loose plug an issue for RPM on prop test?

[suzukidave]

Hi, took my boat out for last time yesterday before winter to test out a new prop. Got it back went to winterize and noticed oily stuff seeping out of #1 cylinder at the plug (maybe a teaspoon worth, not enough to have dripped off the power head yet). Turns out I forgot to cinch up the plugs last time I checked them (1 hour of boat time ago). The plugs were in all the way but would spin freely by hand.<br /><br />I don't think any damage was done (let me know if you think different) but here's a question, would it have affected my rpm on the prop test? I did notice the motor sounded a slightly rougher than usual on start up. I couldn't feel anything different on the test run but I'm not sure I would notice because I was running a stainless steel prop instead of the usual aluminum and the motor sounds completely different with a steel prop. Anyway the steel prop ran out to 5300 rpm and my w-o-t is 5500. The motor range is 4800-5500, and the boat was empty of gas and people at 5300 so I'm very close to going down an inch on the final purchase. Unfortunately, I can exchange the prop now but I can't wait for next year with the dealer, and the boat had to go to bed for the winter and I've run out of boating season. <br /><br />The motor is 2 cylinders, both plugs were loose but only one leaked. I assume I would lose a little compression with this problem, but given the minimal seepage would this be enough to affect rpm? I'm going to take an educated guess here, with your valuable advice much appreciated to educate me.

 

[petryshyn]

Re: loose plug an issue for RPM on prop test?

aloha suzukidave: Compression reduction may not have been the only hinderance. If a plug is loose, heat transfer is minimal and can result in detonation. This can drastically reduce HP if it is severe. (Peek through the plug hole and see if the piston is noticeably cleaner than the other) This condition is usually perceived as a high speed misfire..... If engine felt smooth, disregard me!!!!!....LOL<br /> Please tell me the difference in sound from an alum. to stainless....and maybe why.<br />Good luck!!

 

[suzukidave]

Re: loose plug an issue for RPM on prop test?

Schematic, thanks for the reply.<br /><br />I didn't hear it miss or backfire. The idle was a little rough but not enough to make me worry. I just figured it had the end of the season blues. It didn't sound rough at high rpm. Too late to check the piston -- it's full of fogging oil now.<br /><br />So are you saying that even without detonation you think I could have been light a little horsepower because of compression?If so, would that translate into a few lost rpm?<br /><br />To answer your question about steel props, this is the second stainless I've tried in my prolonged search for the perfect pitch. Both times, my motor is significantly quieter than with the factory aluminum prop or with a composite I also tried -- my motor is usually politely described as "loud enough to wake the neighbours in Japan". With a steel prop it sounds muffled, and is only audible out to about Maui. I suspect it has to do with the thru-hub exhaust.