Compression testing a boat I'm thinking..

[jrp5xd]

I'm going to look at a boat this weekend it has one Re-manufactured merc and one that is original with 500 hours on it. I don't know why the other engine was replaced (I will find out this weekend) but I'm definitely going to have a compression check done on both motors. How much psi should the cylinders hold in a rebuilt 7.4 and what would be acceptable for one with 500 hours. Or is it more of a percent difference between cylinders per motor? I know this is risky buying a boat with one new and one fairly old motor, but it seems to be a good deal. what do you guys think?

[blowngas]

do a "leak down" test along with the compression if possible on all the cylinders and record the readings to see if any leaking valves or rings

[jrp5xd]

how do you do a leak down test?

[EXTREMEBOATS]

First you need to find a bud that owns a leak tester or buy one. I use mine more then a comp tester. It gives a much better idea of the condition of the "inards". You could probly hire a mechanic to test it for you. The cost in my oppinion is worth it. What it does is show in a percentage reading of how much air is escaping and where. You'll know the condition of the valves and or the seats as well as how well the rings are sealing.It works by pressurising the cyilinder from a out side air source and shows on a gauge the amount of leakage. Hope this helps... :D
Mike