91 Mariner 200hp 2.5L Magnum EFI Compression

[Cslone326]

Hello I'm new to this site well actually I'm not really new just never signed up have been lurking the forum for a while and finally need some advice myself. I have a 91 200hp mariner 2.5L The first question is what would ideal compression be on a motor of this age frame ? I did a compression test and the numbers I got don't sound right but they are all very close to each other cyl 1 3 & 5 all had 100psi cyl 2 90psi cyl 4&6 both had 95 psi this seems pretty low to me but I'm most definitely not a mechanic so any advice on the topic would be greatly appreciated. My second question is what size prop would be best for this motor? The motor sits on a 91 pro craft 200pro with a 12" hydraulic jack plate, the prop I'm running now is a trophy plus 13.75 23p 4 blade which is what came with the motor just curious if this is the best option for max speed mid and launch? Thanks in advance!!

 

[racerone]

Remove the port side cylinder head for inspection.------Cylinders 2 / 4 / 6.---- Will cost you pennies for a head gasket.-------Will give you big $$ value information.

 

[ondarvr]

And the actual numbers aren't that important only because gauges can read very differently from one another, being even is more important.

 

[Cslone326]

Ok so I tested again and got a much different results 1-5 were just short of 120 cyl 2-6 were 115 not sure if that makes a difference the first time i tested it i only hand tightened the tester the second time I used a wrench both were tested on 3 cycles of the starter. Them numbers seem much better but still curious what it should have given it's a 22 year old motor. Where would I find a head gasket and how difficult is it to do ?

 

[ondarvr]

Those number are close enough to be good. You should spin the motor until the PSI doesn't increase any further, not just a certain number of cycles.

​Most manufactures don't list what the compression numbers should be, they just say they should be even. Gauges can read differently, so there's a wide range of what could be "correct", you need to know your gauge to get a good idea of what the numbers mean.

 

[Cslone326]

I just followed the direction on the tester I barrowed it from a buddy. I'm thinking I might barrow it again and follow your directions. Also think I'm going to open her up and take a peak inside the gaskets I'm sure have never been changed and couldn't hurt hopefully we don't get any surprises!!

 

[Cslone326]

I pulled the covers off on the side with lower compression everything looks to be normal only thing I noticed was all the piston heads are covered in a black carbon I tried to upload pictures but it kept saying invalid something. What should I be looking for while the covers are off ?