Another Low RPM at WOT thread

[Sous]

So another question, that I normally wouldn't even post given the info already out there. However, my situation seems a bit different and thought I'd seek the expert advice!

Two weekends ago was out with friends and had no issues to speak of. 4 people, 1/4 H20, 1/2 holding, and about 150 gallons of fuel. Made an hour long run and everything was ducky. Cruising at 3500-3600 RPM and zipping along at 24-25 knots... could hit 4000+ coming up on plane. After some lunch and such started the return trip home. Same deal as going up except a bit less fuel. Got out of the river and the bay was a bit choppy and the guests were not fond of bouncing around so I throttled back to about 2300-2400 RPM and plodded the remainder of the trip back to the marina.

Last weekend went to go out and filled up the tanks so now I'm at full fuel tanks, no holding tank, and less than 1/4 H20. Six people on board. Once I cleared the marina I went to get on plane and put the throttles at about 3/4 (my normal get on plane setting). We jumped up but RPMs only got to about 2500 or so. I figured 'eh more people more fuel I might have to goose her a bit more'... unfortunately advancing the throttles to wide open only got me to about 3000 RPM port and 3100 RPM stb.

Once back at the marina, I dove the bottom and all seemed normal. Nothing was wrapped around the shafts or props. Props felt OK (not that I could give them a full inspection) no major chunks or warps. And there was nary a barny to be found all along the bottom.

Not sure what the deal is here. My first thought is bad fuel since its both engines and it happened at the same exact time. I got new filters and will be putting them in this weekend and checking the gas in the filter housing for signs of water. I guess bad gas can happen anywhere, but my marina is pretty first rate and no one else I know has had issues recently from our fuel, so while this is my unscientific guess, my logical side says maybe not so much:huh:

Its the "both engines" thing that it nagging at me. Seems odd that I'd all of a sudden have an issue with any of the other likely culprits like plugs or pumps. So any other thoughts as to what it could be or what I should look for?

 

[FootballFan]

Spark Plugs

 

[mistercomputerman]

Does the boat stay in the water?

 

[480ces]

Fuel Filters and Plugs.

I couldn't believe how much better my 8.1s ran after a plug change...

 

[FootballFan]

Mine had always ran great - right at expected RPMS. When it went to Sea Trial - was only hitting in the 3000's.

Buyer had a compression check, broker gave them new plugs to put back in after compressions check.

After that - immediately hit spec.

Just my experience. It is not getting enough: Spark, Fuel, or Air. Say that cause you said the bottom was clean - and assuming no prop pitch issues.

 

[Sous]

Does the boat stay in the water?

Stays in the water, yes.

Plugs are next on the list, for "can't hurt to change" options... and a fairly inexpensive one at that. I'm just still stuck on the 'both engines at the exact same time' thing... anything is possible I guess, though.

 

[Do Over]

plugs are your issue....same thing happened with my 8.1's....before plugs...3200 both engines....after plugs...4600 both engines.

 

[FootballFan]

mine hit - both engines - same time.

 

[Chief Alen]

Pull the fuel pickup tube out of the tank, check to see it is clear and make sure the anti-syphon valve is working correctly and not plugged up.

Next remove the element from the fuel water separator, dump the contents into a clean clear container.

Now blow thru the hose from the tank side and make sure that hose is clear.

After a hour or so, look at the fuel in the container, is there alot of water / dirt on the bottom ?

If yes your marina put water in your tank or it was already there.

You can pump off the bottom of your tank and get the gunk and just and any water.

Need instructions on how ?

I have a post up on another site, want me to post it here i will just ask.

If your good with no water and gunk clogging the fuel system to the water sep. and your ran the boat if the problem is still there post back we will go from there.

You have electric fuel pumps, maybe your sucking air also could be the problem check good the fuel lines.

Make sure the clamps are tight.

And don't forget the vent.

See a vacuum condition can result from a partially clogged vent.

 

[Ship Tzu]

Any info about spark plugs for 1995 250 DA

 

[CliffA]

agree with others about things that are likely to affect the performance of both engines at exactly the same time:

1. bad fuel
2. clogged (or partially clogged) fuel tank vent
3. clogged (or partially clogged) fresh air intake for the engine compartment

If both engines use the same fuel line from the fuel tank also check for:
4. blocked (or partially blocked) fuel pick-up inside fuel tank
5. clogged (or partially clogged) anti-siphon valve at fuel tank
6. crimp or blockage in the fuel line

not to rule out the possibility of both engines needing spark plugs and/or a good tune-up but IMHO I don't think one or more plugs on both engines would start mis-firing at exactly the same time.....same thing is true for water/fuel seperators being clogged or other components of the ignition system going bad and ignition timing for both engines getting off......but i have no experience with a dual engine boat so this comment is just my opinion which is not based on experience....it just seems logical that your problem is related to fuel and/or air delivery to the engines, not anything mechanical or electrical in nature....

good luck....

cliff

 

[Jimsroc]

Make sure you didn't activate the "docking mode" on your DTS controls. That deadens throttle response around the dock and limits amount of throttle travel/sensitivity. Never tried to plane a boat with docking mode on so cant say for sure what percentage of throttle it provides at wide open but since both engines appear to be running equally I would suspect something is common to both such as this?

 

[Bt Doctur]

MR42T should work

 

[MarkCrusei]

Any info about spark plugs for 1995 250 DA

[h=1]ACDelco MR43T[/h]

 

[BryanOnTheBay]

I have one more reason this could happen, though it was found by going through the excellent checklists here. We were having a problem where in rough seas or hitting a big wake, we would often experience a loss of power, with WOT being 3200 max. I was also experiencing some other power issues as well. Finally, after a horrible limp back into the marina, the fuel pump died. Replace the fuel pump, and things should be fixed. We ran the boat, and low and behold we have a great hole shot, and 4800 rpm WOT.

Then we hit a wake off a big cruiser who is running hard, but can't plane. 10 seconds or so after hitting the wake, the 3500 rpms we are doing drops back to 3200, and no amount of throttle will increase it. While we thought it unlikely, about the last thing to check was the pickup tube in the fuel tank. It was clean, but the pickup had curled into the shape of a "J" and the end of the pickup was facing up about 6 inches from the bottom of the tank. While we haven't had the conditions to really test it yet, the working theory is air in the fuel line when the fuel sloshed around in heavy seas, the other power issues being fixed with a new fuel pump.

While time will tell, we think this problem may be finally put to bed.

Bryan

 

[CliffA]

nice....i hope that fixes the problem....report back one way or the other for the archives....

cliff