Lost Some RPMs

[dmarkvid2]

My boat was planed out at 4500 RPM.(WOT) It was a good smooth ride. After awhile I noticed that the RPMs were slowly dropping, it wasnt much, only about 600. I slowed the boat down for awhile and then I tried again but the most I got out of it was about 3900-4000 rpm, and wasn't able to plane out. Does anyone have any idea what caused the drop in RPMS? Oil pressure and Temp were fine.
The engine is a king cobra 460 330 Hp, pushing a 29' Bayliner.

 

[Don S]

Re: Lost Some RPMs

Sounds exactly like a dirty bottom to me.

Here are some other possibilities.

 

[dmarkvid2]

Re: Lost Some RPMs

When you say "dirty bottom" you mean the "bottom of the boat? Or fuel tank? I just got the boat at the begianing of August, the bottom was painted, but it was last seasons paint, and was wearing off. I believe the prop is a 15/15 or a 13/15, I know there is a 15 in it LOL I have trim tabs, and they help lower the bow. I know the outdrive is all the way down. Is there a combination to use with the outdrive and trim tabs to reach planing?? Anyway, thanks for the RPM check list I never even thought of the bottom being able to drop the RPMS

 

[Don S]

Re: Lost Some RPMs

The bottom of the boat.
Trust me, the bottom getting dirty (green slime, small barnicals, shells etc) all cause the boat to slow down and the rpm drops.
I doubt your props are changing pitch while in the water (unless you hit something) then all bets are off.
Did you do a tuneup in the spring? If not, the old stuff may be catching up with you.
Do you do normal maintenance items like full tuneups, fuel filter changes, oil and gear lube changes each year?

 

[QC]

Re: Lost Some RPMs

If I read this correctly your RPM dropped while you were underway, not over several weeks or so. Is that right? If so it is not the bottom (althought that would cause a drop in speed and RPM as DonS suggests, I just think he missread your post). It would not happen while you were cruising along . . . Sounds like a possibility of many of the others, and the bottom could indeed be dirty, it just wouldn't happen during a trip over 30 minutes or so. My guess is bad fuel, fuel filter, water in the fuel. You still need to do all of the maintenance Don notes, but I think something specific went wrong to cause a problem while you were actually cruising.

 

[dmarkvid2]

Re: Lost Some RPMs

Yes Don, its all been tuned up. It had better be because the engine is a remanufacture and was just installed in early Aug. As for QC, Yes the RPM dropped as I was underway. It was "VERY SUDDLE", no bucking,or hesitation from the engine, which seems to rule out bad fuel filter or gas. It was if I were pulling back the throttle very slowly, and smoothly. It also did it at about 3000 rpm, dropping about 2 or 300 rpm. I'm wondering if mabey the cable to the cabarator was loose.

http://i76.photobucket.com/albums/j37/MarksJoy/b1d5491f.jpg


http://i76.photobucket.com/albums/j37/MarksJoy/100_0684.jpg

 

[Reel Poor]

Re: Lost Some RPMs

Sounds like a possible fuel problem. Check the fuel pressure to the carb. Here is a fuel system test courtsey of Don S.

Fuel System Test

 

[salty87]

Re: Lost Some RPMs

any chance its just the tach? did it feel or sound like you were going slower? gps?

 

[dmarkvid2]

Re: Lost Some RPMs

No, Salty, I heard the sound of the engine starting to slow down, and slowly I lost plane. When I heard the engine slow, thats when I looked at the tach. to noticed that the needle on the tach was going down. As a matter of fact, it was so suttle, that my friends on board didn't even notice it. No skips or jerks, or hesitation from the engine, it was a very smooth transition down. I think its something on the carberator that loosened to bring the RPMs down 600. At WOT, when I was able to get 45-4600, now I just make 4000 RPM