5.7 L Mercruiser low rpm?

[Si Rey]

I have an 89 Sea Ray 24' cabin cruiser with 5.7L Mercruiser. Outdrive is Alpha 1, Gen 1 coupled to a 14.5x17 aluminum prop. Carb is a Rochester quadrajet. At W.o.t. 1600' msl engine only turns 3900 rpm. The "Bible" says 4200-4600 rpm. Haven't checked the tach, compression or timing. Engine runs smooth. Rebuilt carb for other reasons but had this problem before rebuilding carb. Replaced spark plugs, cap, rotor, ignition sensor due to age and condition. Replaced all water hoses, water pump, t-stat, both exhaust manifolds due to corrosion. Checked exhaust butterflies and appear to be o.k. Oil pressure appears to be normal and engine temp appears to be normal. The boat runs at 35-38 mph with two adults and two children, trim tabs fully retracted. Throttle cable travels from stop-stop. Is it reasonable to expect 4200-4600 rpm and if so how do I get there?
Thank you in advance to the pros for any direction in this matter.

 

[QC]

Re: 5.7 L Mercruiser low rpm?

Is it reasonable to expect 4200-4600 rpm and if so how do I get there?

Yes. Try a 15" pitch prop . . .

Also, what's the bottom condition like? Are you trimming out as well as retracting the tabs?

 

[QC]

Re: 5.7 L Mercruiser low rpm?

BTW, great initial info post!! That's a heavy load and a little bit of altitude. Where did that boat come from? Same lake?

 

[paulrfrancisco]

Re: 5.7 L Mercruiser low rpm?

What about gear on board, and water in the bilge, or saturated wood core?

What is the condition of the prop... ?

 

[Si Rey]

Re: 5.7 L Mercruiser low rpm?

Yes. Try a 15" pitch prop . . .

Also, what's the bottom condition like? Are you trimming out as well as retracting the tabs?

Yes, stern drive is trimmed out and tabs are retracted.

 

[QC]

Re: 5.7 L Mercruiser low rpm?

Well if I didn't know the boat since new, I would want to confirm the tach is right. By any chance do you know your gear ratio?

Friscoboy's comments and concerns are good ones too.

 

[Si Rey]

Re: 5.7 L Mercruiser low rpm?

BTW, great initial info post!! That's a heavy load and a little bit of altitude. Where did that boat come from? Same lake?

I live in northern Az which is where boat was purchased. I bought on consignment from dealer. I "Assume" the boat was used in Phoenix as that is where the majority of the lakes are. Your comment of "That's a heavy load and a little bit of altitude" I understand to mean that even that little bit of elevation on a heavy boat could be the issue? I have heard of people having to reprop going from Phoenix up to Powell/Mead.
Thanks for yor comments and remark on first post!!!!!!!

 

[Si Rey]

Re: 5.7 L Mercruiser low rpm?

Well if I didn't know the boat since new, I would want to confirm the tach is right. By any chance do you know your gear ratio?

Friscoboy's comments and concerns are good ones too.

Gear ratio according to the "Bible" is 1:50 to 1.
It appears to be about right for idle on the rpm, but that doesn't mean it can't be off on the upper end. I know that with many instruments including autos if the gas gauge reads half tank and I've gone 300 miles, I drive another 20 miles and the gauge drops to .25 and driving another 50 miles the gauge drops only minutely.

 

[Si Rey]

Re: 5.7 L Mercruiser low rpm?

What about gear on board, and water in the bilge, or saturated wood core?

What is the condition of the prop... ?

The boat is dry stored when not being used and so I don't see any wood saturation or any bilge water. It doesn't seem to make a difference if I have all the camping gear for the weekend or just a day trip to get wet, it still turns 3900 rpm. Prop appears to be decent, I have the little nics filed out and appears to be straight.

 

[rad1026]

Re: 5.7 L Mercruiser low rpm?

hey Si Rey, good initial post. I live in albuquerque at a mile high but do a lot of boating at powell. I have a '89 Chaparral 21' bow rider, 5.7 and alpha 1 gen 1. Most of the lakes I run are about 3500 to 6000 feet in elevation but even Powell is 3600. If you are running in local lakes in the lower elevation of Phoenix, you will have lower RPMs if you go up to Powell. I wound up regearing to 1.62:1 to get my boat in the 4400 range. I can go 16 17 18 depending on what lake I'm at. I bought the boat in Dallas, basically sea level, and it originally had a 1.5:1. I couldn't get it past 3500 RPM on a 19 pitch prop. So I had to reduce prop and regear. Try the 15 pitch and see what that does but if you are planning on spending time at Powell, which is an awesome place I'll be there next week, you may need to think about regearing. You can always go up in pitch if you are at a lower elevation lake. Oh, and I found out my tach is about 500 off on the low side too, so you might look at that.

 

[paulrfrancisco]

Re: 5.7 L Mercruiser low rpm?

Go back to QC's post and move to a 14 1/2 x 15" pitch... or a 14 x 17, or 13 1/2 x 17....

At one time I had a 22' SeaRay and I think with a cabin, and it had 14 1/2 x 15... and it run fine... it was 4500lbs...

 

[QC]

Re: 5.7 L Mercruiser low rpm?

Couple of follow-up notes. If the tach is right, then you need to make the pitch change if there is no concern with the engine itself. If the boat is waterlogged, you need the pitch change, if it is loaded like you will typically use her, you need the pitch change. I would not mess with diameter . . . You're off by 300 RPM + and Diameter is really about load, not RPM/speed stuff.

On the tach, they are usually off by a percentage and not some fixed amount, top or bottom. Double check if there is a switch in the back of the tach for 4, 6 or 8 cyl. If not, you really should check it with a shop tach etc. Tachs read pulses, if the count of pulses is off, then it will be off everywhere. Seemingly less at the lower numbers, because it is, again, a percentage . . .