One bad cylinder

[sandbar]

Have a 1995 Merc 135 (carburated). The engine had good compression in all cylinders (105 - 110) 2 months ago - but less than 1 month later the compression dropped in one cylinder to 65 (all the rest stayed the same). The first mechanic changed all the fluids, the impeller, and removed the thermostats which were in pretty bad shape. That one cylinder came up a little to about 75 psi. Took it to a second mechanic who said the oil injection wasn't working and set it up for pre-mix. He also showed me some slight scoring on the wall of the bad cylinder. The engine is now running between 170 - 190F which is hot according to the Clymer manual I got, but maybe not according to some of the posts I've read here. Am I overheating? Can I run the engine like this if the cylinder doesn't get any worse? I have heard you don't want the cylinders to be more than 5 psi off from each other - is this to identify if there is a problem or can you cause damage to the engine by running with one cylinder that is low? Any additional suggestions besides a total rebuild of the powerhead to keep the engine running good?

 

[Kenny Bush]

Re: One bad cylinder

Is the cylinder scoring in the same spot of the exhaust port? If so, it is most likely combustion temp too high. I use a compression variance of 10% or about 12 to 15 pounds before the powerhead must be rebuilt. When they get out of balance, they will shake themselves to death. As for the temp, it does sound a little high. Also check your timing for max advance. Sometimes if they overheat at Wide Open, you can back the timing off a little to reduce combustion temperature. Also keep your motor decarbed every 100 hours of use. Carbon could cause the rings to stick, and then the piston will begin to score.

 

[fgbutler]

Re: One bad cylinder

What is the best way to decarb an engine?

 

[sandbar]

Re: One bad cylinder

Thanks for the info BushCat. I don't remember if the scoring was in the same spot as the exhaust port - I won't get a chance to take it apart again until this weekend. I have noticed the engine shaking pretty hard lately but it only seems to occur between 2000 and 2500 RPMs and evens out at higher RPMs. At lower RPMs it doesn't shake too hard but seems to have a slight knocking or ticking noise. Can you tell me a little more about what you mean by shake themselves to death - what kind of damage can I cause and how long does it normally last? As far as the timing - the engine seems to overheat more at mid RPMs than at wide open - should I still check the timing? If I decarb the engine could it possibly increase the compression in that bad cylinder? Is there anything else I can do to even out the cylinders to prevent the shaking?