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Missing top-end; stuck at 4200rpm (1996 Mercury 2stroke 75hp ELPTO)

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(i apologize in advance for the novel, i'm trying to provide as much information as possible)

Hello,
I've been having issues this season with my 1996 75HP ELPTO 3cylinder Mercury 2-stroke (Serial No. 0G428168) in freshwater. I've owned my boat for ~6 years and haven't had issues in the past. This has been occurring consistently on numerous trips on the lake this entire season since it first touched the water for this year:

  • OVERVIEW: boat runs 5-7mph slower (usually i get 37-40mph at 4800-5k rpm, now i'm stuck at 30-32mph at 4200rpm and trimming up further causes porpoising with no RPM gain)
  • MISSING TOP-END RPM: motor doesn't reach max rpm at wide open throttle (4800-5k rpm is expected, but it tops out around 4200rpm and porpoises if i try to trim it up further to its normal running trim setting)
    • NOTE: Sometimes, when i'm alone, i'll feel the motor gently surge forward around 4200rpm at WoT (like it WANTS to go) but once it smooths out if i trim it up slowly i can get it to spin up to its max running rpm of 4800-5k. When I add a passenger, this scenario happens maybe 1 in a 20 times and i most always cannot reach max rpm.
  • SLIGHT RPM LOSS AT WOT: motor randomly hesitates while cruising at wide open throttle (me & passenger will be going 30mph/4200rpm at WoT and without touching the throttle or hitting any waves, the motor will drop RPMs and slow down to 27mph and eventually go back up to 30mph)

Background Information:

  • STORAGE: carbs are fogged with fogging oil prior to winter storage
  • STORAGE: fuel stabilizer is added to the gas tank prior to winter storage
  • PROP: running the same prop (19p) for the entire time I've had the boat (6 years) and ive never had these running issues in the past
  • BATTERY: Starter battery is ~5 years old, has a State of Charge of 12.4v which is ~75% (mentioning this incase it could cause an issue)
  • WATER PUMP: Newly installed waterpump this spring before putting the boat in the water for the season
  • THERMOSTAT: Newly installed thermostat this spring before putting the boat in the water for the season
  • STEERING TAB/TRIM TAB: This was adjusted this spring to center out my steering (can't imagine this would cause an issue but still mentioning it)
  • The boat sits in the water at a dock from spring until fall, so it's easy for me to take it out for a spin after modifying something

Current troubleshooting:

  • PROP: inspected by a local prop shop and 1 small curve on 1 of the fins was found but he said at most i'd notice a 1-2mph loss and possible early cavitation but nothing major; he identified the hub in the prop is not spun
  • PROP: tried spare prop in good condition (21p; usually run a 19p) and boat had same issue with early porpoising and not reaching max rpm
  • FUEL: added seafoam to the gas tank (this actually fixed the issue when it first arose last fall, but has not made a difference yet this year after running a few cans through it and the carbs directly)
  • FUEL FILTER: drained, inspected, installed new fuel filter and inlet/outlet fuel lines that connect directly to the filter
  • PLUGS: after running the boat for a trip out, i will check the plugs and they do appear "wet"; i'm told this is common for how a 1996 Mercury 2stroke runs. Taking a lighter to it, it does not ignite the fluid on the end of the plug.
  • TRIM: Sometimes when it's only me in the boat, it will reach max rpm at 37mph/4800rpm; i can trim the motor up to its max trim height before cavitation and the boat runs straight with no porpoising; BUT, always when i add a passenger, the boat porpoises when it reaches its (slower) top speed at 30mph/4200rpm when attempting to trim it up to its usual max trim height and will not get over this "hump"
  • TRIM: (I haven't 100% confirmed this yet so maybe don't take it too seriously) there have been a select-few times where, when the boat would hit its stalling point at 4200rpm and porpoise early, I would make a slight and long left turn and this would seem to prevent the early porpoising when trying to trim it up further, which would actually allow me to trim the motor up and get it to spin to max rpm and then I can straighten the boat out and run it at top speed
  • THROTTLE CABLE: inspected the throttle cable and it appears to be opening up all the way; manually moving it gives me the same amount of travel. i even had a friend try opening the throttle further (there was a little extra play) by hand while under power and no RPM gain was achieved.
  • CARBS: All 3 carbs have been treated with carb rebuild kits, installed by a licensed mechanic
  • HULL: Hull was inspected for any major buildup that could cause the issue, nothing stuck to the hull/no damage.
  • RECTIFIER: Rectifier was burnt out; new rectifier installed as tachometer wasn't working initially; since installing this, the boat started being able to reach max rpm again with just me in the boat (before doing so, it consistently never reached max rpm with just me in the boat, it was stuck at 4200rpm; i don't know how a new rectifier could've changed the way the boat runs...; now i get the behavior as mentioned in the "NOTE:" above in the 2nd bullet point at the top of this post)
    • ​I've heard a burnt out rectifier may cause a stator to go bad; can a bad stator cause a running issue like this?

