Force 120 with low compression seeking advice

[jdamron]

This is a continuation of another thread - testing solenoid and where to get replacement that I started due to starting problems. At first I thought the problem was a starting system issue. After checking starter, solenoid, battery, cables, etc I checked compression as was recommended by other forum members. And I found cylinder one to only have around 110 while 2, 3, 4 were between 149 and 151. I am starting a new thread here as suggested because my problem does not see to be related to starting system.

As recommended by forum I have removed the head. The following is video of the pistons with head removed. It was hard to get good pics of all the cylinder walls so I tried here to get a more complete view with video. I do not really see any physical damage myself, and the scuffs on the cylinder walls are not too bad and not any worse in the cylinder with low compression than the others. However, I do see a lot more hard, black buildup on the top of piston one than any of the others. One member has recommended that I go in from the intake and try to remove piston one to inspect. Or, it could be I have bad Reed valves. I am considering doing that next and looking closely at the reeds to see if that is my problem. Please let me know any thoughts on the condition or recommended next steps. Thanks in advance.

 

[jdamron]

Re: Force 120 with low compression seeking advice

Here is another short video of the pistons in question.

 

[jdamron]

Re: Force 120 with low compression seeking advice

Here is one last short video of the pistons in question.

 

[jerryjerry05]

Re: Force 120 with low compression seeking advice


Bad reeds won't have an effect on the compression.
The head gasket and rings provide a seal that makes compression.
The reeds just stop the fuel from being pushed back out the way it came in.
From the shape of the dome, I'd guess that the rings are carbon'd up and stuck, making the low comp.
From the #s on top of the dome,It's been bored out on the #3 piston. .020
There are no valves in the system.Just reeds.
#1 pic is a reed pushed open.(not Force reeds).
The #2 is a set of good reeds.Closed.
They can be open about .010 and not affect anything..
Like I said,the reason can be anything?
Even the screws holding the transfer port being loose can let it suck air?
Lot's of reasons.

 

[pnwboat]

Re: Force 120 with low compression seeking advice

The top piston looks like it was overheated. The surface of the dome on the top piston looks like it was exposed to extreme heat. That's why it has that cratered affect as compared to the other pistons. Good bet that the rings are froze up in the grooves like Jerry mentioned. With that much heat, it just bakes the carbon hard in the ring grooves and the rings seize. From what I can see of the cylinder walls, they don't seem to be in bad shape.

 

[Big Fish Billy]

Re: Force 120 with low compression seeking advice

I would think good advice would be, since you have it all apart, mic the cylinder walls to see if they are in spec, install new rings, lightly hone the cylinder walls, clean the bolt threads, and get it back together.

 

[Jiggz]

Re: Force 120 with low compression seeking advice

Personally, having gone this far, I would take the piston out which in the process will also allow you to inspect the Reeds, rings, and other part of the piston below the crown. Although it is partially true that bent opened reed may not fully affect compression, it will however, reduce it compare to a perfectly working reed. During the piston's downstroke, the air-fuel mix is slightly compressed and prevented from blowing back out of the carbs by the reeds until the piston clears the intake ports, which then start rushing up to the cylinder without losing volume. With a bent opened reed, the volume of air-fuel mix is reduced due to blow back to the carbs thus resulting is slightly lower compression. So it's very possible bent or broken reed is the source of your problem.

 

[jdamron]

Re: Force 120 with low compression seeking advice

Thanks everyone for the advice. I had another really busy week at work and did not have a chance to work on this until just this afternoon. However, I was able to remove the upper carburetor and reeds and peek inside. It was not as much work as I imagined it would be to remove the upper intake manifold. And I was really hoping to find just an obviously bad reed, but it seems to me all the reeds are laying flat and I do not see any obvious problems with them. See pictures below. However, when I removed the reed plate on the upper left side I found a loose ball bearing just laying there. That can't be good. :blue: So I guess the next step is to try to remove the #1 cylinder so I can get a better look for damage. I am thinking I need to loosen the bolt shown with the red arrow in the picture below - is that the correct procedure for being able to push the #1 piston out? I only see one but I am assuming from the manual diagrams that there should be two that hold on the connecting rod cap. And does the piston need to be in any certain position or gear in forward or anything else special? I have a clymer force book but I can't seem to find it described how to take out a piston that way. Thanks in advance.

These reeds seem to all be laying down flat - no obvious damage that I can see.

Note ball bearing laying behind where reed plate was.

Is this one of the "Connecting Rod cap screws" that I need to remove next? I only see one. Should I rotate the crankshaft to get to both of them?

 

[Jiggz]

Re: Force 120 with low compression seeking advice

That is correct there are two rod cap screws and requires a 12 point 1/4" socket to remove it. The ball bearing is intriguing considering the rod bearings and even the wrist bearings are roller bearings. So this could only only mean on thing and that it came from one of the upper crankshaft bearings which should indicate there is lost of integrity or leaks in the upper crankshaft hence the lower compression on #1 cylinder. I recommend stopping at this point in removing the #1 piston but instead wait for advice on how to inspect the upper crankshaft bearing. Obviously, this will require removing the flywheel, stator, trigger and all those upper mounted gear. Hopefully, that ball bearing was just a debris from previous work but there's only one way to find out and that is to inspect the upper crankshaft bearing.

