Mercury 25 hp misfire - stumped!

[TN_Explorer]

I have a 1999 Mercury Tracker 25hp 2-cycle engine. It will start and run, but has a stutter (misfire) at idle and at all speeds. The engine has always been a good one with no problems. It sat for 9 months while I had surgery, but it was winterized in anticipation of that.

- Compression is 135 on both cylinders.
- Pulling either plug wire makes the miss more pronounced.
- Spark plugs replaced
- Both ignition coils replaced.
- Stator coils replaced.
- Trigger coil replaced.
- Fuel is non-ethanol and clean.
- carburetor is pristine, but rebuilt it anyway.

None of the above made any difference at all. At idle, it lopes like a Harley. It takes throttle, but stutters throughout the rpm range. It will still move the boat up on a plane.

I am normally able to diagnose and correct most any problem, but this has me completely bumfuddled. emoBang

I live an hour away from Chattanooga, and don't know anyone to take it to. Near Tims Ford, if there are any reputable mechanics you can recommend.

Thanks in advance for any thoughts.

 

[silvertalon]

Do you have a service manual for it? Look to see if there's a bleed line check valve that could be stuck. Check the grey wire and/or rectifier/regulator could be an issue. Could be a short in the stop circuit- wiring harness. Ohm test wiring harness on motor. Also, put a timing light on your plug wires and look for a miss. That will eliminate fuel or electrical. Reeds? Could be a reed valve issue- cracked reed valve?

 

[TN_Explorer]

Thanks for the ideas -
I don't know about a bleed line check valve, I will look.
I have checked the rectifier, and that does not seem to be the issue. Removing it from the circuit has no effect.
I disconnected all ground (stop circuit wiring) - that made no difference.

I have considered a reed valve; I need to determine how to diagnose that.

I appreciate the input.

 

[silvertalon]

A loose flywheel magnet? possible/. I'd try that timing light. Index the TDC of both cyls by marking the flywheel and watch if the mark flutters when the engine stutters.

 

[TN_Explorer]

Thanks for the input. I forgot to put that on the list of items I've done -- flywheel was thoroughly inspected when the stator and trigger coils were replaced. No loose, cracked, or damaged magnets. Visual check of the spark is awesome; I am going to get a spark gap tester today just so I can cross this off the list. The whole engine looks immaculate, which is why this has been such a puzzle.

The only things I have not done are to put a timing light on it and to inspect the reed valves. That's on the to do list for later this the week,

 

[TN_Explorer]

Took the reed block off - it looks pristine, can see no light through it, none of the valves stuck. Blew it out with brakleen and compressed air. No change. Going to go back through all the ignition tests once again. . . . .

 

[Gator]

Silvertalon knows his stuff.

 

[TN_Explorer]

Checked the bleeder hoses, the one-way valves work fine. Send Silvertalon a PM - thanks Gator!

 

[silvertalon]

I believe if those coils are not the CDM type. That motor has a switch box. I don't see any mention of changing that out. I can get you a part number if you provide the serial number of the motor. Sounds like the 'stutter' that you mention could be the switchbox dropping one cylinder intermittantly.

 

[TN_Explorer]

Thanks, Silvertalon,
Engine: Mercury Tracker, Model 1999 25EL, Serial: OG829921

The coils in it are 339-832757A 4
I swapped the switchbox for a new one, but it made no difference, so I put the old one back in. 855713A3
It has a new trigger coil, 98450A10
I replaced the stator coils with used ones, no difference.

It's possible one of the replacement parts is bad, but I followed the CDI electronics troubleshooting as best I could and all the replaced parts test within spec. I did not replace the rectifier, but disconnecting it per the CDI guide made no difference. Removing either plug wire makes a noticeable difference, but it still has that "stutter" at idle and at all rpm ranges.

 

[TN_Explorer]

Re: Mercury 25 hp misfire - Solved

Even though I had inspected the reed valves, I replaced them. Got a set from Boyesen. Followed their instructions to the letter to insure the block was flat and installed them in the engine. I finally got to water test it last week and it runs like a new motor. In all my years messing with outboards, I've never had to replace reed valves, so I didn't suspect them. In any cases, all is good. Thanks for the help!

 

[silvertalon]

Guess there was nothing else left to try... Good Job!

 

[WaterChap]

Silvertalon, you are the absolute man.

 

[silvertalon]

WaterChap - 10/23/2018 12:03 AM

Silvertalon, you are the absolute man.

lolol!