'95 Force 120 - bottom cylinder not firing

[Dave K.]

Hello all,
This s a 4 cylinder motor w/ 2 carbs. When I winterized the motor, the plug on the bottom cylinder was like new. The motor seemed sluggish the few times I could use it last summer. 15' boat would normally jump out of the water.
Where should I start?
Thank you

 

[Nordin]

Start with checking compression and spark at all cylinders.
Clean spark plug could be no spark issue, water intrusion in the cylinder etc.
Compression should be 145-150 PSI but down to 120-125 PSI it will run decent.
Most important is that the difference between the cylinders are not more the 10-15%.

 

[Dave K.]

Thank you for writing back. I dont believe the cylinder is getting spark. What could I do to check for that? These are surface gap plugs

 

[Dave K.]

Thanks again

 

[Redbarron%%]

As a quick check I use an induction type timing light.

 

[Nordin]

As you says it is a 1995 year model I think it has the Mercury Thunderbolt ignition system. Earlier engines had the Prestolite system (two blue or black CD boxes and four blue or black ignition coils). All these parts mounted on a plate at star side of the engine.
The Mercury system has one switch box (black) and about 6 inch long and 3 inch wide and four coils in a line.
Start to check for spark by pulling the plugs and ground them.
Run the starter and check for spark at all plugs, if the bottom plugs do not spark try to swap the ignition coil with one of the other three.
Check all wires at the ignition system for good connection.
To troubleshoot it further you need a DVA meter or a VOM and a DVA adapter.
Outboardignitiondotcom has troubleshoot chart for your engine.

 

[Dave K.]

As you says it is a 1995 year model I think it has the Mercury Thunderbolt ignition system. Earlier engines had the Prestolite system (two blue or black CD boxes and four blue or black ignition coils). All these parts mounted on a plate at star side of the engine.
The Mercury system has one switch box (black) and about 6 inch long and 3 inch wide and four coils in a line.
Start to check for spark by pulling the plugs and ground them.
Run the starter and check for spark at all plugs, if the bottom plugs do not spark try to swap the ignition coil with one of the other three.
Check all wires at the ignition system for good connection.
To troubleshoot it further you need a DVA meter or a VOM and a DVA adapter.
Outboardignitiondotcom has troubleshoot chart for your engine.

Thank you and will do. Inclined to think it's a coil pack for that cylinder. Appreciate detailing where it is. I'm also going to do a compression test just to see where the motor is at. Its It's been great for the 10years I've owned it

 

[Dave K.]

Thank you and will do. Inclined to think it's a coil pack for that cylinder. Appreciate detailing where it is. I'm also going to do a compression test just to see where the motor is at. Its It's been great for the 10years I've owned it

Yes, it is the Thunderbolt system.

 

[Dave K.]

Yes, it is the Thunderbolt system.

 

[jerryjerry05]

This works or one like it from Auto Zone