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Outboard compression question

Discussion in 'Boats and Motors' started by Salmonid, May 29, 2007.

  1. A buddy of mine was telling me about his outboard, a Evinrude 40 HP (about 15 years old) and had told me the compression was low on two of the cylinders, I think he said 125 and 105, can some one explain to me what exactly that means and is something like that fixable?,he made it sound like the engine was shot. are those numbers acceptable?? I see a lot of questions on boat sites regarding the compression numbers. What should they be?? and I assume there is a number for each of the 2 cylinders, right? and if a 4 stroke would have 4?? sorry but was hoping someone could educate me a bit here since I have an older outboard myself, would appreciate it.

  2. The numbers are pounds per square inch of pressure. It shows if the motor can compress the fuel/air mixture enough to make it volatile & if the cylinder can withstand the force of the fuel exploding & exhausting the byproducts of that explosion. I'm not real sure what those numbers should be but I've been told 125 is fine for most boat motors. The biggest problem is if there is a difference between cylinders the motor won't run correctly. To repair that the motor must be disassembled, have new rings, seals & gaskets installed. Can be very expensive, even if you do it yourself. You can buy rebuilt powerheads, usually with old sent in as core, for lots-o-$$$$.

  3. Those numbers are fine. Ideally, each cylinder will be within 10% of each other.