Merc in-line. Shouldn't it pull past 5k on a mx-15?

I appreciate some advice.
My dad has a really clean, original and rot free 1981 15' Panther, which is a canadian splash of a checkmate mx-15.... Or v-mate or whatever. It has a narrow 4" pad that is solid and straight, a 1978 merc 115 I6 bolted to the transom. He's had it since '91 (our family boat when I was a kid) and it maybe has got 10h of use a year since then. Motor has even and good compression and runs mint.... When he punches it from a stand still the little thing basically leaves the water. The problem is the motor hits a brick wall at 5100 RPM, 52 MPH and always has. Should it not be able to turn more RPM? He's not into this hobby quite like I am but I feel the boat should do 60MPH easily.... And with me blowing by him at +100MPH with the 'stream these days he's looking for a bit more top end. Do these old 115 (90HP) run out of breath quickly?

Any advice is appreciated.

What prop? What pitch?

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Originally Posted by croSSed

What prop? What pitch?

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with a nothing 19" it will run 48 MPH, with a 24" laser II IT will run 52. Funny thing is as a kid with it strung out with a similar 24 laser it ran 59 once when the starts aligned. Is it just a matter of the motor being too deep?

The later 90s definitely would run out of breath quickly. I don't know about that '78, if it's ported like the mid 80s 90s were. The mid 80s 115s would turn over 6k easily.

Back to the pitch question, I was mostly asking about the rpm when running different pitch props. Will it only turn to 5100 rpm no matter what pitch prop you are running? If so, that is strange. If it turns, say, 5100 w/ that Laser, it should be turning up, like, 6000 with the 19. If it stops at 5100 no matter what, then something is wrong. Too deep? No way. That should have no effect on how high the engine will turn. That is determined by the prop pitch.

If I recall correctly it turns 5200 RPM with the 19 and 5k with the 24. Pulls strong and then just stops like it had a rev limiter.

Look in there and make sure your throttle butterflies are opening up all the way. There are lots of guys spinning 1970's 1150's up to 6k.

1. make sure they butterflies are opening up properly. 2. check WOT timing. 3. make sure its a 1150 powerhead, and not a 90 that someone slipped on there. The 90's fall flat @ 5k rpm.

I bought a new inline six in 78 installed it on a17ft wellcraft and it would run 6K all day long with either 2 or 3 blade bronze prop. Use to tell people, " that Merc has two speeds, wide open and stop" Traded in on a new 85 2L V150, still have it and runs great.

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Originally Posted by milkdud

Look in there and make sure your throttle butterflies are opening up all the way. There are lots of guys spinning 1970's 1150's up to 6k.

1. make sure they butterflies are opening up properly. 2. check WOT timing. 3. make sure its a 1150 powerhead, and not a 90 that someone slipped on there. The 90's fall flat @ 5k rpm.

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Some good info here...
The butterflies are wide open, I'll be sure to double check WOT timing. Funny you mention the powerhead as there has always been some confusion with this motor as to what it really is. Previous owner (family friend) said it was a 115, but the decals always said 90.
How do I tell the difference?

76 Merc brochure shows the 115 turning 4800-5300 and the 150 turning 4800-
Fast forward to my next brochure, 1987, and the propshaft 115 HP is rated at 5000-5500

I know next to nothing about inline 6's, but maybe your motor just isn't ported/carbed/spark advanced enough to rev.

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Originally Posted by David

76 Merc brochure shows the 115 turning 4800-5300 and the 150 turning 4800-
Fast forward to my next brochure, 1987, and the propshaft 115 HP is rated at 5000-5500

I know next to nothing about inline 6's, but maybe your motor just isn't ported/carbed/spark advanced enough to rev.

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1980 brochure states 90 5000rpm 115 5500 rpm 140 5800 rpm.

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Originally Posted by Fullthrottleguy

Some good info here...
The butterflies are wide open, I'll be sure to double check WOT timing. Funny you mention the powerhead as there has always been some confusion with this motor as to what it really is. Previous owner (family friend) said it was a 115, but the decals always said 90.
How do I tell the difference?

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Post the serial number off the motor and we/I will tell you. If its falling that flat at 5100RPM, then I can almost guess its a 90. Can you also post a picture of the motor?

Ive been wrestling with my 1500 and its RPM/Speed as well. My flywheel decals were gone so I had to transpose measurements from a 1500 from a service boat at work to my flywheel (seemed to measure up nicely when compared to pictures in my manual). I used a coworkers fancy timing light to measure my RPM as i do not have a working tach. I had to set my idle RPM to 800-900 to avoid stalling and then I ran my eng up to secondary pick up timing 21 degress BTDC (while adjusting my cams to actuate carbs at the right time), got only 34-3800 RPM. I later spoke with a coworker and old outboard mechanic friend whom said I could adjust my advance til I hear a ping and back it off, which I did - hearing no ping - I ended up backing my screw to the point it did not stop the distributor (although I am now obtaining 4500RPM which is closer to the 5300-5500 spec). I am starting to assume my switch box is not signalling at the right time which my advancement was compensating for. But man oh man does it run now ahaha.

watch out when water skiing and making a corner. with that small pad it fall off it and scares the sh@@ out of you. at least my 15' checkmate did


make sure the tach is working at over 5000.

my 17' Kona, with a '78 1500 turns about 5200 at 48 MPH running a 19" prop. I need a carb job, and a link n sync, then I expect 5500 and just barely north of 50. The trigger can sometimes go wonky on those. CDI has a website (I dont recall the link) that gives troubleshooting instructions for the trigger.

Will