yamaha F250 RPMs falling slowly...

[NikM820]

I have this posted on another site but wanted to ask here too.
Running offshore today with my twin 2005 Yamaha F250's after a hour and a half at 4200 RPMs approx 25 knots I noticed I was having to adjust the port engines throttle much more often than normal to keep the engines roughly synched. After a few miles of this the Port engine starts slowly falling 4200 4100 4000 3900 3800 exc to 3000. At first I could push the throttle and catch the port motor up but eventually I max out the throttle and it was only getting to 3500 and would continue to drop rpms slowly (like over 10 min) down to roughly 2000. If I synch the motors at 2000 the port will start falling from there as well down to basically idle. If I try and rev the engine after the port has fallen down in RPMs to catch back up to the other, it will go up to approx 2000 but max out there. If I take the engine out of gear, the engine will rev right up with the other, like normal to 4400 to get up on plane, but after a couple minutes will begin to loose rpms again. . The motor shifts in and out of gear fine, the engine sounds normal, the fuel filters look fine, new Racors 30 hrs ago. When it occurs the engine sounds perfectly normal. As in no sputtering or surging, nothing like that. It seems like the motor is slowing its self down, not a fuel starvation. If I slow the boat down and take the engine out of gear I can get back up and run on a plane for about 15 minutes or so but after that I have to slow it back down, take the engine out of gear, then rev it back up to get the boat back on a plane. any thoughts?

 

[Slacktime]

The first things I would check are the fuel filters.

 

[Tuna Man]

Assuming your alarm system works fine, I would also suspect a fuel issue. I agree, at the very least swap the port fuel filter and re-test.

I would be very concerned about a potentially larger problem. Although unlikely, if the engine is overheating it could give similar symptoms. Check the obvious first: oil level (look at oil color as well), test the overheat alarm (or have it checked if unsure), maybe get compression checked.

Hope its just a clogged fuel filter but hate to see you have a more serious problem that you may be able to avoid.

Keep us posted.

 

[NikM820]

Assuming your alarm system works fine, I would also suspect a fuel issue. I agree, at the very least swap the port fuel filter and re-test.

I would be very concerned about a potentially larger problem. Although unlikely, if the engine is overheating it could give similar symptoms. Check the obvious first: oil level (look at oil color as well), test the overheat alarm (or have it checked if unsure), maybe get compression checked.

Hope its just a clogged fuel filter but hate to see you have a more serious problem that you may be able to avoid.

Keep us posted.

Thanks for your comments. If it's truly a fuel issue we are all doomed to have fuel issues. I have never had E10 in the tank, either did the previous owner. I Keep the tank full at all times and use the boat very frequently. At least 2 times a month I burn 150 gallons of fuel on one trip and week day cruising down the river is common, so the fuel never sits. I had the tanks cleaned when I bought the boat a year ago just for good measure so I assume no issues there. I had replaced all my filters 50 hours ago and there is not a drop of water, dirt, sludge anywhere. The engines "pee" is the same temp as the other and you can touch the motor and know shes cooling. Also it does it just cruising down the canal 30 seconds after starting at 800 she will decrease to 600 obviously cold. Compression was checked when I bought the boat, lets hope thats still fine and the oil looks good.I had two seasoned yamaha guys stumped today. Its going on the computer Friday so hopefully at that point we will have some answers. First time any Yamaha has given me any issues, I DONT LIKE NOT HAVING MY BOAT!

 

[BCLII]

Sounds like you could have a couple of injectors going down.

 

[Grog]

Check ALL the fuel filters (there's a lot of them) then if you have priming bulbs, be sure it's not collapsed then see if you can squeeze it and see what happens to the power. If the power comes back, the low pressure pumps are going.

 

[NikM820]

The VST was the answer. Mechanic said the vst had some corrosion inside and those particles were plugging up the screen. The answer is a new tank. The boat sat for about a year before I bought it and he believes thats when it occurred. He has to wait on the parts and get it back together before I know for certain but he is pretty confident that's the issue. I am having him tear down the other motor too, just in case.
Coincidently another boat was there with the same corrosion and symptoms. However, the owner had been just cleaning out the VST's for 2 years to keep the boat going and now has a bunch more corrosion eating through the motor. Injectors ect. Might be a good thing i'm not so mechanically inclined. I would have probably been doing the same thing and ended up with a much bigger issue.
Thanks for all that contributed and hope this helps someone in the future.
Nick

 

[Tuna Man]

Glad to hear is was nothing major with the engines. If I understand correctly, the fuel tank is the root cause of the vst problem? I guess particles from the tank are causing hacvoc inside the vst tank.

 

[NikM820]

No the gas tank is fine it's Inside the vst (tank) some pitting started a while ago and pieces of the vst are clogging up the screen. I am hoping the other vst tank is good but having it serviced is probably a good idea anyway.
Nick