1991 Force 120hp Bottom Cylinder No Compression

[PaulO]

I compression tested my engine today (basic hand turning the fly wheel with compression test meter plugged in. I could feel strong compression on 3 cylinders.) The bottom cylinder had none and there was, I guess, aluminiam buildup on the spark plug core (definitely buildup of some kind.). The rest of the cylinders had compression. When I bought the motor the owner told me there was a clunking sound in the gear box but now I have discovered this #4 cylinder I am not so sure.

I await my clymers manual.

Can any gurus offer insight to the spark plug and lack of cylinder compression?

Paul of Oz

 

[johnfarrow]

Re: 1991 Force 120hp Bottom Cylinder No Compression

G'day Paul, How much compression were you getting?
Another way of testing compression is to take out all plugs, push in the compression gauge and use the starter motor to crank over the engine (1 or 2 seconds would be plenty. This may give you a more accurate reading.

The compression problem could be any number of things.
Head Gasket,
Cracked Head,
Piston rings,
Deep scrach on the cylender wall, ECT...

Most common problem would be the head gasket.
Altho this is good because it is easy to replace & a large marine shop would stock this item, and it is not too mean on the hip pocket.

The first thing to do is crack open the head. Have a look at the gasket for cracks and breaks. this might be the answer to your problems.

now have a look at the head itself, look for hairline cracks, altho if is has no compression as all it could be a decent sized crack

Next thing inspect the cylender wall for deep scraches. if you fing any, more than likely it is a broken piston ring. it has damaged your cylender wall.
If this is the case it is pretty major, cylender block will need replacing along with piston ans rings. But it might be ok with a rebore.

Just something to get started with.

 

[PaulO]

Re: 1991 Force 120hp Bottom Cylinder No Compression

G'day John,

Compression assessment was very basic but there is definitely a problem with number 4 cyl. I haven't got the battery hooked up so I couldn't get a true picture of the good cylinders compression

What took my interest was the metal deposit melted on the spark plug tip. I will bring the engine to my place to start working on it. It may take a while but I will inform you in the near future as to it state.

Paul

No4 Spark Plug

I would be interested to hear others point of view.

 

[johnfarrow]

Re: 1991 Force 120hp Bottom Cylinder No Compression

Defenitaly a problem with cyl #4

U need to crack the head to find out more

Good luck

 

[PaulO]

Re: 1991 Force 120hp Bottom Cylinder No Compression

1. There appears to be no problem with Gasket. Note: Still on head
2. There was no thermostat in the head
3. There is minor cratching in bottom cylinder with one area of metal buildup highlighted and piston pitting

Based on the oil marks on the head for all cylinders is it fair to presume the bottom carburettor is running too lean?

My local engine shop said they would re-bore if possible but were not interested in sleeving. Fingers crossed. I'm in Australia too.

 

[PaulO]

Re: 1991 Force 120hp Bottom Cylinder No Compression

From Frank Acampora via prvate message

"G'day! #3 piston shows slight aluminum throw-off due to detonation. #4 piston shows heavier aluminum throw-off due to detonation. The melted aluminum is what clogged the plug gap. This can be a blessing in disguise because with the plug not firing, detonation and damage stops.

Detonation is always related to a too lean carb setting whether the needle is set too lean or the carb had a low float setting or the carb had a clog-- doesn't matter, detonation occurs.

NOW the last photo shows aluminium ( British LOL) rub-off on the cylinder wall. It is not "welded" and it IS possible to scrape this off with a sharp knife or chisel without further damaging the cylinder liner.

It is difficult to see in the photos, but if the cylinder scoring is light, meaning you almost can not feel it if you run your nail over it, (less than .001 deep) Then it is possible to just re-hone the cylinder and not over bore it. But remember: factory tolerance for wear etc is only .002 so honing must of necessity be light. If you are careful, this can be done without disassembling the engine Remove the carb and reed plates and remove the head. Working through the opening, remove the rod cap screws and push out the piston and rod.

If you do need to overbore, it is only necessary to do the one or two cylinders affected. All four need not be done; the engine will never notice it."