1998 Mercury 75 HP 2-Stroke RPM Issue - Possible Stator Problem?

[dustychops]

Hi everyone,

I’m hoping to get some advice on an issue I’ve been dealing with for the past two years on my 1998 15’ Boston Whaler Dauntless, powered by a 1998 Mercury 75 HP 2-stroke (ELPTO, Serial OG622***). Here’s a detailed breakdown of the problem and what I’ve tried so far:

Initial Condition:​

• When I bought the boat two years ago, it would only go up to 22 MPH, and the RPM gauge was broken.
• The first thing I did was replace the RPM gauge, rebuild the fuel pump, change the spark plugs, and replace the primary and secondary fuel filters.
• After these changes, the boat could reach 4200 RPMs and 30 MPH. However, on my next outing, it dropped to 3500 RPMs and 25 MPH.

Steps Taken to Fix the Problem:​

1. Carburetor Rebuild: I rebuilt all three carburetors, but this did not improve the RPMs.
2. Fuel System Overhaul: Replaced the primer bulb, all fuel lines (tank to carburetors), and fittings. Still no improvement.
3. Propeller Change: Swapped the heavy stainless steel 20P prop for a lighter aluminum 17P Black Max prop, which improved the hole shot but didn’t affect RPMs.
4. Timing Adjustment: Set the WOT timing to 20 degrees BTDC, per the service manual. I experimented with slight adjustments but found no improvement.
5. Compression Test: All three cylinders showed 120 PSI, which I’ve heard is acceptable for a 2-stroke. The manual, however, states that anything below 120 PSI could indicate significant engine wear.
6. Ignition Coil Replacement: Replaced all three ignition coils, but there was no change in performance.

Notable Event:​

One day, while leaving an island, the boat unexpectedly gained and lost power multiple times at full throttle, as if a switch was being flipped on and off. This made me suspect an electrical issue or perhaps a cylinder cutting out.

Diagnostics and Testing:​

1. Comparison Test: My neighbor has a 2004 16’ Key West with an identical 2004 Mercury 75 HP 2-stroke. His boat reaches 5250 RPMs and 38 MPH. This ruled out any fundamental issue with the propeller or engine type.
2. Stator Resistance Test: I used a multimeter to check the stator (red type with two yellow wires for charging and green/white and white/green wires for ignition). All readings were within the manual’s specifications.
3. Advice from Local Dealer: A Mercury parts dealer suggested that Ohm readings might not tell the whole story, and recommended a DVA voltage test for a more accurate diagnosis. He also ruled out reed valves as the likely cause, noting that engines with bad reed valves often idle poorly but still reach high RPMs.
4. Spark Test: I used an inline spark plug tester to check each cylinder and recorded the spark consistency using my iPhone 12 at 240 FPS slow-motion. The results showed inconsistent firing intervals at idle (≈1500 RPMs). Below is the data I gathered:
   

   My Engine (1998 Mercury 75 HP, 3500 RPM max):​

   Sparks Fired (seconds) - Time Between (seconds) @1500 RPM
   4.00 - 0.08
   4.08 - 0.04
   4.12 - 0.08
   4.20 - 0.03
   4.23 - 0.05
   4.28 - 0.03
   4.31 - 0.04
   4.35 - 0.04
   4.39 - 0.11
   ...
   
   

   Neighbor’s Engine (2004 Mercury 75 HP, 5250 RPM Max):​

   Sparks Fired (seconds) - Time Between (seconds) @1500 RPM
   4.01 - 0.04
   4.05 - 0.05
   4.10 - 0.04
   4.14 - 0.04
   4.18 - 0.04

The discrepancies in the firing intervals on my engine suggest a potential electrical issue. However, I’m not certain if this is due to the camera's limitations or an actual problem with the spark.

Next Steps:​

I purchased a DVA adapter for my multimeter to perform stator output tests and trigger tests while the engine is idling. I’ve also made a makeshift harness to help with this testing, using a diagram I found online. I’m trying to avoid spending $400 on a new stator unless I can confirm it’s the problem.

Question:​

Based on this information, do you think the stator is the culprit, or could there be another underlying issue? I appreciate any insights or suggestions on what to check next.

Thanks in advance, and feel free to ask if I’ve left out any details.

 

[Faztbullet]

Most likely a failed CDM module

 

[dustychops]

Most likely a failed CDM module

That's what I figured too. I replaced all three CDMs and it didn't fix it. I did buy the after market modules as the OEM were significantly more expensive.

