250HP Ox66 Issues - The Hull Truth

aj bruno · 05-08-2006, 12:45 AM

Here is what has been done so far.... Had about 140 gallons of last years fuel in the boat which was run out on a shakedown trip to about 10-20 gallons left.... The boat @ that point had a slight miss @ full throttle, anything below 5400 was running like a top. A compression test had been done on the engine all cylinders were within 1-2lbs in the 115 - 117 range.... from some research thought the high end miss was a 02 issue. The O2 sensor and passage were pulled from the engine, tons of carbon on them. They were cleaned with ring free and looked brand new... Reinstalled and a shake down showed she was running great.... smooth power throughout the power band...
She was then fueled all the way up and ring free was added to the fuel, plugs were changed, old plugs looked normal. She fired right up yesterday... and proceeded to run fun for about 1 hour of run time with no issues... Saw that a storm was coming... fired her up and dropped the hammer. About half way back to the landing @ 4700 rpms she started to lose rpms abruptly... to 1000 rpms for about 1-2 seconds....
then she powered back up. Tied here up to the dock... checked the racor... no water.
Gonna pull the plugs next week to see if I can see anything, and change the racor, and do a decarb. Do you think it could be an ethanol issue? I need to call the marina I fueled up @ to see if they have switched over to ethanol yet. Any advice is appreciated.

Think Tweiss · 05-08-2006, 02:19 AM

sounds like a fuel filter issue.....not a water separator issue.

HeadHunt · 05-08-2006, 06:34 AM

If you follow the maintenance schedule here

http://66.221.91.35/TechPhotos/Yamah...I%20Models.pdf

you will be in good shape. The items on the chart are hours OR years, whichever comes first. You could have a dirty VST filter or a bad low pressure fuel pump. Both of those are pretty cheap fixes. I would change all 3 low pressure pumps every three-four years even though Yamaha's official stance is to fix them when they break. They have a pretty high failure rate after a few years of age.

Just because you cleaned the O2 sensor, it does not rule out a bad sensor. A proper test of an O2 sensor is under a load but a simple test that gives some indication of its health can be done in the shop. You need to find a mechanic who understands the procedure. Most do not.

Good Luck,
HH

Nestor · 05-08-2006, 11:08 AM

I am having the same issue. I have done the Seafoam (Dunk), new low-pressure pumps and filter, new plugs, etc. Each time, I keep coming back to a dirty VST filter. I clean them up, and run 10 more hrs no problem. I have ordered 10 micron racor's which I hope to put in this weekend. I too do not believe I have a water problem, just too much getting by my current water-separator and low pressure filters.

good luck