1988 Mercury 25HP Bogging - Misfire

[tdrudd87]

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

 

[achris]

Re: 1988 Mercury 25HP Bogging - Misfire

Hi Terry,

I would start with going through the fuel system first (cheaper and a bit easier). Those engines are pretty good as far as the electrical side of things go, don't see too many dead switchboxes or stators.

I would pull the carb and strip it down and thoroughly clean it, using new gaskets and diaphragms.. Also check the filter and its 'O' ring. Sometimes they don't get screwed on quite right and can leak air...

Chris.......

 

[tdrudd87]

Re: 1988 Mercury 25HP Bogging - Misfire

Chris,

Thanks for the response. I did disassemble and clean the carb tonight, will redo it with a rebuild kit this winter. 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

 

[achris]

Re: 1988 Mercury 25HP Bogging - Misfire

Hey Terry... You've done some excellent diagnostics there, well done... Certainly makes my job easier .... Simple answer is it definitely sounds like a switchbox...

Reasoning... Not stator... Only one stator providing power for the switchbox. If stator faulty, both cylinder effected... Same reason for the trigger. The trigger is just one coil of wire. There is a single 'ring' magnet around the crankshaft that fires 2 pulses per revolution. One is positive and the other is negative. Both from the same coil, down the same wires into the 'box. The positive pulse fires one cylinder, the negative pules the other cylinder. If the trigger was faulty, both cylinders effected.

Coils... you ruled that out by the swap (same as I would have done).... :thumb:

Only one thing left in the system.... Switchbox.... Here's a test that might proof helpful. Remove the 'box and toss it in the fridge for a few hours. Then put it back on the engine and see how long it takes to fail. If it takes longer than before, pretty conclusive...

Chris........

http://www.iboats.com/Boat-Parts-Ac...5--search_in_category.f--session_id.813302939

 

[25xs]

Re: 1988 Mercury 25HP Bogging - Misfire

Stator: I've replaced many, many of them on that engine. I've owned maybe 20 of the 18/20/25hp Mercurys and LOVE THEM! Stator

You should also unplug at the switchbox, the black/yellow wire and retest... You could also have a short in the tiller handle stop circuit wiring at the tight bend under the front of the engine where it comes out of the tiller handle. Check that first, the I would head straight for a new stator (used is a waste of your time).

 

[ANFAC76]

Re: 1988 Mercury 25HP Bogging - Misfire

I had that problem myself and it was due to the flywheel magnets, one of them was loose and could be moved with my finger around the disc. If that is the problem you just have to pull out the flywheel, sand it and glue the magnet back again the magnet. Be carefull not to break it whem pulling out the flywheel. Hope it helps.

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