 

[airshot]

OEM parts are usually more expensive because they work better than the Chineseium counter parts. When it comes to boat motors, I never use aftermarket !!!

 

[dustychops]

Update:

My DVA adapter arrived on Thursday, so I took the opportunity this weekend to remove the flywheel and stator for inspection. The magnets on the flywheel, as well as the stator and trigger, appeared to be in good condition. However, the trigger wires looked questionable—the colors didn't match the original harness (the purple wire in harness to gray wire on trigger, and the white wire on harness to tan wire on trigger). The wire jacketing on the trigger was puffy (unlike the wire harness), as if it had absorbed moisture, and was easy to stretch off the wires. When I cut off the factory connectors and stripped back the insulation for my makeshift testing harness, I found that the wires were tarnished. I'm not sure if this matters, but it’s worth noting.

I proceeded with the voltage tests. According to the service manual, for outboards with CDMs and no switch box, special testing is required. You either need a specific harness or splice in a custom one, but that can compromise the waterproof connections (I plan on using those Vaseline wire nuts, and a lot of tape to secure the connections). The manual only provides voltage output values at cranking or idle (675 RPMs), not at 300–1000 RPMs or 4000 RPMs like it does for engines with switch boxes.

I had the engine idling around 850–1000 RPMs (higher than the specified idle RPMs which is ≈675). Here are the readings I got:

• Stator to ground: 242V (manual specifies 200–350V)
• Stop circuit to ground: 243V (manual specifies 200V or higher)
• Trigger (white wire) to ground: 4V (manual specifies 2–8V)

It looks like everything is within the specified ranges, but if anyone sees anything unusual, please let me know.

I was hoping the issue wasn’t with the aftermarket CDMs I purchased. I paid $99 for a 3-pack from The ROP Shop on Amazon, but I’m beginning to believe that @Faztbullet and @airshot were correct in their initial assessment. OEM CDMs are sold out everywhere, including Boats.net, but I did find a replacement option: a CDI-Electric module (part number 114-7509) for $103.30. This is significantly more than the $33 per module I paid for the ROP Shop ones, but I’m considering ordering it and retesting the engine using the iPhone slow-motion camera to see if the spark timing returns to a consistent 0.04-second interval.

I would borrow a CDM from my neighbor for testing, but I’m hesitant since I can’t find any OEM replacements if something goes wrong.

Another potential issue is the part numbers on my old CDMs. The modules I removed from the engine had the part number 1347-827509-A8, while Boats.net specifies 1347-827509-A4 for my serial number. Even the CDI-Electric replacement lists compatibility for versions A1, A3, A4, A7, A9, A10, and T7, but not A8. I’m not sure why my boat has A8 modules installed—it's possible that Boats.net’s information is outdated, or maybe someone replaced the modules with the wrong parts.

One other sign of previous work on the engine is that the voltage regulator's factory waterproof fittings were replaced with crimped connections that were heat-shrunk and taped.

Any insights or suggestions would be greatly appreciated. Thanks for the help so far!

 

[w2much]

Not real technical here but have you ran it at night , cowl removed. Look for and listen for a short . Perhaps when on the water if its calm enough and safety measures observed hook up a timing light to each cyinder in turn try to find a trouble spot on acceleration under load. When is your neighbor leaving town ? Try his switchboxes. Your wires sound suspect.

 

[dustychops]

Not real technical here but have you ran it at night , cowl removed. Look for and listen for a short . Perhaps when on the water if its calm enough and safety measures observed hook up a timing light to each cyinder in turn try to find a trouble spot on acceleration under load. When is your neighbor leaving town ? Try his switchboxes. Your wires sound suspect.

I have not run it with the cowling off on the water. I'm fairly new to outboard engines (this is the first powerboat I've owned beside a tender for my sailboat that has a new 2.3 Honda 4 stroke), so I'm not sure about the DOs and DON'Ts. I figured that if a splash of water somehow jumped into my air intake that it might get water mixed with the air and forced into my cylinder and could potentially bend a push rod or something.
It's been about a season since I last changed my primary water separating fuel filter. I'm going to put a new one on, and then I'm going to remove the makeshift wire harness this morning and hook the wires back up with the permanent waterproof wire nuts and tape it up good and take it out on the water again and try the test where I get to WOT and I'm at 3,600 RPMs, squeeze the primer ball until firm, and then push the key in to initiate the choke and see if my RPMs climb higher. If this works, then I'm probably misdiagnosing my spark problem, and I really have fuel deliver problem. If that still doesn't work, I'll probably order one of the $103 dollar CDMs. If that passes the consistency test, I'll order the other two.

