compression testing Suzuki 175 4 stokes

[SHOWBOAT]

Have a buddy looking at a boat with twin 175s, surveyor is there today and expressing some concerns with compression. Apparently compression testing is different in 4 strokes than 2 strokes, can anybody explain that difference? Why would a surveyor not be able to test a 4 stroke?

Boat is a 24' off-shore boat. When tested last September, cylinders were 145, 145, 140 and 150 starboard. Port was 150, 145, 145 and 150. After ring free treatment, compression significantly increased. Starboard went to 175, 175, 170 and 170. Port went 175, 175, 165, 170.

Any info would appreciated re: what factory specs should be, why the surveyor can't recheck 4 strokes or anything that may be setting off the surveyor's red flags.

Thanks in advance

[FarhFrumWurk]

the numbers look fine, could be the difference in testing methods, some test at WOT some do not. the testing should be done at WOT

[Chalk]

You can do one - but a leak down seems to be the best - http://forums.iboats.com/yamaha-suzuki- ... 49331.html

[FUTCHCAIRO]

MAN, I WOULD NOT WANT TO BE AROUND THAT ENGINE IF YOU HAD A TESTER ON 1 CYLINDER AND HAD IT WOT, IT WOULD PROBABLY THROW A ROD OUT THE BLOCK AND METAL WOULD GO FLYING EVERYWHERE. I DO NOT KNOW ABOUT TESTING SUSUKI ENGINES, BUT I DO KNOW ABOUT TESTING EVINRUDES, JOHNSON, SEA HAWK, AND MANY OF THE OLDER ENGINES. I JUST TURN THE ENGINE OVER ABOUT 20-25 REVS. WITH THE IGNITION OFF. YOU CAN MAKE A ENGINE HAVE GREAT COMPRESSION IF YOU SQIRT SOME OIL IN THE PLUG HOLE JUST BEFORE TESTING BUT THAT WILL NOT BE A CORRECT READING. I HAVE KNOWN OF DEALERS PULLING THE OLD OIL IN THE PLUG HOLE JUST BEFORE TESTING TO MAKE THE ENGINE HAVE A HIGHER COMPRESSION READINGS. MOST OF THOSE OLD DEALERS HAVE BEEN LONG GONE PULLING STUNTS LIKE THAT. I HAVE NOT HAD TO DEAL WITH ANY DEALERS LATELY SINCE I DO ALL OF MY OWN WORK, THEN I KNOW IT IS DONE RIGHT.
PA
SEMPER FI

[Flint River Pirate]

I can imagine having duals is literally twice the headache on a boat.

[Chalk]

Wot testing is with the kill switch pulled - mercury suggest pulling all plugs, inserting compression tester.....turning engine over with throttle at WOT again with kill switch pulled....doing this for each cylinder

[fishinfool]

The reason for testing with the throttle open is so there is no restriction of the air going into the engine. Obviously this is done with spark disconnected, as you cannot do a compression test on a running engine. Same for cars, motorcycles, etc...It gives a more accurate reading. If the throttle plate is closed less air enters the combustion chamber and consequently the compression is lower.
As far as the numbers you have provided, I would have been happy with the first set. The second set is what I would expect from a premium (91 octane or higher) fuel engine. If they are accurate, you might want to start burning a higher octane fuel, at least mid grade. This will give you a cleaner and more efficient burn. In turn the need for ring free or other additives will be reduced. Your dollar to mile ratio (as opposed to miles per gallon) will increase also.