'04 Merc 25hp dead(ish) cylinder

[nola mike]

Been chasing some poor running for the last few weeks. Originally was running well, but had a couple of episodes where it died at speed, difficult to restart. Noticed fuel leaking from somewhere around the carb. Rebuilt the carb with a crappy kit. Took it out once and all the gaskets disintegrated (impressive fail). Got a sierra kit and rebuilt again. Couldn't get it to run right, went back and set the timing/idle/cables from scratch, which didn't improve anything, especially at idle. Took carb off multiple times (it seemed like it was running lean, and idled better with the primer knob pulled out). Tried to verify that I didn't screw anything up, primer circuit seemed like it was working, needle seating, all idle ports (I found 3) rodded out, etc.

Today I had it idling pretty well on land for about 30 minutes before it started acting up. I pulled the plugs and the #2 cylinder was fuel soaked. Noticed this consistently whenever it stalled out.
Interestingly, whenever I pulled the primer out the engine would speed up, and it will keep it from stalling for a bit.

I put a spark plug tester on both cylinders, and had a good spark on both. The engine ran the same with tester on either plug (so the other cylinder was working). Again, on one cylinder it liked the primer knob out.

Other pertinent info:
1. Wires replaced with OEM
2. A piece of magnet (maybe 1") broke off the outer rim of the flywheel. I could see this being an issue, but the stator output is still good, and the poor running preceded the problem. Although I'm getting good spark on both cylinders individually, wondering if I'm not getting enough capacitor charge to fire both? IDK.
3. Compression was 100 in both cylinders. Multiple issues with 2 different testers, but I measured it last year at 60, which was what I got when I used the same tester/tubing this go round. Also my starter crapped out so I was turning it over by hand for the test. I know it's low, but I'm fairly confident that it hasn't dropped since it was running well.

So what am I missing here? I'm tempted to chalk this up to compression, but I don't think this has changed, and both cylinders are the same. Again, it seems that only the lower cylinder is dropping out when both wires are connected but will run at least for a while on its own. So I don't know if the wet plug is the cause or the effect of the misfiring.

Reed valves? Would that explain differential cylinder fueling and these symptoms?
I haven't changed the plugs out yet, but the symptoms are the same and don't follow the plugs. New ones on order...

 

[nola mike]

TL;DR : I think I need a new switch box.

Alright, a few developments.
1. When I took off the carb again, the gasket was ripped and covering part of the intake. That wasn't the initial problem, but when I replaced it I was able to get it idling normally (well, sometimes on 1 cylinder) without any priming.
2. New wires, new plugs, new coils didn't change anything.
3. Today I got it out on the water with the timing light. Had it hooked up while running normally. When the cylinder dropped, so did the spark. After shutting off for a few minutes it would restart and run on both cylinders for a bit. If I kept going on one, sometimes the spark would come back but the cylinder wouldn't. If I then pulled the plug it was soaked. Cleaning/reinstalling the plug and all would be well for a bit.

I was fooled by the spark being present but the cylinder not firing sometimes. I think that running on 1 cylinder would soak the plug, and then if the spark returned the plug would still be wet and not fire. Soooo... although the parts cannon is getting a good workout, I can't think of anything left but the switch box at this point. The fact that 1 cylinder is unaffected (it's always #2) means that the charging coils/stator are OK. Any other thoughts?

 

[nola mike]

Wow, I'm totally lost at this point. Confirmed that I'm intermittently dropping spark (via timing light, difficult to see the light on an inline tester). Switch box, coils, plugs, wires changed with no change. If the charge coils/stator were bad it should affect both cylinders. Only thing I can possibly think of at this point is the trigger signal? To troubleshoot would I need an oscilloscope on the trigger signal to see if it's firing both negative and positive (or maybe a dvm with min/max ac volts?). Any and all thoughts greatly appreciated.
(Pretend that I'm able to post a pic of the ignition wiring)

 

[flyingscott]

Run with a timing light. You could have a bad CDM module or bad pick up under flywheel. Tests need to be done.

 

[nola mike]

Run with a timing light. You could have a bad CDM module or bad pick up under flywheel. Tests need to be done.