 

[w2much]

I hope you get her done. The processs of elimination can be frustrating but for some of us it beats bringing it to the shop and spending a whole lot of $$. Plus some of us somehow enjoy the frustration but when its fixed it is the joy of the win. Ignition issues can be difficult just the nature of electronic components. Fuel issues may mimic electrical problems . You need to know that it is firing on all cylinders all the time , but you also need to know that fuel is reaching the cylinders al the time. My suggestion may be impractical if alone without someone at the helm but should be doable if on a lake vs the sea. If posible you may want to replace a peice of fuel line with a clear temporary one to watch for flow or air bubbles. Puffy wires can be a sign of heat or corrosion causing intermiitant firing as your Noteable Event suggest. The noteable event is your hint at your issue whether fuel or ignition. You sound savy enough to figure this out so please post as you move along .

 

[dustychops]

Update:

After revisiting the troubleshooting section in the manual, I began to suspect that the stop circuit might be contributing to the intermittent spark issue. I decided to inspect the kill switch, and it turned out to be heavily corroded. It seemed plausible that an intermittent short could cause the engine to receive mixed signals about whether to run or stop, potentially affecting the spark. So, I cut out the old kill switch and replaced it.

Coincidentally, my neighbor with the 2004 Mercury called to borrow my oil extractor, as he’s dealing with some fuel tank issues on his Key West. While he was heading over, we bumped into the new neighbors who recently bought the waterfront house across the street. I had already introduced myself the week before, so I listened as my '04 Merc neighbor introduced himself. It turns out the new neighbor is a former outboard mechanic with 20 years of experience. He casually mentioned, “If y’all need any help with outboards, let me know.” Naturally, I took the opportunity to fill him in on my troubleshooting journey.

After hearing my story, he clarified that using the choke and observing RPM increases doesn’t necessarily indicate a fuel delivery problem; it’s more likely related to the carburetors. He explained that a true fuel delivery issue would show up if squeezing the primer ball caused the RPMs to climb.

With that in mind, we went out for a test. After leaving the no-wake zone, I pushed the throttle to wide open. Right away, my '04 Merc neighbor noticed that the engine sounded like it was “missing.” Once on plane, I was able to reach 4,600 RPMs with the engine trimmed just right, which is strange since I seem to gain RPMs every time I replace the water-separating fuel filter. The weather was significantly colder, so I’m also wondering if the lower humidity and denser air might have helped.

At wide open and around 4,200 RPMs (with the engine trimmed low), I pressed the choke, but instead of increasing RPMs, the engine bogged down and lost power. This seems to rule out a carburetor or fuel delivery problem and points more clearly to an ignition issue.

We looped around the bay and into the bayou near the new neighbor’s dock. He listened as the engine ran at wide-open throttle and agreed that it sounded like it was only running on two cylinders. This is puzzling, given that the inline spark tester showed spark on all three cylinders.

Any insights on what might be causing this or additional tests I should try?

The new neighbor mentioned that he has used CDI Electronics parts and trusts their quality. I’m planning to order one new CDM module. He’s on the fence about the likelihood of all three CDMs failing but suggested that, since they share a common ground, a single bad CDM could affect all three. Given that I initially bought cheaper CDMs, I’m considering replacing all three with higher-quality ones to finally resolve this issue once and for all.

Thanks for all the help so far.

 

[dustychops]

Update:

I took the boat out today after investing $350 in three brand-new CDI Electronics CDM modules. Unfortunately, this didn’t resolve the issue—my engine is still unable to reach over 4,000 RPMs. It looks like the CDMs from "ROP Shop" weren’t the problem after all. The downside is that the new CDMs are non-returnable, which makes this outcome especially frustrating.

At this point, I’m considering replacing the OEM wire harness that I modified during my DVA tests, even though it will cost another $85. I’m also planning to buy both a new trigger and a new stator. The reasoning behind this is that if I replace just one (either the trigger or the stator), I would still need to cut into the new harness to connect it, which defeats the purpose of replacing it in the first place. Alternatively, cutting the connectors off a new trigger or stator isn't ideal either.

Given this ongoing challenge, does anyone have any thoughts on whether there could be an underlying issue outside of ignition components? I’m wondering if there’s a less obvious factor that I might be overlooking.

