1999 Force 120

[Alan407]

Hello all,
I'm having an issue with my Force 120 that I can not figure out for the life of me

As stated in the title it is a 1999 120 HP
This weekend I got stuck in some mud and ran the engine in the open to clear any sand that might've got sucked up. I was cruising at WOT later in the day and apparently over heated (cant hear buzzer at WOT) The engine lost power and stalled out. Tried cranking it to let it idle and cool down and all I got was a slow crank.

I got home and tried to isolate the slow crank. Here's all I've done so far
Fully charge battery
Clean ground on block and solenoid connection.
Apply direct power to starter
Tried 3 different starters
Tried cranking with out lower attached (water pump is still good btw and spins easily by hand)

All of the above still produced a slow crank
I can turn the engine by hand with the plugs in. I removed the plugs and it spins freely by hand, starter will spin it with no issues. I peeked through the plug holes and pistons looked okay as far as I could see. No aluminum on electrode.

Today I will adjust timing. adjuster was missing a nut and I was able to turn screw by hand
I will also try to get compression numbers.

 

[jerryjerry05]

Change the buzzer to a car or motorcycle horn.
Load test the battery?
Do the comp and spark test and post results.
One cyl. with low comp gives the same symptoms??
OR?? the drive bearings seizing can give them too. Check the oil for shavings?

 

[Alan407]

Cyl 1-3 low compression (about 50) Cyl 4 has 125

No deep scoring in walls. Cyl 3 has some, barely catches finger nail the rest even have some evidence of crosshatch still. Pulled exhaust cover off and snapped 8 bolts in the process =( rings in 1-3 seem to be seized.
I have a 90 hp parts motor with good compression. Can I use those pistons? I think I read somewhere the 3 and 4 cyl have the same piston size? Also the plate behind the exhaust cover seems warped, is that big deal?

 

[Jiggz]

The bore and stroke for the 90's come in two flavors which is also the same for the 120's. Depending on the year here's what it looks like:
1990-90HP and 1990-120HP = 3.312 x 2.8
1990-99 (90HP) and 1990-99 (120HP) =3.375 x 2.8

Since the 120 you have is 1999, hopefully the part motor is also of the same year group. The other thing to check is if the 99-120HP pistons and the 90HP pistons are still of the same size meaning if both are still original sizes and not over bored yet.

The other option is to buy brand new piston and ring combo and a wrist kit from Wiseco, if the part motor is not the same size. Of course, you still want to find out if the pistons are still the same original size or if it's over bored to what size. Usually, there's a stamped number on the crown to indicate if it's still original or not.

Personally, even if the piston sizes are still original and the same on both motors, I would at least replace the rings.

 

[Alan407]

The 90 HP is a 1996 but I will definitely take a caliper to the pistons first to make sure. As far as rings I may run these for now just to see if this will work and baby the motor for the rest of the season. If it holds ill come back for a proper rebuild.

I've read its not necessary to remove the power head to replace pistons but with those bolts being broken off on the exhaust cover I think the only way I can get to the bottom ones that snapped is by removing the power head. Any tricks to this? I plan in welding a nut or bolt to the snapped off bolts.

I know Force isn't really something I should dump a lot into. Here's my shopping list so far
Exhaust gasket x2
Head Gasket
Carb bowl gasket (Had a leaky carb for a while)

I'll try to get pics posted either today or tomorrow. Im new to boats and outboards but have been into cars and on car forums forever, I know pics always help make it an interesting read and a may help someone in the future

Also, I'm concerned with what might have caused the over heating in the first place, I pulled the head and all passages are clean, impeller looks fairly new. Maybe at WOT I was revving too high or trimmed too high?

(Any input on the plate behind the exhaust cover? BTW great idea on a car horn)

 

[Jiggz]

Whenever an outboard, especially this Force motors, get stuck in the mud the first thing that gets clogged is the t-stat. And at WOT by the time you hear the overheating buzzer, damage to the cylinders, rings and pistons is already happening.

Yes, you can remove pistons by dismounting the carbs and intake adapters and v-blocks (reed) to access the rod caps. If you decide to go this route, make sure you do some reading first. There's a lot of thread regarding this topic.

Dismounting the powerhead is another option but you can also access the lower exhaust bolts by unbolting the lower cowl and rotating it.

 

[pnwboat]

Rings seizing up in the ring grooves after over heating is a common problem with two stoke motors. Sometimes you can take the rings off and clean the ring grooves up (remove the baked on carbon) and you're good to go. Sometimes you have to replace the rings and maybe the pistons if there is some damage.

Make sure you check the water passages in the exhaust cover. If sand gets into the exhaust cover, it may restrict the water flow as the water passages in the exhaust cover are pretty small in some spots.

 

[jerryjerry05]

The 96 and 99 should have the same size pistons.
The warped stainless plate in the exhaust is normal.

IF??? you can scratch the inside of the cyl and catch anything with your fingernail??
It needs to be fixed.
If you don't it won't last.

 

[Alan407]

I got the 90 HP pretty torn down last night funny thing is the crank looks BRAND new obvious oil staining on the rods but the crank literally looks like you can eat off of it. Either way, I didn't have a 12 pt for the rod caps and am still trying to figure out how to get to the exhaust bolts that broke without pulling the power head on the 120 which at this point seems impossible. Of course the hard to reach ones broke below surface. Anyone have a link they can direct me to that shows powerhead removal?


Cyl 3 has scratches that just barely catch, I have to move my nail across at just the right spot at the right angle. This can be honed?

 

[Jiggz]

Yes, it can be honed. In fact, if you are changing to new rings you are required to hone to de-glaze the cylinders to seat the rings perfectly.

 

[jerryjerry05]

Since Mercury changed the drive shaft housing(95), it's harder to drop the cowl and access the bolts.​

You'll probably need to remove the powerhead to access the bottom bolts that are twisted off.
It's much easier to work on when it's on a bench.
Then you can take it to a machine shop and ask them about the grooves in the cylinder walls.

The block/exhaust chest and cover, the mating surface can be hard to reseal.
I have the shop cut them down and use sealer when reassembly is done.

 

[Alan407]

Just a quick update, after honing most of the scarring went away, just a few lines left. Im going to have to remove the power head to get those exhaust bolts off, I should've never unbolted it but hey learning curve.