Third Cylinder Compression

[Simply Slinky]

Alright, much as I was afraid, the third cylinder on my 94 SL 650 has low compression. They read 115-115-80psi. All plugs were wet when I pulled them out (they look like that after actually running it too). I'm wondering what I need to do at this point. I know the carbs need adjusting because I think it's running too rich if the plugs are wet all the time, but I'm wondering if the lack of some compression in that third cylinder is going to be too detrimental to continue running it that way for a while. Although I've never done anything like it before, I could probably rebuild the top end of the engine myself, but I just hate to risk screwing something up that does in fact run at the moment and hasn't given me any problems other than being a little slower than my wife's Yamaha. What suggestions do you guys have. Thanks....

Nathan [img]smile.gif[/img]

[John Sharp]

I dont think it would be wise to run with a bad cylinder. You haven't noticed any loss of performance? Are you sure you got an accurate reading? Is so I would pop the head and look for scoring..probably time for a top end job..if you hurry you might get back out for a little bit this year.

John

[Simply Slinky]

John,
Is it possible to just rebuild one head? I was under the impression that if you popped the head on one cylinder you might as well do them all. I've never rebuilt the head on an engine before. I'm pretty good with wrenches, but I've never tackled such a monster. :(

[tomsl750]

If the scoring is minimal, then you *MAY* be able to get away with new rings and a little bit of honing. However, if this were my ski, depending on the severity of the scoring, I'd redo all of the cylinders.

[pcalonge]

I had a similar problem this spring, on the first outing of the season. My son rocked an impeller, then made the mistake of limping it back to our camp, overheating the motor. Anyway, after a pump repair showed that the engine had a problem, we discovered the compression was 122/125/62 psi in a 1050 domestic.
As we had just replaced the motor in our older 1050 with an SBT reman., I wasn't really eager to buy another new engine so soon, although I am very happy with the new SBT engine. I decided to look into the original motor, which had only 36 hours on it.
Removing the water jacket and dome (head) on the bad cylinder revealed a badly pitted dome and piston crown, with what appeared to be minor scuffing on the visible cylinder wall. Removing the intake/exhaust and the bad cylinder, the damage to the bore did not appear to be too horrible, some scoring with a minimum of sacrificial aluminum from the piston. A very-very-very carefull inspection of the crankcase with a strong flashlight shoed no aluminum pieces-parts around the fragile crank bearing. As the other cylinders appeared unhurt, we decided to try the cheap fix, hoping the motor would last at least through the season. Figured we could replace the engine during the long Wyoming winter.
Anyway, replaced the bad piston $105.00, gaskets & o-rings were $64.00, I luckily had aquired a spare set of stock domes and so had one on hand to replace the pitted one. I had enough of the various gasket sealers left from the SBT install kit to button everything up.
I had to hone the cylinder bore quite a bit to clean it up, and carefully knocked a rough edge off the exhaust port and one of the transfer ports to prevent the rings from hanging up (don't try this if you are uncomfortable with a dremel, it is important not to raise or lower the port unless you really know what you're doing!) The piston clearance was about at the max, but the replacement piston is a WSM and I've heard they need a fairly loose fit.
After hand turning the engine about 50 times with lots of oil in the bore to ensure the rings would't hang, I fired it up. The hour meter is now at 72 hours and the motor is as strong as it ever was- I am extremely lucky.
Conclusion- you can sometimes get by with a cheap fix. We parked the boat without running it after the impeller replacement did not fix the problem, my engine had low hours, the other cylinders were ok, the dealer had the parts I needed, I had a replacement dome, and I had the time and the tools to do it myself.
I would not continue running the boat untill you look at the bad cylinder- you could cause damage that would make the motor unusable as a core. If you have a bunch of hours on the motor, the cheap fix might not last very long when the other jugs fail, so a remanufactured engine would be more cost efficient.