head gasket or rebuild

[jay1216]

Hi ive got this 120 force 4 cylinder ob that was running great till late last fall it started to idle really bad and was harder to start. So this spring i gave it a tune-up, decard,adjusted the carbs and so on. still running like junk. So i ordered a compression tester and spark tester. Did the comp. test on the ob with it being warm and the results where #1 125 #2 125 #3 125 and # 4 was 35 psi. So not good right what would be my next step? you know the boat runs great at midrange and wot,just idles like crap. Dont know if its the cylinder or just a headgasket. Also ive notice after running the ob that the bottom plug is really covered in black oil. But the rest are a tan residue.

 

[Frank Acampora]

Re: head gasket or rebuild

Well, Jay, you said it in the title! Your next step is to take off the head and see what the damage is. It could be as simple as a broken head gasket or as complex as a scored cylinder and melted piston. You won't know until you look.

 

[jay1216]

Re: head gasket or rebuild

yeah, well i was hoping it could be something minor. i hate to take the head off for nothing. Thanks for the advice, but it seems like the cylinder is probaly shot huh. If it was just a head gasket wouldnt the compression on all of the them be low not just the one cylinder. But like you said time to pull the head

 

[Frank Acampora]

Re: head gasket or rebuild

35 PSI is not a minor thing. You need to remove the head. It is not difficult and at this point it is not for nothing--you absolutely need to examine the head gasket and cylinder. And no, a bad head gasket will not affect all cylinders. If all were low at 35 it would indicate major engine damage with a probability of no repair.

Once you remove the head, look at the metal ring inside the gasket at the affected cylinder. Then look inside the cylinder for scoring. If the only thing you see is a broken gasket ring or the fiber part burned, then a new head gasket will probably cure the problem.

Do not assume the cylinder or piston is damaged before you examine it.

 

[jay1216]

Re: head gasket or rebuild

ok frank the manual i have says if you have a low reading on one of the cylinders do a wet compression test on that cylinder and see if it rises, it went from 35 to almost 70 when i did the wet test. Could the ring just be stuck from carbon? I did a decarb on it and it smoked like a mother, but the plugs looked normal except the one from the problem cylinder the bottom cylinder it was really fouled with black gooy tar looking stuff.

 

[Frank Acampora]

Re: head gasket or rebuild

Even at 70 PSI the compression ratio is still way too low and still too far from the other cylinder's values.

A wet test with values as low as 35 psi is almost worthless. What the manual means is if one cylinder is more than about 10% different (in your case 12 PSI), do a wet test.

While it could be a stuck set of rings, I doubt it. If it were my engine, I would have already removed the head without bothering to do any other testing.

What you have is an engine that is basically running on three cylinders. At high throttle settings, the power produced by the three good ones overcomes the drag of the bad cylinder but at idle where all cylinders are producing little power, the dead cylinder is apparent and you will get possible stalling or hard starting, or crappy idle.

 

[jay1216]

Re: head gasket or rebuild

Well looks like we gonna need a rebuild!!

 

[Frank Acampora]

Re: head gasket or rebuild

Whoa! Not so fast!

Your engine has broken a ring and damaged the piston. However from what I can see in the photos, the scoring is not so bad that it would need to be rebored oversized. What does the cylinder look like where the ring broks and flipped out through the piston land?

If that were an engine that I were repairing for a customer I would recommend overboring because I know that it is a sure cure.

However, if that were MY engine, and the scoring I see in the second photo was the limit of damage to the cylinder I would remove the carb, manifold, and vee block plate for the bottom two cylinders. I would then remove the rod big end cap through the vee block opening and push out the piston being certain to account for all 16 rollers on the rod big end. I would lightly hone the cylinder then insert a new or good used piston (3.375) into it. After placing the rollers back and attaching the rod cap I would align it then torque to 180-185 inch pounds. If you choose this option, search for other posts describing how to do it.

Way easier, cheaper, and very likely to be a satisfactory repair.

All Seafoam users also take note! Oh, by the way: Notice how well the decarb cleaned your piston? Stuff really works, doesn't it? Piston is nice and black just like a new one---NOT!

 

[steveclv]

Re: head gasket or rebuild

I wish we could start a thread of 2 stroke myths.....

1. You do NOT need to decarbonize your motor - period!
2. You do not need to use the most expensive oil - the cheapest oil to the spec is fine
3. Reving higher than the recommended rev range uses more fuel and generates less power
4. Rebuilding a carburetor is not needed - new needle valve and gaskets + a good clean is all that is required

and probably a dozen more

 

[jay1216]

Re: head gasket or rebuild

well the second photo is actually the #2 cylinder, but that cylinder has good compression for now. The photo with the melted piston which is #4 bottom has very little damage @ the the melted piston area, but like i said hell i almost couldnt feel the score so its not that bad considering the way the piston looks. Cylinder # 2 is way worse than that. I started tearing the engine down last weekend, Do i need to pull the powerhead off to get the intake off. Ive got everything unhooked i just need to fig. out how to get the powerhead and intake off. Im gonna try and fix this thing but gonna be hard with the way i work!! Thank you for your 2 cents worth

 

[jason32038]

Re: head gasket or rebuild

Yes in order to get the crankcase off you need to remove the powerhead and also remove the matchplate? You also have to remove the flywheel..stator and trigger and also the crank seal holder thingamajig. Look for the areas to pry it apart it's obvious. I used a cro bar. Dont forget about the 2 smaller block bolts on the opposite side of the other block bolts. Sorry if this is confusing I cant understand myself sometimes. EDIT...sorry my 85hp had 2 bolts on the other side..not sure about a 120.

 

[jay1216]

Re: head gasket or rebuild

ok so had some time to mess with this thing, got everything disconnected,powerhead, stator,trigger,crank seal holder. My question is how do we get the intake off?

 

[Frank Acampora]

Re: head gasket or rebuild

Remove the match plate and exhaust down tube as an assembly. There are four 3/8 nuts under the match plate which hold the two crankcase halves together.

The white arrow pointing up in the first photo is pointing to the center of the front half of the crankcase.

In the first photo, the white arrow pointing down appears to be pointing to the lower manifold and reed plate assembly. To remove the front of the engine case, you need to remove all the bolts holding both manifolds and remove them and the reed plates, Inside each manifold right where the semi-circular bumps on the outside are you will find two 3/8 allen head capscrews. You remove all the 9/16 bolts and the 9/16 nut off the top stud. Remove the four allen head capscrews then remove the two fillister head screws shown in the second photo. They actually function to clamp the lower seal but will hold the block together. Finally, remove all the flange bolts--I think 6 per cylinder. Remember the one that goes in from the back behind the starter mount and I think the other side too. Now drive out the two locating pins in diagonal corners of the block and finally using the pry points, pry the block halves apart.

 

[jay1216]

Re: head gasket or rebuild

Ok those two with the slots are being a pain!! they are soaking in pb blaster for about 24 hours now and now ive got to find a 3/8 allen socket. do i need to take that whole piece off just to replace the piston. You said the piston walls didnt look that bad. You think i should replace the rings on the other cylinders?