1994 Yamaha 115 compression question

[Clinton37]

Hi again everyone...I have a 1994 Yamaha 115hp 2 stroke (C115) powering an 18ft. Ebbtide Dynatrak fish n ski boat. The motor looks, runs and sounds perfectly normal. The only thing that concerned me was that the top speed is 40mph which I thought was kinda slow.. (I used to have a 2000 90hp Johnson that pushed an Astro 17ft fish n ski 47mph at WOT). The prop on the Yamaha is an aluminum 21 pitch which allows it to reach a max of 5500rpm ..which is the max rpms Yamaha recommends. So, I performed a compression test and found the compression on all 4 cylinders to be 92,90,92,89. Isn't this way too low? This is the only comrpession test I've ever done on it. I did warm the engine up for a few minutes before the test and I did use a new but cheap Harbor Freight compression gauge. Thank you for any thoughts or suggestions.

 

[QBhoy]

Hi.
Couple of things maybe.
47 mph if gps is mega fast for a 90hp on any boat of even the fastest pedigree hull. Was this gps ?
secondly. The ebbtide will be heavier and often more beam perhaps.
Lastly, I’m sure the terrific v4 you have there should be about 130-140 psi new and anything under 100-110 perhaps on the edge of acceptable.
One thing I would say though is. You have fairly closely matched numbers. Unusual for them all to be down. Especially on separate v bank engines. I’d Be suspecting your gauges accuracy first. I have a cheap gauge and it has read 15 psi low from new, compared to a calibrated snap on gauge.
All the best.

 

[rejesterd]

Those compression numbers are low, but they're consistent. Did you do the test with the throttle wide open? I don't know if it would make a ton of difference, but that's what my Yamaha manual says to do. Also the gauge could just be inaccurate.

But as QBhoy noted, they're all very close in value. That typically means worn rings. which is somewhat normal for an old engine. I wouldn't keep pushing that thing past 5000 rpms under any circumstances.. the redline decreases as the engine ages and the rings wear more and more. That means you might want to consider a lower-pitch prop. 21" seems high for a boat that size.

 

[Clinton37]

Great advice and insight, QBhoy. Thank you! The speeds for both boats were GPS (the boat speedometers were within 1 or 2 mph). I should have mentioned that. Yes, the Astro/90 Johnson would run 47 at max recommended rpm but it blew a hole in a piston top after nearly 20 years of use. BTW, I only run WOT occasionally.
Anyway, the 4 closely matched psi numbers and lack of any symptoms on the 115 Yamaha are leading me to question the gauge accuracy as well. I guess for now I will have to be satisfied because it is running fine to the best of my knowledge. Meantime I will hunt for another gauge. Thanks again!

 

[Clinton37]

Thank you rejesterd! I just saw your reply. I did not have the throttle wide open. I will definetely try that and see if it makes a difference. I wondered about the worn rings, too. Would worn rings cause excessive smoke..like a car engine maybe? This Yamaha certainly doesn't smoke more than a normal two stroke. Thanks again for your advice!

 

[rejesterd]

I think with only slightly worn rings, you probably won't notice more smoke than usual. That only happens when they're really worn. If you keep up with maintenance, it will probably run a long time before you'd have serious internal issues.

 

[dfrants]

I'd borrow a different compression gauge and try that test again.

 

[racerone]

I do not believe the rings have anything to do with the amount of smoke on this engine !!

 

[robert graham]

I do not believe the rings have anything to do with the amount of smoke on this engine !!

I agree 100%.....smoking from warn rings is a 4 stroke problem?.....