As always, any input or insights would be greatly appreciated.

 

[racerone]

Cylinder sleeve out of position ?-----There is no mention of a new water pump impeller.----That motor needs a new one about every 5 years.

 

[dustychops]

Update and Response to @racerone :
Thanks for your reply and suggestions! I should have mentioned the additional maintenance I've done on the engine. I previously outlined these on another thread when dealing with the warning horn not activating when the key was turned to the “on” position. For context, the 1998 75 HP 2-stroke has some unique features, including a combined overheat and low oil alarm that gives a constant warning. This configuration allowed the removal of the warning module, which turned out to be the cause of the missing warning sound.
To address your points:

• Water Pump: I did replace the entire water pump assembly and impeller, so that part is already taken care of.

In light of your comment about the cylinder sleeve, I picked up a borescope from Harbor Freight this afternoon to check if there were any alignment issues inside the cylinders. The inspection showed that everything appeared in order, with no signs of misaligned sleeves. I even put the engine in gear and used the prop to rotate and view the intake ports. The clearance on either side looked consistent for each opening.
I then revisited the basic principles of “fuel, spark, and air,” as my neighbor reminded me. With my DVA tests coming back normal, I decided to inspect the air intake and removed the airbox cover. And here’s where I discovered the real issue—my butterfly valves were not fully opening at full throttle! At the console, they only reached about 60% open.
Throttle Adjustment: The throttle assembly has three key adjustments:

1. A spring-loaded idle adjustment with a finger knob.
2. The WOT (wide open throttle) timing adjustment.
3. A full-throttle stop screw to prevent over-stressing the carb linkage when pushing the throttle hard.

The stop screw's nut was loose, allowing it to move and limit the butterfly valves’ range. This oversight kept them from opening completely. I tightened the nut to secure the stop screw and adjusted it to ensure the butterfly valves now open fully, but with minimal extra force to protect the carburetors.
Thoughts on the CDMs: While it might seem that replacing the CDMs was an unnecessary step, I can't dismiss the possibility that some ignition issues were at play earlier. The mechanic neighbor confirmed that he heard misfiring, and my initial tests with an inline spark tester showed inconsistencies that have since cleared up.
Next Steps: I plan to take the boat out tomorrow morning to test these adjustments and see if they solve the RPM issue once and for all. Fingers crossed this is the final piece of the puzzle!

---

Any additional feedback or thoughts would be greatly appreciated. Thanks again for everyone’s help!

 

[dustychops]

Final Update:

I took the boat out this morning, hopeful that my latest adjustments would solve the persistent issue of being stuck at 4,000 RPMs with the engine missing intermittently. Unfortunately, the issue persisted.

Thinking that I needed to check my WOT timing under load (instead of the method I previously used with grounded spark plugs and the engine off), I set the timing to 18 degrees BTDC. This adjustment did not resolve the problem either.

A friend suggested pulling the spark plug boots one by one to identify the "trouble" cylinder by listening for changes. When I tested this, the engine bogged down each time a boot was pulled, indicating that all cylinders were contributing.

However, I discovered a significant problem that has led me to abandon this project altogether. A while back, I noticed that my new spark plugs were rusting prematurely, and the bolts on the water jacket were also becoming rusty and ionized. I decided to replace the water jacket gasket and thermostat, but during this process, six of the 18 bolts holding the water jacket to the engine block snapped. Attempts to remove these broken bolts with EZ-out bits failed, and I had to drill and tap the holes. I secured them with smaller bolts, washers, and liquid gasket.

When I tested the engine on a garden hose, I noticed minor leaks around the water jacket, which I thought were manageable with WD-40 after each use. However, during today’s test, I saw that under higher pressure (around 3,000 RPMs with the cowling off), water was gushing out like a geyser near the middle spark plug. The entire area was getting soaked, which likely contributed to a loss of spark power and engine misfires.

In an attempt to troubleshoot further, I grabbed a spark plug boot while the engine was running, not realizing my friend had left to get a tool for that purpose. I felt a tingling shock, which suggested that 45,000 volts were finding a way to "float" through the water. This reinforced my suspicion that the water leak was causing electrical interference and misfiring.

Given the extensive issues and failed repairs, I've decided to dismantle the engine and sell it for parts on eBay. Thank you to everyone who provided advice and insights along the way.

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Feel free to adjust any details or add more context if needed!