WOT rpm range loaded vs unloaded

[carolinadiver1]

Hello all,
I've been trolling the boards over the last couple of weeks as I was considering a 97 Maxum 2700 SCR. Well, I bought the boat and need some of your thoughts now. Boat is in great shape other than caulky glass that I will be cleaning up soon.

During the sea trial the boat performed great. Up on plane rather quickly, RPM's (4500) at manufactures spec. ect. Now.....the boat is running at WOT rpm's of 3900-4000. It will plane, but takes more time. Below are the differences for the two trials.

Original trial: 4 people, 20 gallons fuel, Salt water, calm seas, engine sounds ran great.

At home trial: 2 people, full tank (105+gallons), fresh water, calm seas, engine sounds ran great.

boat specs: Mercruiser 7.4 carb, 18.25x19 prop (new replacement same as original), painted bottom

Issues already addressed: Water/dirt in carb....removed/cleaned/replaced. Fuel filter removed/checked/replaced. Fuel pump ok. Fuel flow ok.

The only real change is the approx. 500# of fuel. Could this affect the rpm this much?? Could I still have carb problems?? Prop??

Thanks!

[shrew]

hi Carolinadiver1, Welcome to the site.


Water: (8lbs/gal.)
Gasoline: (6.07/gal.)

Was the water tank full or empty during the seatrial?
How about after you got it home?
Was the boat loaded with gear when you seatrialed? Is it now?
How much did the 2 extra people weigh who were onboard during the seatrial?
Where you at the helm during the seatrial or was someone else?
Are you trimming the drive out?
Are tabs in or out?
Where either drive or tab position different during the seatrial?

The short answer is yes. Consider what happens when you drive a truck, then hook a trailer up to the truck and compare performance. There are many, many variables to consider here.

[seapuppy]

also what is the state of tune on the engine...try giving it a full up tuneup...plugs..distr. cap/rotor/points(if you have them)....replace filters and oils ....what is the condition of the prop??..bottom scum??(if any)....as shrew said...weight of people/gear/gas/water/san tank full??...
also what was the water and weather like...barometric pressure will change the performance of a carb from one day to the next...
all this adds up...

and welcome to the zoo..


SP

[carolinadiver1]

Thanks for the reply shrew....

Was the water tank full or empty during the seatrial? **Minimal water...approx. 8 gallon or so
How about after you got it home? **No change
Was the boat loaded with gear when you seatrialed? Is it now? No changes other than already listed. Just safety gear
How much did the 2 extra people weigh who were onboard during the seatrial? **350#
Where you at the helm during the seatrial or was someone else? **Yes
Are you trimming the drive out? **After the fact....same as before
Are tabs in or out? out. **no change from original trial
Where either drive or tab position different during the seatrial? **No

Sea trial fuel - 15 gallons @ 6.07=91#
Currently - 105 gallons @ 6.07=637#
With two less people the weight difference should be neglible......200+/- pounds.

I'm leaning towards a carb problem, but am open to more suggestions.

[carolinadiver1]

Engine has been sitting for a while. Survey was good and engine runs very smooth....then and now.....but it is due for tune-up. Time frame from original trial to at home trial was only a few days. I would think the tune condition would have affected it on the first trial. Prop brand new. No scum. Weather the same (HOT and HUMID!)

[ss3964spd]

So there was only a few days between the 1st trial and the 2nd? The new prop was on during both?

Hard to imagine anything mechanical happening in the span of a few days that would cause the boat to lose 500 RPM.

Water/dirt in the carb: did you do that work after the 1st trial but before the 2nd? After the 2nd?

Dan

[carolinadiver1]

The only changes were the extra weight and fresh water rather than salt water. I'm thinking..... that the boat sat for awhile empty of fuel. We put 15 gal. in for sea trial. The boat ran great. It sat for a few days and loosened up any trash in the carb. I filled the tank and got some residual water in the carb along with the loosen dirt which caused the clogging of the jets. I did a quick cleaning of the carb and it seems ok other than the 500 rpm (big issue)......so maybe I need to get someone to throughly clean the carb. Any other suggestions?

[ss3964spd]

Just so I'm clear, you did the carb work after the 2nd outing when you realized you'd lost 500 RPM's? Naturally, when ya run into the "one day it was workin fine and the next day it isn't" ya try to determine what exactly, if anything, changed between the two days.

How long was the boat sitting before filling it with the initial 15 gallons of gas?

I know the engine isn't EFI, but it does have electronic ignition. Do you know if the engine has a knock sensor on it? If so, perhaps the new gas is of a lower octane rating and the knock sensor is retarding the timing, thus lowering the WOT RPM.

Dan

[seapuppy]

replace all the fuel filters...the spin on one and the one in the carb....also make sure the pickup screen is clear in the tank..



SP

[carolinadiver1]

It sat empty for a year. I will check on the knock sensor. Pickup screen is clean. Hope to change the filters this weekend. Took the boat down to Washington (NC) last night for dinner. Boat ran great just a few hundred rpm short at wot. Nice smooth cruising at 3200 and about 28mph. Really enjoyed that little trip. Had the outdrive oil overflow at the resevoir......got to figure that one out now also.

[ss3964spd]

Maybe, before you do anythng else, you should go ahead and burn though 80/90 or so gallons of gas and then duplicate the conditions of the 1st sea trial. She show she does.

Might just be the nature of the beast.

[carolinadiver1]

I think that's what I will do. It's running so smooth now I don't want to tweek anything to much. I plan to go to the coast this weekend and do some diving offshore, so that should take care of the fuel.

Thanks for the help everyone!