Many thanks for your reply, just looked at the work shop manual and your right 15PSI is within limits, it also goes on to say that the difference between cylinders should be no greater than 25% (lowest reading and highest reading). The manual then states that the cylinder head design on V6 models was modified to enable better cooling and should be measured between pairs of cylinders which share the same crankshaft throw. The readings should be top 1/2 same, middle 3/4 same, bottom 5/6 same, it then goes on that the middle two 3/4 should be the highest as to cool the head the most.
This is exactly where my 26 year old engine is at and why when i asked Power Products Marine to fully check the engine out last year, and i know they did a compression test, nether Richard or Andy said they was a problem, just saying it was good for its age.
I was alway told and read that the compression across the cylinders should be equal, just 10 minutes reading a Yamaha work shop manual goes to prove that a little knowledge is a dangerous thing.