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Mercury 115 HP Four Stroke Engine Issue - Help!

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Slainte

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We bought a 20' Bennington SLX Tritoon with a Mercury 115 HP Four Stroke motor new in 2017.
We are having intermittent issues with performance. At about 5000-5100-5200 RPM the motor will run fine for a bit (Maybe 30-45 minutes) and then will "lug" -slow/fast/back and forth. After a bit it clears and runs fine again. Reminds me of when a clutch in a car is slipping or you have a transmission issue. Had dealer out; they said it was a prop issue (turns out they had not inserted the white ring (sleeve prop bore) when they installed a new prop in the spring. They put the sleeve prop bore in; still same issue. Then they said it wasn't the correct prop (although it is the same type prop that came with the boat new when boat was running well) and they replaced the prop with a four blade prop. Same issue and boat not as fast and used more gas. We put a brand new three blade prop (like it had originally) back on the boat. Same issue.
At this point of course I will contact the dealer but I am looking for some help as to what this could be.
Thanks!
 
I say the motor is mounted too high and pulling air down into the prop.-----Or try trimming the motor down to see if that changes the way it behaves.-----Water temperature might have something to do with it as well if it ran fine in the spring.
 
Is your motor a short shaft or long shaft? A short shaft is 15 inches long, a long shaft is 20 inches long. I think you measure from the transom clamp to the middle of the prop shaft but that's a guess. On my pontoon with a Merc '92 40hp 2 stroke, I have a long shaft and cruise at about 3500rpms - fast enough; 4000 if I need to get home to pee. I don't attempt to win any speed trophies with a pontoon.
 
I doubt you find a 115 HP 4 stroke MERCURY as a 15" shaft !!----And no it is not measured from the transom clamp to the middle of the propshaft.
 
racerone said:
I doubt you find a 115 HP 4 stroke MERCURY as a 15" shaft !!----And no it is not measured from the transom clamp to the middle of the propshaft.
Click to expand...
Oops - I defer to the expert! Found this:

  • Place the tip of a yardstick on the top of the mounting clamp bracket that secures it to the boat.
  • Read the measurement at the bottom of the cavitation plate. The cavitation plate is the flat area that is horizontal above the propeller.
 
Slainte said:
We bought a 20' Bennington SLX Tritoon with a Mercury 115 HP Four Stroke motor new in 2017.
We are having intermittent issues with performance. At about 5000-5100-5200 RPM the motor will run fine for a bit (Maybe 30-45 minutes) and then will "lug" -slow/fast/back and forth. After a bit it clears and runs fine again. Reminds me of when a clutch in a car is slipping or you have a transmission issue. Had dealer out; they said it was a prop issue (turns out they had not inserted the white ring (sleeve prop bore) when they installed a new prop in the spring. They put the sleeve prop bore in; still same issue. Then they said it wasn't the correct prop (although it is the same type prop that came with the boat new when boat was running well) and they replaced the prop with a four blade prop. Same issue and boat not as fast and used more gas. We put a brand new three blade prop (like it had originally) back on the boat. Same issue.
At this point of course I will contact the dealer but I am looking for some help as to what this could be.
Thanks!
Click to expand...
This is EXACTLY what my 115 HP is doing now. Has anyone figured out what this is?
 
Are you running with more weight up in the bow than you used too? Does it do this when you're running over a wake or in a turn? Can you trim the engine in to get rid of the issue? I have a S21 Bennington pontoon with a 115 Yamaha SHO with a 4 blade 15" pitch and it is sensitive to bow weight and trim angle and will ventilate under certian conditions but can always be corrected with trimming in.
 
Mine did the same thing. or would stall, take a few tries to restart. or just act like it was flooding/starving. after several trips to the dealer with "no issues found" results, i decided to take a look at it myself. There's a high pressure fuel cannister on the engine. It regulates the fuel going to the injectors. Float inside was sticking. Replaced the assembly and it worked like new.
 
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