Whaler Central - Boston Whaler Boat Information and Photos

Posted by Mike on 05/08/08 - 5:58 PM

I am going to check a boat for someone as part of the Whaler Shopper Network and thought I'd be able to get some technical advice on how best to go about this compression test.

I have read that the motor should be warm and I have read that it doesn't matter. So I guess I'd be interested in some opinions on why it does or does not matter if the motor has been warmed up.

Thanks,
Mike

Edited by Mike on 05/08/08 - 6:07 PM

Posted by Derwd24 on 05/08/08 - 6:47 PM

Hey Mike,

Here's the original THREAD

Did some additional research and verified that because it's a 2 stroke, the throttle does not need to be wide open, compression numbers are the same either way.

Also found out if the engine is an OMC V4 or V6 looper (+/- 80's era) that the overall compression numbers are low normally, around 90 PSI whereas other engines are higher.

And readout numbers can vary significantly depending on the tester gauge used, which isn't a big deal if you get all higher numbers, but if they're all low it may merit checking with a different gauge before drawing any conclusions.

As far as testing an engine warm or cold, the concensus appeared to be that testing a cold engine was fine. Warm engines may give you different overall numbers, but you don't want to see variations of more than 5-10% between cylinders and that's going to be the same regardless of whether the engine is warm or cold.

And finally, how the engine runs when started cold can yield clues to the overall condition (Eg. Hard starting and low overall cold compression numbers don't bode well). Hope that helps some...

Edited by Derwd24 on 05/08/08 - 6:51 PM

Posted by Mike on 05/08/08 - 7:03 PM

Derwd24,

Thanks - that helps a lot. Now that I see your original thread I remember reading your original post but didn't go back and read the entire thread.

I just want to do a decent job inspecting the boat. This is valuable info! I'm going to give Jeff's inspection checklist a go too.

Thanks again,
Mike

Posted by Derwd24 on 05/08/08 - 7:11 PM

Glad that was some help! I meant to also add that if the engine hasn't been run this year since being winterized, the fogging oil in the cyl's will raise the overall compression numbers and could mask an issue. So if that's the case, you may want to ask if they can get it running before you look at it, and have it cold when you get there.

Good luck!

Dave

Posted by Jeff on 05/09/08 - 3:05 PM

MIKE,

My checklist has been updated however the new size even zipped is larger than the 92kb allowed so I can not upload it. I will send it to you in a PDF if you could please find a place to upload it here that would be helpful.