could it be the prop?

[Si Rey]

Got an 89 Sea Ray 220 Sundancer, Mercruiser 5.7L, ALpha 1 Gen 1 outdrive with 14.5x17 aluminum prop. Engine was turning only 3900 rpm @wot, sooo I went down to 15x15 ss prop (Quicksilver). Rpm's came up to 4700 + rpms. Here's where it gets interesting. I now go up to 14x17 ss prop (Stilleto) to try and bring rpms down a little. The rpms go down to 4500 @ wot. I shut the engine off for about 15-20 min to go swimming. Start the engine up, rpms @ wot are 3900. where did the 600 rpms go? I didn't touch a thing- honest! Engine runs fine. Don't think it is the tack in error as I can hear the difference in engine rpms between 3900 and 4500. I noticed that the rpms during the rest of the day on the lake would vary 100-200 rpms without rhym or reason. I'm new at boating and this is my first boat, had it about 3 months. Any ideas from anyone would be greatly appreciated>

 

[QC]

Re: could it be the prop?

The rpms go down to 4500 @ wot. I shut the engine off for about 15-20 min to go swimming. Start the engine up, rpms @ wot are 3900

Same exact trim position? Same water condition? Same wind condition? Bimini up or down in both cases? Does the boat take on any water? Do you need to run your bilge pump? If the answers are truly absolutely no difference, then it is probably some sort of engine issue.

 

[Si Rey]

Re: could it be the prop?

yeah, I touched nothing, same trim, same people, bimini etc. The wind and temp are things that don't remain the same, however should the rpms vary 600 after the engine has been off for 30 min. What it riddling about this is when I initially switched from 17" prop down to 15" rpms jumped up 600 to 4600. When I went back up to 17" rpms stayed approx the same, 4600, then after about 30 min of no engine run time is when it changed to 600 rpms less, 3900. I have thought of tach issues, carb, outdrive trim but nothing makes sense. The rpms do change, I am judging this by the sound of the engine to verify what I'm seeing on the tach. Engine runs fine, no missing running rough, timing is where the book says-28btc degrees@3400rpm.
Help!

 

[QC]

Re: could it be the prop?

I'd repost in the Mercruiser section ^^^^^^ definitely not prop. There's a good troubleshooting tree floating around up there.

 

[RubberFrog]

Re: could it be the prop?

You engine is not running right.