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'87 Mariner Speed Question/Help

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christopher425

Member
Hey Everyone! I've got an '87 Mariner 45hp 2-stroke sitting on a 20 foot 1990 Riviera Cruiser pontoon with an 11.1x13 prop. This is the motor that came with it when I purchased this past summer. Going at WOT on the lake with one other buddy on board, we reached an average speed of 14mph using a GPS app on our phones. My question is: Does this speed sound about what I should expect with my rig/combo? I know she's not gonna go lightning fast and I don't want her to. I just wanna make sure everything is running in good order on the water. Any advice/help would be greatly appreciated. Thanks everyone!
 
Sounds about right.-----But you can do some simple checks to ensure motor is running properly.------Spark on all 4 leads.----Compression at 120 PSI on all cylinders.-----Or test run on a dyno at a shop.
 
racerone said:
Sounds about right.-----But you can do some simple checks to ensure motor is running properly.------Spark on all 4 leads.----Compression at 120 PSI on all cylinders.-----Or test run on a dyno at a shop.
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Thanks for the reply racerone! Motor sounds really good and feels good on the water. You've helped me out with some troubleshooting in other threads I've started on here and I am very appreciative. Spark is definitely good (checked it with a gap tester several times). Compression number are as follows from top to bottom. I know the bottom cylinder is a little lower than the other 3, but it's still in spec. Kind of expected from a 35 year old engine.

Cylinder 1:120
Cylinder2: 125
Cylinder 3: 120
Cylinder 4: 105
 
Well---105 compared to 120 is NOT in spec in my book.-----Parts are precision made these days.-----A 15 PSI difference tells me that something is / has gone wrong.
 
racerone said:
Well---105 compared to 120 is NOT in spec in my book.-----Parts are precision made these days.-----A 15 PSI difference tells me that something is / has gone wrong.
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Let me walk what I said back a little. I know a 15psi difference is not what you want to see, but here's what the Merc service manual says, so it's for sure at the bottom end of being 'acceptable' (I use the term loosely) for a two-stroke this old. The motor runs great now, no overheating, no misfire or rough running, etc. Should there be anything arise in the future, I have added a deeper dive into the bottom cylinder to the list. Again, I appreciate your expertise racerone.
 

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I just wanted to post a follow-up to this thread b/c I know that step can sometimes be forgotten. Long story short, I bit the bullet and bought a used carb body for my lower one and cleaned it really good. The motor has never run better! No misses, surging, or loss of rpm. I actually gained 4 mph in speed and my rpm also went up to within the acceptable range as it should be. I put a new prop on (11.8x10) and that helped my acceleration out of hole shot tremendously. I really appreciate all the help from everyone on here and sometimes I suppose we just have to go step by step until we get our issue resolved. Slow down...think it through...and use your gut. That helped me out tons. Can't wait to get it out on the water in the spring. Happy & safe boating everyone!
 
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