1988 Mercury 25HP Bogging - Misfire

Hello all,

Recently purchased a boat with a 1988 25hp Mercury, serial 0B254011.

Runs great for a few minutes, then bogs down to a VERY reduced power level. Rough idle, only reaches 5-7mph at WOT. Cleaned plugs on the water, didnt help. Randomly ran good for 2-3 more minutes, and then repeated the problem. Headed back to the dock, it would "blip" more power for a second or so, then continue bogging.

Brand new gas tank, lines, and fuel, bottle of seafoam in fuel. Ran great when it wasn't misfiring.

No DVA meter, but I checked ohms from CDI's troubleshooting guide, stator ohms are low as follows:
Black/Yellow - 3060 ohms, spec 3250-3650
Black/White - 136 ohms, spec 150-250

Trigger ohms good. I dont have specs for the coils, but they are identical to each other, ~1ohm + to -, ~950 ohms plug lead to -.

I'm looking for any opinions, if I should order just a stator, or switch box as well. Anything else to check?

Thanks in advance, been awhile since i was on here.
Terry

Your specs look fine.

It (could) be the switchbox but given this is an 88 model there are other things I would suspect simply because of it's age (I work almost exclusively on 6 thru 25 horse 2 stroke Merc's, so I am seeing some patterns).

Mid/Late 80's models have been turning up with the following issues (simply due to age) and many present with the symptoms you note:

First, since you may not have a good history on the motor and it's "new to you" I would clean/rebuild the carb. A kit is not expensive and simple to install with basic tools (including a manual).

Also, at least change the water pump impeller unless you "know" it was done in the past year or two.

Some issues affecting these models because of age.

1) the coils - even if they are in spec they develop small spider cracks in the cases - the winding is fine but electricity "jumps" out of them through the cracks (run the outboard at night with the hood off and look for fireworks)

2) intake gaskets go bad - not the carb base gasket but the intake gasket on the side of the motor up near the top of the cylinders. Let's gas out and air in resulting in a "sneeze" (similar to a backfire) and drastically reduced power when you try to throttle up.

3) upper crank seal goes bad. Similar situation as with the intake gasket - gas out, extra air in, runs like crap - no power, poor idle, stalls

4) bad powerhead base gasket - sneezes, coughs, backfires - might idle ok, no power - backfires when you give it gas.

All of the above are fairly inexpensive fixes (for the parts) compared with a 200 dollar switchbox, so check the "cheaper" stuff first.

A Seloc's manual (35 bucks) can walk you through all the maint that this model may require...

(should have added. I have worked on these for over 25 years and have never personally seen a toasted switchbox on a small Merc - have heard of one or two but it is very rare)...

Graham, thanks for the info.

Cleaned the carb tonight. No rebuild kit, but torn down, sprayed down, and reassembled. Ran great in the barrel, until it had warmed up over the next 10-15 minutes, then same symptoms. Eventually wouldn't rev up, and stalled. Both plugs were dripping wet after it died.

It would restart, but seemed to be running mostly on 1 cyl. Waited for it to get dark to check for shorting. Started it up, noticed the white ceramic insulator of the plugs got a blue hue to it when the plug fired correctly. Never seen this before, sound OK? Could tell the lower plug wasn't glowing as often. Eventually as the engine warmed, the lower plug went nearly always dark, and the engine was definitely running on 1 cyl.

Switched plug wires and coil wires from the switch pack, problem stayed on the bottom cylinder, did NOT follow the coil. Checked Hz of the wires from the coil + to engine gnd. Firing cylinder had a signal on the green/yellow, dead cyl was getting nothing on the green/yellow from the switch. Ohmed the stator and trigger, same results as last night, or a touch higher, with a new DMM.

I believe they don't go out often, but is there anything I should still be suspecting other than the switch? Haven't pulled the flywheel for a closer inspection, but from what you can see from underneath, both magnets are attached to the fly, and there isn't any discoloration on any of the visible coils.

Thanks again,
Terry

Sounds like switch box. Several on eBay cheap.

Jeff

Yes, agree - does sound like the switchbox.

Like I said, I have heard of a couple going, just never had to change one myself (and I see quite a few small Mercs).

Because of that I can't advise about aftermarket parts in this case, but as Jeff notes, try ebay. A used one could well last 20 more years and it would be a fraction of the cost for new OEM (about 200 bucks if I'm remembering right)....