Yamaha 115 2 stroke - Prop question

[wkerber]

A little help, if you can. I've got an 07 Yamaha 115 2 stroke. It came with a 21p aluminum prop and last fall when I first got the boat it was turning right at 5500 rpms and stayed there until spring. As the air temp rose into the low 90s along with the humidty, it dropped to 4900 rpms. Well, I don't fish much in the summer because it's too darn hot in SC, so the next time I ran it was Labor Day, which was still 95 degrees and I was still at 4900 rpms. I purchased a 19p prop which I finally tried last weekend when the air temp was 50 degrees in the morning and it turns 6100 rpms. I wasn't expecting that, a 1200 rpm change. Can the temps have that much of an effect or a 2p change in prop or the combination? Plan to try the 21p again Thanksgiving week to verify the temp impact. Both props are aluminum and I've never hit anything with the 21p to bend it. Is it possible for an aluminum prop to flex out and not return to its normal pitch? Thanks for any help.

Bill

[Sigma]

Read through this thread.

http://www.bbcboards.net...rform

[wkerber]

Thanks, Mike. That helped. I was thinking a 2p change would only change rpms 400-500, but this shows me that it can be more than that. I'm going to try the 21p again next week, but may have to split the difference and go to a 20p. Thanks, again as I missed that post.

Bill

[Sigma]

Remember Bill,

There is a differences between a 21p alum and a 21p SS

I say try the 21p ss

[Nova Kaw 650]

I know this is an old post, but the RPM issue in hot weather is not a prop issue; it's a jetting issue. Go down 1-2 sizes on the main jet to compensate for the less dense hot air, and you may find that it still runs well in cold weather.

[Sigma]

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Nova Kaw 650 &raquo;</TD></TR><TR><TD CLASS="quote">I know this is an old post, but the RPM issue in hot weather is not a prop issue; it's a jetting issue. Go down 1-2 sizes on the main jet to compensate for the less dense hot air, and you may find that it still runs well in cold weather. </TD></TR></TABLE>

You are absolutely right but I don't know of anyone that re-jets a outboard for the season change. Too much of a pain in the but. Definitely need to make sure winter jetting is set properly to minimize the effects of running rich in the simmer.

[Nova Kaw 650]

Going from the 90's to the 50's really shouldn't need a jet change, unless you are looking for optimum performance. That being said, the engine in question sounds like it's jetted too rich, even for cold weather. A drop of one or two jet sizes would probably make for an all-around better running engine.