Cannot reach max RPMs when at WOT

[swatkinz]

2000 Dancer 270 with 7.4L Mercruiser Bravo III outdrive. Multiport FI engine Manual says RPM should be b/w 4600-4800 and 37-41mph. Last season (first season that I owned the boat), a mechanic had the boat at 40mph and 4400RPM. This season, I cannot get the boat to exceed 4100RPM and 33Mph. I have done the following to remedy the situation:

changed fuel/water separating filter
changed spark plugs
changed distributor cap and rotor
checked all plug wires with ohm meter for continuity.
compression test shows all cylinders reading b/w 120 and 150psi (this was with throttle in neutral position. forgot to put throttle into wide open position)

There hasn't been a prop change since last season's performance test. Can anyone offer a suggestion on what to check next? The boat cranks easily, idles very good, and accelerates very well just can't get to max rpm.
Help

 

[comsnark]

Most likely culprit is bottom fouling. What do the undersides and props look like?

 

[swatkinz]

boat has only been in the water since May 1. Bottom fouling is minimal. Props are in same condition as last year when better numbers were attained.

 

[osd9]

Are you still running on last year's fuel....???? Have you bumbed the props on anything...including even a slight bumb on the bottom????

 

[Dave S]

The only other things I can suggest are to make sure your flame arrestor is nice and clean and to make sure your throttle is actually wide open at the throttle body itself when the controls are in that position.

 

[Asureyez]

Where is home port

Just wondering. Is the boat wintered out of the water? Is the boat carrying more weight now?

1. Corroded injectors,

2. Dirty air filters,

3. There seems to be a wide difference in compression 30# from top to low ... what is the spec on this?

4. Are you smoking more, buring oil or spewing raw gas in the exhaust?

5. Check any vacuum lines for ignition advance, that still may have been mechannical in that year.

Just some more places to look.

 

[boatrboy]

I just looked up the WOT spec and for our motors and it is 4200-4600 rpms. I think you are pushing it at 4800 rpms IMHO.

My boat has 22p props and usally hits 4200. I once hit 38 mph.

 

[Dave M.]

Here are some numbers from the above posts. The numbers are:
Speed, RPM (thousands), Speed per thousand RPM.

Last year 40, 4.4, 9.09
Boatrboy 38, 4.2, 9.05
This year 33, 4.1, 8.05

So last year your boat got the same speed per thousand as boatrboy, within 1/2% (9.09 compared to 9.05). This year your speed per thousand RPM is down over 10%. The props are turning, the boat is not moving as fast. I would look for something holding it back. And I suspect whatever that is is loading down the engine so you can't get to max RPM.

Others already mentioned several items, bottom fouling, bent props. Also, are you sure your trim tabs are retracting? Boat heavier for some reason? Sure seems to me you don't have a lack of power, you have something holding you back.

 

[PlayDate]

A few questions:

1) Was the tune up done in between last year's test drive and this years test drive?

2) When you did the tune-up and replaced the distributor cap what did you set the timing to?

3) Did you check the total advance at 3000 rpm?

4) What type plugs did you use (what was the gap set at)?

5) What did the old plugs look like? (oil, fouled, gray, tan)

6) How many hours are on the engine?

Any other changes or mods between the test drives?


-John

 

[swatkinz]

OP here.
Thanks for all of the comments/suggestions. The one thing that has jumped out is that perhaps I've not tested with the trim tabs all the way up. Dang I must be losign my mind. I'll be at the lake this weekend and will try that, but would that effect RPM as well as speed?
Also, this is the first sterndrive I've owne in some time and the first MPFI. When I crank the engine and especially when the boat is coming up on plane, the engine sounds like a jet at take off. I'm guessing this is air being sucked into the engine. Does anyone else notice this and is this normal?

 

[swatkinz]

OP Here again,
I forgot to ask some of the questions that were asked from some of you folks so here goes:

Last year's readings were done after last year's tune up
When distributor cap, rotor and plugs were changed, timing was not adjusted.

Plugs were replaced with Champion (can't remember the exact number, but they were the replacements suggested by Clymer's repair mainual) Gap was set at .040 per the manual.

Last year's fuel is long gone. I had stored the boat last year on the trailer with full tank and stabilizer. When I put in for the sping, I again topped of the boat and added appropriate amount of Sea Foam.

808 hrs on the engine

As far as the compression readings go, Clymer's says that lowest reading should be within 70% of the highest reading. Remember that when i did this test, I forgot to set the throttle at wide open. Will this matter much? If I repeated would I expect the same variabiity with just overall higher PSI readings?

 

[Dave M.]

OP here.
Thanks for all of the comments/suggestions. The one thing that has jumped out is that perhaps I've not tested with the trim tabs all the way up. Dang I must be losign my mind. I'll be at the lake this weekend and will try that, but would that effect RPM as well as speed?

If you put more drag on the boat, so the engine has to work harder to move it, then that will lug the engine down some, and will lower the max RPM you can get to. How much is totally boat dependent, and now much load the tabs can really cause. The change you are seeing is a lot. Whether the tabs could cause it I can predict.

But normally a boat is most efficient in the water when the bow it angled up about 4 degrees. If the tabs force the bow down into the water, then you cause increased hull drag, in addition to the drag of the tabs themselves.

 

[tphinney]

Swatkinz;

also noticed you mention this is the first sterndrive in a while.

make sure the sterndrive is trimed up for best performance along with the tabs being all the way up. finding the sweet spot on the drive makes a HUGE difference in performance.