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Bf90 low compression

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Arctic

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New to boating so go easy on me. Looking to buy a 18 ft jetcraft with a 1996 bf90 with 430 hours on it. Took it to a shop and they did the compression test and it got 90 psi across all 4 cylinders, some history the engine was overhauled about 5 years ago, but no idea what was done, the current owner has had it for close to 3 years but only ran it on a lake 3 to 4 the first couple of years, the last year it has sat unused. Talked to the shop tech and he said even sitting unused it should be close to the low end of 180 psi, he said you could take it out and run it for awhile and it might bring it up, but he didn't sound convinced. Shop did say it started fine and ran well, oil was clear. Cant run it in water for a couple of months, boat is in great condition. Should i pass it by, or is the some other tests that could be done on the engine?
 
Two observations...

1. I've seen some so called "shops" who don't really know how to properly do a compression test on an out board. It's very important to have a really fresh battery, all plugs removed, throttle fully open, engine as warm as possible, compression gauge pigtail fully screwed in and sealed in the plug hole and crank the engine for at least 10 seconds - 15 is better in order to get the oil circulating.

2. A good de-carb with Yamalube Ring Free, Sea Foam, or a similar high quality carbon buster will often bring the compression up considerably.

Hopefully, some more experienced folks will jump in on this post.
 
Run it on a dynomometer or with a test prop.-----That will tell the condition of the motor.--------I doubt the 90 PSI values as well if it starts and idles OK.
 
Sounds fishy to me.....in my opinion it is not typical to be low compression across ALL cylinders and still run and start fine......... Typically when there is a compression issue it is 1 of the 3 cylinders. I am with Chawk and Racerone, you can buy a compression guage for under $20 on amazon. Run a mixture of ring free, seafoam through it over a few hours. Then do the compression test with the engine warm and cylinders lubed. With the amount it has been sitting, the rings a probably a little gummed up. Engines are made to run, it is not good for engines (marine, auto or small) to be sitting for extended periods of time without use. Frequent use and proper maintenance is a combustible engines best friend.
 
Thanks for the reply's, after much reading i was thinking like mentioned by you guys that 90 across all 4 cylinders is a bad reading, ill do as suggested ringfree and seafoam, run couple of hours, then do my own compression check,

Thanks again, feeling better about the future purchase
 
I'm not a mechanic.. I always engage the kill switch before running any compression tests, I was told you can mess up the ignition or stator? if the electric has no where to go. but on a lawn mower or similar engine I just make sure plug is grounded against the engine some where.. A friend of a friend said he was running his boat and wanted to change the running battery..he accidentally switched his batteries from 1 or 2 to off, he said it killed his ignition... My 1997 BF90 has 2012 hours on it and compression is 170-175 according to the local Honda dealer, my shop manual says compression should be 212? psi. I do plan on checking it myself If I can get over to my cousins car shop.
 
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