Where di my rpms go?

[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 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>

 

[Zackman]

Re: Where di my rpms go?

Sounds like your tachometer is having troubles and giving inaccurate and inconsistent readings, which is fairly common.

 

[Si Rey]

Re: Where di my rpms go?

bump

 

[Cuyose]

Re: Where di my rpms go?

Sounds like you are loosing fire in one of your cylinders. You are dropping almost exactly the amount of rpms that would drop had you pulled one of the plug wires.

Check for spark with a timing light next time it's not working. Or if its on its trailer, pull each plug and find the wet one, provided it was still acting like this from its last outing.

 

[Haut Medoc]

Re: Where di my rpms go?

Sounds like you are losing fire in one of your cylinders. You are dropping almost exactly the amount of rpms that would drop had you pulled one of the plug wires.

Check for spark with a timing light next time it's not working. Or if its on its trailer, pull each plug and find the wet one, provided it was still acting like this from its last outing.

I like that advice....
When was the last time new wires were installed?...