Posts: 3469
Location: Elk River, Minnesota | Hiya,
When testing compression, a gauge is screwed into the spark plug hole. Then the motor is turned over which compresses the air/fuel mixture in the cylinder. The gauge will read how much pressure the cylinder is able to hold. Each cylinder is tested independently to determine if there are any potential issues with the motor itself.
In the case of this motor, one cylinder is reading 120 psi, while another is reading 105 psi. psi standing for pounds per square inch..
When checking out a used motor, the general rule of thumb is that each cylinder is within 10% of each other. In this case, take 10% of 120 and you get 12. You would not want any more than 12 psi difference between any two cylinders you compare. In a nutshell, each cylinder should be holding a minimum of 108 psi up to 120 psi. The situation people are worried about here for you is that one cylinder is performing at 105 psi, which would be outside that minimum 108 psi value. That becomes an indicator that something may be amiss with the motor that the seller might not be aware of. I would be interested in knowing the individual compression values of each cylinder to see just how uniform the other cylinders are. If the one cylinder is showing 105 while the others are all 110 or higher, but not over 120, I would be raising a red flag and would not hesitate to make that known with the seller.
The last thing I would ever want to do is purchase another person's problem. If the seller is trying to hide something, a quick question to ask would be can the two of you together take it to a reputable mechanic (who has nothing to gain or lose) and have him or her go through the motor and perform various tests to determine the condition of the engine. If the seller hesitates at all on that request even if you as the buyer are willing to pay for it, this would be a huge red flag to consider and to be truly honest, a boat to walk away from.
Any seller worth their salt would not hesitate to allow a potential buyer to have the motor tested (at the buyer's expense, or an agreed expense).
My last rig I sold (in 2005) the buyer requested this and before he was willing to close the deal, asked if he could have the lower unit tested as it had been replaced and the color of the lower unit was a tad different. Knowing it was in great condition, I was not worried at all about it. For him, it was money well spent for peace of mind. Very much worth it for him.
Steve
Edited by VMS 10/22/2014 7:40 PM
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