 

[jerryjerry05]

Re: Force 120 with low compression seeking advice

Gonna have to disagree with the ball bearing falling and low comp.
The rings and head gasket make compression when the crank is turned.
The seal above the bearing could be bad and sucking air and that screwed the piston.
I agree the motor's gonna need to be taken apart.
Bring it over and well work on it

 

[jdamron]

Re: Force 120 with low compression seeking advice

Thanks guys for the replies. Taking apart the motor was not what I was wanting to hear, but I know you are right. At some point when I get some time off I think I really will take Jerry up on his offer and drive over to the coast for help. In the meantime, what are the first basic steps to teardown that you would recommend? One thing I am confused about is how much to take off while the engine is still on the boat. Should I start disconnecting linkages and all connections from motor to boat and remove the engine mount bolts next? I am assuming I need to first put boat in forward gear and drop the lower unit. Or does anything else get taken apart before removing the engine? Also, do you have to get an engine hoist for an outboard that will lift it high enough out of the boat? Or can you take enough apart such that the engine is light enough to be removed by a couple of guys. This boat is in the backyard in the grass so I am concerned with logistics of how to get the engine off the boat and into the garage for further work.

 

[pnwboat]

Re: Force 120 with low compression seeking advice

The power head can be lifted off of the lower unit while it's still mounted on the transom. The problem is that it's pretty awkward to do. One person can lift the power head, but trying to do any delicate maneuvers with it is pretty tough. Two average guys can lift it off without too much trouble, but it's still awkward to maneuver around especially if you're standing on the stern of the boat. I would guess that the power head weighs around 150LBs or so.

I've seen all sorts of home made contraptions used to lift the power head off. Everything from 2X4 A-Frame to 8 or 10 foot step ladder.

 

[jdamron]

Re: Force 120 with low compression seeking advice

Thanks for the info pnwboat. I have a couple of really good neighbors so I think I may try to enlist their help after getting everything loose. My plan at this point is to 1) level the outboard while trim/tilt motor is still attached 2) put gear into forward 3) disconnect everything else that is connected from the powerhead to the cowling, remove all the bolts holding the power head on 4) see if myself and a couple of neighbors can get it onto a workbench in the garage. If we can't do it safely then I guess I could rent a engine hoist or build some sort of frame as suggested. I think if I can get it into the garage it will be an interesting project to disassemble. I know I will need help from someone like Jerry to fix it, but I would like to attempt the disassemble part myself.

Also I found this other thread on iboats that has some good info on teardown for engine similar to mine so I am using that along with my manuals for reference. At this point the engine is not working so I figure I can't make it much worse anyway.

 

[pnwboat]

Re: Force 120 with low compression seeking advice

Just so you know, there is an adapter plate bolted to the bottom of the power head where it sits on the lower unit. It's about a 1/2 inch thick. When you go to separate the power head from the lower unit, the adapter plate stays attached to the power head. The parting line is between the adapter plate and lower unit. Depending on what year, there also may be a bead of silicone sealant between the leg and adapter plate. It's pretty tough stuff, and can really be a bear to get the two separated. This is where a lift really helps. Trying to do it manually is very hard. Other than that, it's pretty straight forward.

 

[jerryjerry05]

Re: Force 120 with low compression seeking advice

You have a 96?
If so the lower unit dosen't need to come off.
The mid section(plastic) drops and exposes the lower bolts and nuts.
The rest should be reaced from the top.
The older powerheads had the exhaust connected to the adaptor plate yours doesn't.(good)
If you get it all unhooked it should lift off the base or mid section.
The Block weighs about 110#

 

[hundee r1]

Re: Force 120 with low compression seeking advice

That ball bearing is too small to be out of the upper crank bearing, and seems to be foreign to the engine
Id still only pull the top slug out to check compression loss issue.

 

[Jiggz]

Re: Force 120 with low compression seeking advice

That ball bearing is too small to be out of the upper crank bearing, and seems to be foreign to the engine
Id still only pull the top slug out to check compression loss issue.

Actually, it's not a bad idea to go ahead with the piston removal before deciding to go forward with the powerhead removal just to make sure the cause of the low compression (albeit slightly low only) is not from broken rings. Hopefully, as previously mentioned, the ball bearing was just a foreign debris from previous work accomplished. But you are absolutely correct, the size of the ball bearing is quite way smaller to those of the crank upper bearing.

 

[jdamron]

Re: Force 120 with low compression seeking advice

Thanks everyone for the ideas. I have had another busy week and just today had a little bit of time to spend on this project.

I actually just saw these last couple of posts suggesting removing top piston first or otherwise I might not have done the work I have done so far in disconnecting the powerhead. I should have checked this thread again before I started.

However, disconnecting did seem to go faster than what I thought it would. I have since disconnected all the linkages and cables and unfastened everything I believe except for the eight locknuts and washers securing the powerhead to the motor keg. Those looked pretty tough so I just sprayed with penetrating oil and I thought I would let them sit a while before trying to loosen. I also ordered a lift ring that should be here in a couple of days. So am not sure - should I proceed as planned to remove the power head or should I try to just remove the top piston first as suggested in last couple of threads? I guess I could do that while waiting on lift ring to come. Also, any special instructions for loosening those 8 final locknuts and washers?

One last thing - speaking of foreign debris - I found a large chunk of plastic just inside the lower intake manifold after I removed the lower carburetor. See pictures below. I am not sure if this managed to make its way in all the way through the carburetor or if this is from something else inside that is coming apart.

 

[jerryjerry05]

Re: Force 120 with low compression seeking advice

Wonder what the heck that's from??
Rubber? Plastic?

 

[jdamron]

Re: Force 120 with low compression seeking advice

Wonder what the heck that's from??
Rubber? Plastic?

Yes I was surprised to see it there. It feels like hard plastic. I am thinking maybe it is from the upper cowl. The upper cowl has some areas that look like have been repaired. Maybe a piece came off and was sucked in?