what will low compression do?

[jgtrev]

I have a 70's mercury 115hp inline 6cyl. When I bought it, I was told that there was low compression in one of the cylinders, but still had enough HP to get around in the 17 foot runabout. The boat was sitting for about 2 years after I bought it (would have tried to fix earlier, but was sent to Iraq). I have gotten the motor to turn, and the next step would be getting gas to it, and seeing if it will turn over. My question is 2 parted:

1. Would it be worth it to try to fix the motor up (don't have a lot of money to blow in the boat, and seems that for every step closer I get to starting the engine it costs another 50-100 bucks.) Or just scrap it and sell parts on Ebay.

2. If I keep it and get the motor to run, I was told that the low compression would just be a loss of HP, but the 115hp would still be pumping out 65-75hp, which would be enough for the fishing I intended the boat to be used for. Is this true, and if so, is there any long term effects on the engine being used with low compression?

Thanks for any help

 

[JB]

Re: what will low compression do?

Running an engine with a bad cylinder will ruin it, jgtrev.

You don't have to spend any money to do a compression test and tell us what you find.

 

[j_martin]

Re: what will low compression do?

Low compression indicates failing mechanical parts. When they totally fail, they usually do very serious damage. Notice I said when, not if.

hope it helps
John

 

[mthieme]

Re: what will low compression do?

Low compression indicates worn parts - not necessarily failing. Big difference.
You're talking powerhead here. Big bucks on a tower of power. You would probably be better off financially to scare up another used motor in better condition. Sounds like you have no other issues with the running of the motor. Eventually when the compression gets too low, it will let you know. Would this be the bottom cylinder by chance?

 

[jgtrev]

Re: what will low compression do?

Ok, me and a friend checked the compression. In cylinders 1 and 3 the compression is around 65, and 2,4,5, and 6 are 85-95. There is a spot along the head gasket where it looks to be possibly leaking compression. We are going to go ahead and tear it apart and replace all the gaskets, and the cylinder rings, and see where that gets us. My friend is an airplane mechanic and knows a little more about engines than I do, so we will see how this works out. Curiously though, I have not been able to find a head gasket. I have already gotten the power head gasket set, but it said that the "Head gasket sold seperately" but I am unable to find one. It isn't even listed on websites as existing, it is usually just a blank space where the part number would be listed. Any suggestions? It would have to have a head gasket wouldn't it?

 

[Chris1956]

Re: what will low compression do?

Inline sixes do not have cylinder heads that are removeable, hence no head gasket is listed. They do have a water jacket cover over the top of the cylinder, that comes off.

Your compression readings are horrible. I suspect your gauge is inaccurate. I recommend you get a better compression gauge and retest. Cyliinders 1 and 3 are the most likely to fail, and when they do, the compresion is 10 PSI or so. The other cylinders should be 100+PSI.