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Mariner 60 RPM issues

C

Coxn

Member
I've just picked up a boat with a 95 Mariner 60 2-stroke on it, bit it seems too be having an RPM issue. The previous owner had filled the tank for the winter and added stabilizer. I put in new fuel/water separator just in case. The telltale was a bit weak, so I blew through the tell tube to check the cooling lines were clear and then changed the impeller out. I also added new leg oil. I did the idle and WOT timing (cranking). So...

She starts up fine. She idles fine. She even runs at WOT out at sea fine...for the most part. After running for a few minutes at WOT, however, the revs just cut in half. The engine runs smooth enough. There is no spluttering. She just seems to drop off to half throttle of her own accord. If I drop the lever back and idle for a few minutes, I can open her up to WOT again. But then at some point she'll drop to half revs again.

Any idea where I need to look to fix this?
 
Sounds like it's dropping a cylinder. When it does this again, keep it doing it long as possible, then shut it right off--no idling allowed. Then pull plugs and see which one is different.

Jeff
 
Took the boat out for another spin today, after doing a little more work. Amongst other things, I changed the in-line fuel filter and cleared out the corrosion which was plugging the grill over the fuel tank vent. She ran like a top for about an hour, much of it at WOT. Then came another brief moment where the engine dropped its revs again. She picked up quickly afterwards though so I didn't get a chance to pull any plugs whilst she was misbehaving. Then another 15 mins on the plane with no problems. Thinking maybe that this is just a dirty fuel issue...? Or is there an overheat sensor on these engines which might be kicking in/acting up? The engine sounds fine, but the telltale still looks a little weak.
 
..."She ran like a top for about an hour, much of it at WOT."

Yikes! That's not an electric motor. You should never, ever maintain wide open throttle for that long. Get the boat up to the speed you want at full throttle, then back off and "breath the motor" at less than WOT.

Give the poor thing a break.

Jeff

PS: My good buddy had to learn that lesson the hard way. Put a brand new 4 stroke Merc on his party barge and used WOT from the time he cleared the slip until he got back. Was surprised that the poor thing was worn out by season's end.
 
No worries. When I said WOT, I really meant 'at high speed on plane'. I do usually back off a few revs once I get going :)
 
Be sure water pump is fully operational, no bad fins. Could eventually be overheating and ecm cutting back rpms to save engine damage. Also, oil pressure drop could cause an issue.
 
I changed the impeller and there's plenty of water exiting the fwd outlet above the ventilation plate. Plenty of 2 stroke getting through I think and I don't see any signs of overheating after a couple of hours running time. Seems to run pretty smooth in fact, apart from this occasional lag. Boat was sitting for nearly a year though. New owner gremlins.
 
OK, back to this WOT thing. I don't actually have a tach on the boat. WOT in this case is just my term for 'control lever all the way forward'. After my first two runs though, it seemed to me that the engine wasn't getting anywhere near max revs. Sound more like 4000 than 5500. I've taken a look at the throttle linkage again, and turns out I'm right. The butterfly valves aren't even close to fully open when I have the control lever all the way forward.

The barrel on the cable is set so that the throttle mechanism returns everything to idle when the control lever is returned to the idle position, but pushing the control lever all the way forward only opens the valves about 3/4. I can adjust the cable barrel so that the valves open fully with the lever forward, but then the cable won't return the throttle mechanism to the idle position. Maybe I need new cables? Or a throttle cam return spring? Which of the two is primarily responsible for returning the mechanism to idle? Does the spring pull, or the cable push?
 
Think I have this cleared up. For anyone googling for answers, I basically put some seafoam in the fuel tank and smoked her out. I also backflushed the tell tale outlet and she's pissing like a trooper now, so the weak tell was probably just a bit of shell lodged in the system. I then took apart and re-built the controls, and that seems to have sorted out my cable issues.

Plus I've installed a tach, so I can see her climb to about 5800rpm - and yes I back off to 5000rpm for cruising ;-)

Now I can play with the prop. She's running 22knots with 3 adults at 5000rpm, and shoots out of the hole no problem. Might see if I can increase the pitch and squeeze a couple more knots out of her...
 
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