1993 4.3L Merc LX Alpha One - Higher RPM/Lower Speed than normal

[Jeff_C]

I've got a 1993 20' Stingray running a 4.3L Merc LX Alpha One 180hp. I've been using it all summer, and typically my RPM at WOT has been 4800 with the speedometer bouncing around between 47 and 50. This past weekend though, my RPM was hitting 4900, and the speedometer wouldn't go above 45. The only changes I made between this weekend and the last is that I replaced the distributer cap and rotor, and I switched from using 87 octane gas with ethonal to 91 octane non-oxygenated, ethonal-free gas. Can changing gas affect the RPM like that? As for the loss of speed, I've noticed the hull is starting to get a bit grungy below the water line. (Its been in the water in a lagoon since Memorial Day.) Plus, I normally go out with half a tank of gas while this time I had a fully topped-off tank. Can some grunge on the hull plus 20 extra gallons of fuel in the back push the speed down like that? Or is there some way that the new distributer cap and rotor could be affecting things?

 

[silver_power]

Re: 1993 4.3L Merc LX Alpha One - Higher RPM/Lower Speed than normal

My friend, the fact that your engine reaches more rpms with the same propeller and the same time the boat speed is lower means according to me 2possible things. 1)The propeller spin has been raised because of possible propeler hub damage, or 2)you trimmed up a lot and the propeller went out of waterline and could not produce thrust while reached more rpms.
I thing that there is nothing to do with the changes you made at the fuel or at the rotor cap.
You see at a boat ,for example if you have 25mph@2500, you will have 50@5000. Nothing more , nothing less generally, except if something goes wrong with the propeller as i described.
I hope i helped you.

 

[Bondo]

Re: 1993 4.3L Merc LX Alpha One - Higher RPM/Lower Speed than normal

Can some grunge on the hull plus 20 extra gallons of fuel in the back push the speed down like that?

Ayuh,... Sea life will nearly Stop yer boat if not tended too....
The added fuel is another passenger...

Doing a Tune Up, could/ should bring Life to the motor,.. Especially if it were Ill when ya started...

Go Clean the hull,+ come back with yer results,... I'm guessin' you'll be Happy....

 

[Jeff_C]

Re: 1993 4.3L Merc LX Alpha One - Higher RPM/Lower Speed than normal

Ayuh,... Sea life will nearly Stop yer boat if not tended too....
The added fuel is another passenger...

Doing a Tune Up, could/ should bring Life to the motor,.. Especially if it were Ill when ya started...

Go Clean the hull,+ come back with yer results,... I'm guessin' you'll be Happy....

Yeah, I'll be cleaning the hull at the end of the season. I cleaned it with an acid wash in the spring, but like I said in the original post, the dock I have it at is in a lagoon. It gives me great protection from wind and waves, but I think the standing water in the lagoon is letting the grunge build up pretty fast.

 

[Blk-n-Blu]

Re: 1993 4.3L Merc LX Alpha One - Higher RPM/Lower Speed than normal

Put on the swim fins and get a scrub brush and go to town, or let the girlfriend do it LOL