examine the damage.
Previous owner over cooked it when the water pump failed. I could tell by the deposits on plugs #1 & #2 and by visual inspection that a small amount of those domes have vapourized. Sea foamed and checked compression and it was still fine in 4 holes and slightly down in 2 but not the top 2.:$
It seemed to run ok but kept fouling plugs. So I jetted down to the stock 074's from 076's:$.
Still fouled plugs so I changed to LT77JC4's from L76V'
s. Finally, it stopped fouling plugs and ran better overall it seemed. Must of ran 10 tanks of gas like this no problem.
Then just to check, I pulled the plugs and checked compression and both were ok.
So i fired it up and sprayed some carb cleaner through it
and i let it run for I while when I noticed that there was some water flowing out from around the top plug and a little less from the 1 below it.
Decided to throw some JB Weld on there to seal it and boy did it ever do a good job!
Took it out the next morning and man, was it ever hard to start.
Smoked like crazy. Must of been the carb cleaner.
Ran it around 3500 rpm out to the fishing grounds for about a half an hour and it ran well even when I felt a cylinder drop out. Must be residue from the carb clean.
Trolled on the kicker for a few hours and then could not restart the 115. Pulled the plugs and noticed that they were all very clean and dry but with some white deposits on a couple. It was as though no gas was getting to the cylinders because I verified good spark all around and the plugs were so dry.
Gas was pouring out of the carbs when I sqeezed the ball and it stayed hard.
It kept trying to catch on when i cranked it but would keep dying.
Finally able to start it with a fresh battery but it would not get full power as we made a run back to the dock. After only five minutes it just packed it in losing power until we could no longer plane and then bang bang bang and it stopped along with my heart.
Now having pulled it apart today, I see 4 bad pistons some which have melted to the rings and 2 pistons with cylinder damage and which were missing one of their rings altogether.Broken rings on 2 others and 2 were fine.
Looking for replacements but all the ones I see listed are 2 ring and mine are 3 rings.
Any ideas anyone?

 

[Chris1956]

Re: 1986 merc 115 wot

Xmen, There are two and three ring pistons for that motor. The two ring pistons have keystone rings, and are therefore not interchangeable with the three ring piston rings. If you look aftermarket, you should find some slightly lower prices vs the dealer. I cannot say I can follow the narrative you provided on the failure scenario, however, all contribution to the failure need to be fixed, or they will repeat. You need to make sure the waterpump is good, the carbs are clean and the timing (ink and sync) is correct. Have the bad cylinders bored oversize to match the OS pistons. Install std size pistons in the non-bored cylinders.

 

[croSSed]

Re: 1986 merc 115 wot

Here's what I would suggest: Get a rebuilt powerhead from Wayne Canino, or ship yours to him and have him rebuild it. His e-mail is cc-marine@earthlink.net. He is dedicated to keeping these old in-lines alive, and he really knows his stuff. One of the most helpful knowledgeble people I have met when it comes to these motors. Doesn't it seem that the most knowledgeble people are pretty tight-lipped when it comes to their knowledge? Wayne shares his knowledge. He has helped me out of more than one "sticky wicket" when it comes to my motor over the phone.

TG