1991 Force 120 Hp Outboard Problem with Cylinder #2

[Force 120]

Hi,
I am new to this Forum.
My 1991 120 Hp Motor FORCE ran fine till I was 100 yards out from the marina and it started to act up and quit. The flywheel got locked. I removed the head and see what in the pictures
Anyone, Please give me a clue what happened to my outboard.
My 30 year marine mechanic come out and look at it and told me that the cylinder #2 was not firing. The outboard motor needs to be rebuilt for $1500.00.
It is more than the motor worth.

 

[pnwboat]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Looks like you melted a piston. Usually from running too lean. Carb problem can cause this.

 

[Force 120]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

One carb. feeds 2 cylinders. Why only one cylinder had the problem but the other was not. Could I just remove that piston and replace by new piston?

 

[pnwboat]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Many variables involved to explain why only one cylinder was affected. Most common cause for melted piston is lean fuel mixture which is controlled by the carb. Also pre-ignition caused by timing too far advanced or poor quality fuel (low octane or contaminated). You may be able to get away by just replacing the piston and rings. Depends if cylinder walls are damaged. If there are any grooves in the cylinder wall, it may have to be bored and oversize piston and rings installed.

 

[Force 120]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Thanks a lot for your info.
Other people told me that the water get in too the cylinder but upon inspection of the gasket and head, I don't see that water could get in. I have inspected the cylinder wall. It is OK, no visible groove in the cylinder wall. There is a light mark on it but it can be cleaned up with honing.
I have read other thread the instruction of how to remove one piston without dissemble the whole engine. Could I do it with honing one cylinder. Or I have to disassemble the whole engine and hone all cylinders as well. As per the pictures, Is the head still usable? The piston wall is still in good shape except the melted head. Could I just clean it up and reuse again ?

 

[tonycrespo]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Force 120, looks like something hard was floating around in there don't it.
Maybe you'll see more when you remove the piston. What did the plug look like?

 

[pnwboat]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

I doubt very much that the damage was caused by water. Looking at the picture, it appears that this piston was subjected to very high temperature. Looks as though there is some melting. Also if you notice, there is absolutely no carbon build up on the top of the piston. It looks like the piston got so hot, it burned the normal carbon build up right off. The head is probably OK. You should check for any sharp edges caused by the pitting. Any fine sharp edges could get hot and cause a pre-ignition. Head surface in combustion chamber area should be smooth. The pits are probably OK just as long as no sharp edges around them. As far as just replacing the piston and rings...well it hard to say from pictures. Actual measurements will tell you for sure. If there is no damage to the cylinder walls, then you are probably OK, but the cylinder should be measured just to be sure. I doubt that you need to rebuild the whole motor...That's just my opinion. Maybe some other folks will chime in with their thoughts.

The next problem you need to resolve is why did this happen? Like I said before, most common cause is lean fuel mixture. Most likely cause is a problem with the carburetor. Mixture was set too lean, piece of debris plugged up the small fuel passages in the carb, needle vale sticking etc. You need to fix that too after you replace the piston. Other wise you be seeing another melted piston. To prevent debris from entering the carb's, you should have an inline fuel filter between the fuel pump and carbs. Good insurance to prevent this from happening.

Timing should also be checked.

 

[actionpaintball]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

They does not appear to be any scoring on the cylinder walls. Pull all pistons and verify each cylinder diameter per specs. Hone if needed. You may get lucky and just need one new piston and ring kit, and go through your carbs. I am with everyone else-probably running lean or not getting good jetting on that carb.

You have to pull that piston anyway-just crack the case and you will know the whole story. It is possible that one cylinder just happened to be more affected by the lean condition-I am betting the other paired cylinder would follow but it was caught in time.

 

[Force 120]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

One thing still puzzle me that the cylinder #1 has a lot of carbon and oil when I removed the head and the cylinder #2 likes completely washed as well as spark plug. My theory is that the carb. feeds 2 cylinders OK but the reed valve for the cylinder #2 somehow did not function properly which stopped feeding gas/oil to it. I will post an update when I tear it apart.

 

[moparman]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

It looks like something has been floating around in the chamber,maybe part of the top ring if it got caught on the transfer port, but really hard to tell by the photo.I have also seen piston damage similar to this from water intrusion into the cyl,so it could go either way. Keep us posted as you tear it down. MOPARMAN

 

[Force 120]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Just got a other 1989 125 Hp Force without a lower unit for $200.00 which have piston #4 shot but have much better condition than mine. I bought it so I can take the piston to swap on mine.
Here is what the owner told me "the bottom (4) cylinder was not firing..there was spark, so the head gasket was shot and it may have been taking in some water".
It seems to me that these motors have the same failures. May be the design of the block ? My neighbor have a late 1970 120 Hp Chrysler with 4 cylinders and the same block as Force still runs good.