Thanks for the reply. As mentioned, the #2 cylinder is the one that drops when using a timing light. The switch box has been replaced. The pickup (=charge coil?), trigger, and flywheel are the only things left, but I can't see how any of those would affect only 1 cylinder? Which tests would you recommend at this point?

 

[flyingscott]

Doesn't that have CDM modules?

 

[nola mike]

Doesn't that have CDM modules?

No, it's got a (capacitor) switchbox that controls both cylinders and 1 separate coil per cylinder. Just ohm'd out the ignition stator and trigger, and both were within spec. But IDK if that's a good enough test. And again, I don't know if a bad trigger/stator could cause a dropped spark on only 1 cylinder.

 

[flyingscott]

Where did you get the switch box from?

 

[nola mike]

Where did you get the switch box from?

NOS Quicksilver. Be strange if both failed in exactly the same manner, intermittently on the same cylinder.
Yesterday finally got out there after dark so that I could use a proper spark plug tester. Even when the timing light showed firing, the spark was noticeably weaker esp at low idle (only bottom cylinder). Even though the cylinder was firing, the spark would occasionally improve and the engine would change tone. I'm circling back now to the flywheel--I'm wondering if the small broken magnet is enough to not charge the capacitor, and if it would only be on that cylinder. I'd LOVE a way to swap wires around to see if I can get the fault to change cylinders. But I'm a bit leery of frying something.
I ordered a DVA so that I can hopefully get a better handle on the stator and trigger outputs. Would an oscilloscope do a good job on this? Been looking for an excuse to buy one for a while.

 

[flyingscott]

right gaskets in the carb. how many magnets on the flywheel

 

[nola mike]

right gaskets in the carb. how many magnets on the flywheel

Carb has been apart so many times... It feels like a fuel problem so it took me a while to isolate to spark. 4 magnet flywheel/red stator.

 

[flyingscott]

check your stator

 

[nola mike]

check your stator

Resistance check OK, waiting on the DVA to check voltage output. Ordered a new flywheel since I don't think enough of the magnet is gone to substantially affect the capacitor charging/stator output, but who knows. I thought that the 2 stator wires together put out the charging volts, but if each wire is responsible for 1 capacitor in the box then that could do it. After the flywheel though, I'll only be 2 parts away from replacing the whole system.

 

[nola mike]

So I got the flywheel and installed with no change (my torx bit sheared trying to swap the ring gear, so now I've got to drill that out...). I did get the DVA. It was giving me some strange readings before it completely failed, but off the stator I was getting 310v off one wire and 80 off the other. Very unstable readings, so I don't know how accurate those are. However, on closer inspection of the stator when I swap the flywheel, it appeared that it was rubbed through. Couldn't tell if it was just the plastic or actually made it into the coil wiring, but I guess that's the next step. I'd love to upload a picture, anyone have a workaround?

 

[nola mike]

Just to hopefully close this out...
Got another DVA that read a bit better, and made the troubleshooting easy. Those stator readings were accurate. 300v on cylinder 1. Cylinder 2 would stay around 100v, where it made an inconsistent, weak spark. Enough to sometimes trigger the timing light and make a weak light on the inline tester, but not enough to detonate. When the voltage occasionally hit 200 the engine smoothed out noticeably. New chinesium stator installed and it ran great for 20 miles...until I started overheating.
Replaced water pump and saw that a tiny piece of impeller vane was lodged in the housing. Also had water in the gear lube and a corroded sleeve in the gear housing that needs to be addressed at another time in another thread. Running great for now.

 

[Mad Dog 2]

Just to hopefully close this out...
Got another DVA that read a bit better, and made the troubleshooting easy. Those stator readings were accurate. 300v on cylinder 1. Cylinder 2 would stay around 100v, where it made an inconsistent, weak spark. Enough to sometimes trigger the timing light and make a weak light on the inline tester, but not enough to detonate. When the voltage occasionally hit 200 the engine smoothed out noticeably. New chinesium stator installed and it ran great for 20 miles...until I started overheating.
Replaced water pump and saw that a tiny piece of impeller vane was lodged in the housing. Also had water in the gear lube and a corroded sleeve in the gear housing that needs to be addressed at another time in another thread. Running great for now.

Thanks for the update.