Thank you very much for any/all help! I'm not sure which way to go from here. Ideally, as everyone is aware of, throwing parts at it and hoping something sticks is not the way I want to go with this. I'd at least prefer to have a priority of what to consider first.
 
johnnygjr said:
Are throttle plates opening to a horizontal position?
Click to expand...

I'm assuming the plates in each carb is what you're referring to? Hopefully and if so, below are pictures of all 3 carbs (click to enlarge). Motor off, throttle moved to wide open position. Thoughts?

Carb #1 (top):
boat_carb1.jpg

Carb #2 (middle):
boat_carb2.jpg

Carb #3 (bottom):
boat_carb3.jpg
 
Last edited:
UPDATE:

PROP: Installed a brand new 17p prop on 8/2 (Thursday) for a couple reasons; both to rule out that my current prop was causing the issue and because I wanted a 17p prop for better hole-shot anyway.

RESULTS: The boat got on plane quick and ran at max rpm and reached it's normal running top speed when I used it on Thursday and Friday. With adding 1 passenger, the boat still ran to its full potential. BUT I took it out on Saturday, 8/4, and it ran like it's crummy self again. I added a passenger and it still ran like crap. It's been running slow again all weekend.

So the prop doesn't appear to be the issue even though the boat ran great the first two days of having the new prop on, yet somehow now it's running slow again. How frustrating!

Any ideas???
 
racerone said:
What does a compression test show?-----Seems to missing in all the information posted.
Click to expand...

Just did a compression test with the following variables:

- Warmed up motor taking it for a few laps on the lake (it actually ran fine this time out, which is not how it ran yesterday when it wasn't reaching max rpm but was short ~800rpm)
- Enabled the Kill Switch to prevent spark
- Removed spark plug wires and spark plugs
- Opened throttle to Wide Open Throttle

Compression readings:
- Cylinder 1 (top) read 122psi
- Cylinder 2 (middle) read 120psi
- Cylinder 3 (bottom) read 120psi

Any thoughts regarding this? Could these compression readings vary on the days when it's running poorly? Or is compression in 1 case the same readings i'll get tomorrow if it were to be running poorly then?

Any idea where to go from here?
 
UPDATE:

Adjusted the carb butterflies to ensure they're opening all the way (see my pictures a few posts up) as I wasn't sure my top carb was fully opening. The boat will hit a cruising plateau around 4200rpm at WoT with trim all the way up and then on its own, with no change made to the throttle position, after 3-6 seconds it will increase in RPM and spin up all the way to 5k.

This is a similar behavior i've always had on days the boat would decide it'd want to reach max rpm. So i'm not positive the carb butterfly adjustment made a difference. But regardless, they should've been tuned so it's not a loss. I plan to do a full "sync and link" this evening per my service manual, because my carb adjustments last night increased my idle rpm and now it shifts hard.

QUESTION:
Based on my first paragraph with the hesitation/surging at WoT and then it takes off on its own and spins up an additional 800rpm, is that indicative of anything? (it's like a rpm "hump" it tries to get over and sometimes it can, other times it can't; adding a passenger perhaps adds just enough weight to the boat that it can't get over this rpm "hump")
 
Some will argue.---Does spark jump a gap of 7/16" or more on all 3 leads, yes or no ?-----I would install a fuel pump repair kit.----I would run with a timing light hooked to # 1 and go for a spin.----Observe light.----Repeat that test with other 2 cylinders.------I would use a strong flashlight to look into throttle bores to compare amount of fuel coming up the main nozzle.------Is oil injection in service or bypassed ?
 
