Correct prop pitch/size for high altitude on 3.0l alpha 1 mercrusier

Thorn

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Jun 6, 2012
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Great forum! I have a 1999 Bayliner capri classic that I am the original owner of. The prop that came on the boat is a 19 pitch. The boat is pretty doggish when I go to my favorite spot which is Flaming Gorge in Utah/Wyoming. That body of water is at a bit over 6000 feet (GPS) and my boat with three people and gear can only manage 23 mph @ 3600-3700 rpm at WOT. That is actually a bit less than WOT because it backfires (pops) at WOT, which I am assuming is from a rich condition, which is a topic for another thread maybe.

Anyway, I want a new prop which I will order from this site. I was looking at a 17, because it might give me a wider range if I want to go to Lake Powell (3600 feet), but 90% of my boating is around 6000-6500 fee so I was looking at a 15 or 16 pitch. What brand and pitch do you guys/gals recommend?
 

fishrdan

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Jan 25, 2008
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I wouldn't go with a 17P as it will only give you around 400RPM, when you need to make up around 800rpm (ideally make up 1000RPM). I would go with a 15P 3-blade as this should put your WOT RPM around 4500.

Pay close attention to the WOT popping as it's most likely "lean backfire", which can do nasty things to your engine like melt holes in pistons.... Probably time to replace all the fuel filters and maybe rebuild the carb.
 

Bondo

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That is actually a bit less than WOT because it backfires (pops) at WOT, which I am assuming is from a rich condition,

Ayuh,.... Welcome Aboard,.... Actually, that's a sign of a Lean condition, 'n probably Why it's down on power,.....

Fix it 1st, then figure out which prop gives ya 4800 rpms at Wot,.....
 

Thorn

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Thanks guys for the responses. I will probably go with the 15P then.

I changed my carb fuel filter and installed a water/fuel separator last year and it helped cut down the popping/backfiring quite a bit. I treat my gas with Marine Stabil as well.

My plugs don't look like the engine is running lean (more on the black side) and I was assuming rich because of the altitude. i checked my jets and they were the correct size for my altitude. I have been reading about a lot of issues with the accelerator pumps on these engines too.
 

Bondo

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and installed a water/fuel separator last year and it helped cut down the popping/backfiring quite a bit.

Ayuh,..... So pull the filter, 'n check it's Contents in a clear bag or bowl to look for crud, 'n water,.....

Then throw on a new filter, 'n try it,....
 

Sea Rider

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Sep 20, 2008
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Has the engine been already re jetted for 6 K ft high altitude ? You plan boating at sea level and high altitude as well ?

Happy Boating
 

Thorn

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Jun 6, 2012
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The carb has the proper jets for high altitude. The fuel/water separator filter gas looked very clean and I did not see any signs of water in it. I did notice that there was a kink in the new gas line I put in last year with the fuel/water separator. It was hidden in the hoop that the line goes through that is mounted on the engine doghouse. I took about 6" off the line and straightened it. I hope this is the source of my WOT popping.

This boat will never see sea level. The lowest would be around 3300 feet. I just ordered a 15P Solas Amita 15" diameter prop from this site. This prop appears to be about an 1" bigger (15") than what I am running now, which should help a bit too.

I changed my plugs back to the OE AC Delco plugs as I was getting quite a bit of corrosion on the NGKs. I think the Ethanol is really screwing things up. I noticed it ran better and didn't seem to pop at WOT when I filled up this past week with 3/4 tank of high octane non-ethanol. I think I will start filling up with that from now on.

I am not looking for it to become a barn burner, but I would like more low end to pull a big skier (me) up and to get on plane faster.
 

Thorn

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Update... I put a 15P Solas Altima 3 on it and went back to the Gorge and it was a different machine. I was getting 4100-4200 rpm at WOT and 3-4 more MPH on the top end depending on wind/chop conditions. It had much more low end take off. I should have done this years ago! I can actually get up on a nice plane for the first time! My trim range is much broader now too. Only con is it is a bit nosier with the engine reving that high. I plan on putting some more insulation in the dog house to help with that. Thanks gents!
 

airshot

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Jul 22, 2008
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Well the new prop performance is good news....however you just put a bandaid on your problem rather than fixing it. Your RPM is still to low, and if the miss is not corrected soon, your next posting will be about engine rebuilds or repacements. Just because a book says those jets are the right size for your altitude doesn't mean your engine will like them. From the sounds of things you have some engine issues to deal with before you worry about a prop.
 
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