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75hp Mercury Outboard won’t go over 3500rpm’s

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briansfridley

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I bought a pontoon boat with a 2001 Mercury 75hp 2 stroke outboard. It starts easy and idles fine. However, it will not go above 3500 rpms. Everything I have read says it should run up to 5000+ RPM’s. The gas tank and pick up screen have been cleaned. New plugs and wires. Carbs removed and rebuilt. New primer bulb, gas line and filter. Compression is 90psi on all 3 cylinders. It runs great up to approximately 3500 rpms and then it just won’t accelerate any more. It isn’t missing or sputtering and again it starts easy and isles great. It isn’t like we want to run this pontoon full throttle all the time but last year we got stuck out on the lake during a lightning storm because we couldn’t get back in time (14mph top speed right now). Is there a rev limiter? What should I check next?
 
Jeff, I don’t know a lot about props but what I have read says that for every 2” of pitch equates to approximately 400 rpms. That would mean that the prop pitch is off by 10-12”’s...could it be off that much?
 
First I would run it without the cowling so I could verify the carbs butterfly valve was fully opening. If that was OK I would change the prop and give it a test. The right prop should give you around 5500 RPM. Are you sure you don't have the trailer still attached (sorry, I couldn't help myself)?
 
Lol. I feel like it sometimes. That’s funny.

I will definitely check the pitch on the prop and run it without the cowling. It just runs so great until it hits 3400-3500 RPM’s then it doesn’t matter how far you push the throttle it just doesn’t change the speed of the motor and it doesn’t sound different...it’s almost like it has a governor on it...
 
One can't (shouldn't) assume that, just because the motor has a prop, it's the correct one. And you're not running a runabout.

Jeff
 
fastjeff said:
One can't (shouldn't) assume that, just because the motor has a prop, it's the correct one. And you're not running a runabout.

Jeff
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I called SunTracker. With my pontoon and motor they told me the prop should be 13 3/4 x 15. I will check and see if the one on the boat matches. I guess that is a great place to start.

I forgot to mention I did replace all 3 CDM’s when I replaced the plugs and wires. I could have absolutely gotten a bad one. What is the best way to test the CDM’s?
 
briansfridley said:
I bought a pontoon boat with a 2001 Mercury 75hp 2 stroke outboard. It starts easy and idles fine. However, it will not go above 3500 rpms. Everything I have read says it should run up to 5000+ RPM’s. The gas tank and pick up screen have been cleaned. New plugs and wires. Carbs removed and rebuilt. New primer bulb, gas line and filter. Compression is 90psi on all 3 cylinders. It runs great up to approximately 3500 rpms and then it just won’t accelerate any more. It isn’t missing or sputtering and again it starts easy and isles great. It isn’t like we want to run this pontoon full throttle all the time but last year we got stuck out on the lake during a lightning storm because we couldn’t get back in time (14mph top speed right now). Is there a rev limiter? What should I check next?
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Does it throttle past 3.5k RPM in your driveway? Does it only happen under a load? If it only happens under a load, its most likely fuel restricted or not getting a strong enough spark. I would really inspect your carbs. Ive seen floats have holes the size of pin holes. I would then inspect all of your fuel lines. As everyone knows ethanol destroys fuel lines faster than you'd think. Sometimes fuel lines can collapse when at throttle, restricting the flow.
 
Reving up in driveway is worthless and tells you nothing!!! A 3 cylinder motor with a cylinder down will rev to the moon without a load!!!
 
^^^^----Big round of applause on the above post #13-----Not sure why folks insist on revving the engine up in the driveway.-----Just more proof that folks do not understand the terms ---POWER and WORK--Post pitch information off the prop.------Do some real trouble shooting including inspection of cylinder sleeve position.-----What is condition of the boat?-----Any water in the logs / tubes ?----How big is this boat ?
 
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faztbullet said:
Reving up in driveway is worthless and tells you nothing!!! A 3 cylinder motor with a cylinder down will rev to the moon without a load!!!
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If it won't rev up with a load but it does without, tells you a lot.

You'll be able to tell if you have a cyl down pretty quick regardless of this testing. If it revs up in N it steers you towards fuel restriction and not ignition.
 
Jeff,

Since I posted this I verified with the factory that I have the prop that came on the motor for this boat. All 3 carbs were rebuilt and verified they were opening all the way. I put 3 new CDMs/wires on. I noticed that the top 2 cylinders really are not dropping down when I unplug the wire from the plug. I switched the CDM’s around thinking I may have a bad one but still the top 2 aren’t firing like they should. Compression is the same across all 3 cylinders. Correct plugs/new. Any chance this could be reed blocks or stator? Starts and idles fine. Top 2 cylinders just not firing like they should.
 
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