In response to your questions/comments (please excuse my ignorance):

-Does spark jump a gap of 7/16" or more on all 3 leads, yes or no ?
- I'm not quite sure what you're referring to. Initially i thought you meant the gap of the plug (j-style) but I run surface gap plugs (the flat ones). Are you talking about the distance from the bottom of the spark plug, mounted, to some point in the combustion chamber? More info on this would be appreciated as I did not find much via Google.

-I would install a fuel pump repair kit.
- I will look into doing this, or also find a way to test my current fuel pump.

-I would run with a timing light hooked to # 1 and go for a spin. Observe light. Repeat that test with other 2 cylinders.
- I will search for a timing light tool to borrow and add this to the "To Do" list

-I would use a strong flashlight to look into throttle bores to compare amount of fuel coming up the main nozzle.
- Are you referring to taking the airbox off that covers the carbs and watching the needles/jets as they spray fuel into the carbs when the throttle is opened?

-Is oil injection in service or bypassed ?
- The motor is mixing the oil into the fuel itself, pulling oil from the reservoir. I am not pre-mixing oil/fuel manually and dumping into the gas tank.

Thank you for your help!
 
I believe the question about spark is very clear !----It does not ask or mention about the plugs you are using !---It is a test.----So does spark jump a gap of 7/16" or more , yes or no ?
 
racerone said:
I believe the question about spark is very clear !----It does not ask or mention about the plugs you are using !---It is a test.----So does spark jump a gap of 7/16" or more , yes or no ?
Click to expand...

I'm not quite as familiar with engines as you are. Therefore, I needed to search online for more information. However, i'm only finding results pertaining to gaps of a spark plug, not the spark itself. I just went through my full service manual to see if this was documented, and also searched online, and could not find anything unfortunately. Is there an alternate terminology that I could search for to find instructions for how to perform this test and tools needed?

I will continue to investigate the other mentions you had in your reply. Thank you.
 
I have not forgotten to run the above tests, but had a hard time sourcing an adjustable Spark Tester so I had to order one and it will arrive in a few days. Also, I spent the weekend determining that my warning module went bad on me this past Saturday, so will be replacing that while I wait for the Spark Tester to arrive.

Thank you.
 
Quick update:

In the near future i'm leaving for a 9-day trip and won't have time to continue troubleshooting the boat. That, plus the Spark Tester not arriving quite in time to run that specific test myself, has led me to dropping it off at a local Mercury dealership for repairs. So, it's currently out of my hands with the hopes that I've done a decent amount of legwork for them and once I return from my trip I will have a properly running motor.

Thank you all for the help thus far, I will certainly follow-up with the outcome for anyone else who may stumble upon this thread with a similar issue.
 
UPDATE:

I've had the boat back for about a month and a half now and it has been running significantly better. According to the dealership, my 75hp was only dyno'ing out around 40-45hp when I brought it in. After performing a LOT of troubleshooting and a small handful of repairs/replacements, they left me with it dyno'ing around 65-70hp.

1) A few things were changed related to the fuel line, such as a gasket in/near/around the fuel pump (i can get specifics if necessary), as well as the connector that connects the fuel line to the front of the motor.
2) New spark plugs
3) What they believed to make the biggest difference - "Check balls" were missing from the carb(s) and the mechanic is not sure where they went. This was causing a rich condition and thus, my bogging at the top end and unable to reach max rpm. This was repaired and now my motor spins like a top.

The reason they did not go any further with repairs is the next step was to essentially take the block apart for analysis and most likely a rebuild. The mechanic mentioned some slight wear marks were noticed in the cylinder walls (I believe) which I'm guessing will eventually lead to lost compression (my compression currently tests out well)

So, a big thanks to everyone who led me along the way of this process. It's a good thing I brought it in because I don't think I ever would've opened up a carb considering they were "rebuilt" with the usual gaskets/needles/cleaned shortly prior to me deciding to bring it into a shop.

Hope this helps someone else, should they run into a similar issue. Good luck and happy cruising!
 
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