 

[pnwboat]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

If you look at the other forums, these failures are common to all brand of outboards.

You'll probably have to swap the piston and rod assy. If the pistons have pressed in wrist pins, you'll be surprised how much pressure you need to press the pin out. You needs tons of pressure. When you go to press the pin back into the rod, you need a special spacer tool to prevent damage to the wrist pin bearings. Pretty much impossible to do without the special spacer tool.

Most shade tree mechanics get around this problem by purchasing replacement pistons that use "floating" wrist pins. Floating pins are held in place with wire keepers and require modest pressure to insert into the rod.

One other thing to check is the piston size. Force motors use two different piston diameters. Your 1991 motor may take a larger piston than the 1989 model. 1989 std. bore is 3.312 inches. 1991 std. bore may be 3.375. In this case, pistons are not interchangeable.

 

[Force 120]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Hi Pnwboat,
2 motors are identical, could I carry the whole piston and rod assembly from one motor to other. Just other thought

 

[pnwboat]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Yes you can if the piston diameters are the same. Blocks may be the same but pay close attention to the cylinder size. The 1991 may have a larger bore than the older 1989. I believe the change in cylinder bore size came about in 1990, but you need to verify.

 

[Force 120]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Last night, I tried to remove the piston out but since the flywheel could not turn so could not see the piston rod bolts in order to remove from intake side.
I have to assemble the whole engine in order to remove the pistons. All the reed valves looked clean and good. I tried to remove the flywheel but I did not want it turn so the damaged piston could move (this piston got jammed now so the flywheel could not turn by hand). So I have to hold the flywheel solid and unscrew flywheel bolt. The problem is I don't have the tool to hold the flywheel. Does the impact gun work? Any ideals?

 

[pnwboat]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Impact gun will take the flywheel nut off, but you will probably need a flywheel puller, propane torch, hammer and a few choice words to get the flywheel completely off of the crankshaft.

Again I can't stress that you need to check the cylinder bores to verify the diameter between the two blocks. I suspect that the diameter of the 1991 block is 3.375, and the 1989 block is 3.312. This is only .063 of an inch difference. Enough to prevent you from using pistons from the 1989 block in your 1991 block. If you do, and the cylinder bores are different, you will be disappointed with the results.

 

[Force 120]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Does anyone use this method to unscrew the flywheel nut without a impact gun? Put the rope inside one of the cylinder which is at TDC and bolt the head back so the rope is sandwiched between piston and the head. The rope stops the piston moving but does not damage the surfaces.
Yes, I will check the measurement of pistons of both motors to make sure they are the same. I have looked up the spec. in internet. 1991 120 Hp is 3-3/8" & 1989 125 Hp is 3-5/16". So there is 1/16" difference.
Question both of them have same stroke but 1991 has 1/16" bigger bore and 5 Hp less ???

 

[pnwboat]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

The flywheel nut is torqued to either 125 foot pounds or 90 foot pounds depending on serial number. Yes you could try the rope method, but you may need help from someone to hold the block in place while you're cranking on the flywheel nut. Believe it or not, taking the flywheel nut off is usually the easy part. Getting the flywheel unstuck from the crankshaft is usually a task in itself.

 

[CharlieB]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Did the 91 still have the two ignition modules, each firing two cyl's?

Before Force adopted the Merc ADI/CDI coil ignition they used blue boxes, then their black box ignitions, sometimes had a nasty habit of 'double-firing' one side of the box, the 2nd spark occurred during the exhaust/intake phase of the stroke and caused leaning of that cyl, melting the piston if the power drop was not noticed in time. And it doesn't take long at WOT.

Once the motor is back together and running on the hose, check timing on each cyl with a light. You should only find the timing mark in ONE place per cyl. A double firing box will show the timing mark in two places, 180 apart.

Change the box and break the old one so that no one can ever try to use it again.

 

[Force 120]

Re: 1991 Force 120 Hp Outboard Problem with Cylinder #2

Remove the fly wheel nut was an easy part. I put a cloth on top of an piston at close to TDC and bolted the head back. This method prevented the piston moving as well as the fly wheel. Remove the fly wheel with harmonic pulley was easy as well. But it took me a week to figure out how to remove the power head out of its lower part. Previous owner put them together without gasket in between. He/she just used a sealant to glue them together. It was a hell to take them apart. I hammered, heated etc. and broke a pry bar. Finally, I had used a vibratory hammer to separate them. I am at almost final stage to remove the pistons out but it is too dark so I leave it for tomorrow. I found out that my 1989 125 Hp pistons are 3-5/16" with 0.020" stamp on the top of piston. So swapping pistons is not an option. So I decided to rebuild both power heads as well so I can have one for spare.
My 1991 still have the two blue ignition modules, each firing two cylinders.