Yamaha XO 66 250 issue

[gradywhite248]

Thoughts on the issues I had this past week on my Yamaha XO 66 250.

While running smooth an at speed say 5000 RPM she rolled back to 4400-3700 rpm's. I pulled the throttle back and still hesitating at power or surging. but would clear up for a few hours and come back? I change the main fuel filter, clean as could be. By reviewing the web it appears that the VST filter might be the issue or carbon build up in the O2 sensor? I'm thinking of ordering a new VST filter and changing along with cleaning the O2 sensor area of carbon. I also thought of changing the three LP fuel pumps since there so cheap.

Thoughts from the gang other than buying a new motor?

 

[family affair]

All of those are the usual suspects. I'm in the process of changing my LP pumps for the 3rd time in 8 years. Complete BS that they don't last longer.
I'd check the filter and change pumps (if leaking) before changing the o2. Inspect the o2 port and clean the o2 before replacing. If you need to replace, buy the automotive equivalent and splice it... Unless you don't mind flushing an extra $200+ for the Yamaha part.

 

[seasick]

Thoughts on the issues I had this past week on my Yamaha XO 66 250.

While running smooth an at speed say 5000 RPM she rolled back to 4400-3700 rpm's. I pulled the throttle back and still hesitating at power or surging. but would clear up for a few hours and come back? I change the main fuel filter, clean as could be. By reviewing the web it appears that the VST filter might be the issue or carbon build up in the O2 sensor? I'm thinking of ordering a new VST filter and changing along with cleaning the O2 sensor area of carbon. I also thought of changing the three LP fuel pumps since there so cheap.

Thoughts from the gang other than buying a new motor?

I suspect the VST filter. If you are going to buy the screen, you might as well get a new gasket. Sometimes the old one can be reused but getting it to fit in the groove can be a challenge. If I remember correctly there are three lp pumps. One is a tougher to get to than the others. If they haven't been replaced in a while ( years) it may make sense to change them buy I really don't like the 'change everything' approach to troubleshooting.
I do not suspect the O2 sensor given the symptoms. On that motor when the O2 get fouled, the motor runs rich. That can foul plugs and may cause black exhaust smoke, rough operation but not the surging. You might also want to try running at speed with the gas filler cap loose. It is possible that you have a fuel restriction due to a clogged vent. In that case I would expect the primer bulb to go soft and/or collapse when the motor starts to die.

There are some 'hidden' filters in the 250 that are not in the smaller motors. I think there is a small filter in the output side of the VST.

I have seen clogged VST's that caused surging when in rough seas where the hull gets a bit pounded.

 

[gradywhite248]

Thanks for your responses. Ill follow up with the fix and so others who might need this info in the future.
SH

 

[ttles714]

change filter # 65L-13905-00-00... a small screen located on the ouput of the vst .. behind and on top of vst. .. Good . low maintenance engines ... Do not use aftermarket LP fuel pumps !! I learned the hard way !!

 

[Harpoon]

If you haven't changed the LP filters in a long time, it may be worth doing- even if they aren't the cause of your problem. It sounds like an issue I had and resolved by cleaning vst screens in the past.

 

[gradywhite248]

I would like to report a fix for the surging issue on my XO 66 250. Believe it or not after having the issuer for two years every now and then and replacing most everything I could from fuel pumps to filters and so on. Here is what we have found and can recreate every time.
If I top the boat off with fuel I have the surging / fuel starvation. Evidently with the tank topped off the Vent system is clogged or fills with fuel. After burning a few gallon the issue goes away.
Has anyone ever seen this?
I just don't top off or expected to happen or I loosen the fuel cap and it seams to help.

 

[DennisG01]

Excellent detective work - and thanks for posting back!

The first thought that comes to mind is that the vent hose is not vertical enough. It may be running too close to horizontal, or even have a slight dip in it.

Now, in terms of "topping off" - if you end up filling the fill hose, even partially, then you're putting too much fuel in. In that case, you're also filling up the vent hose. The correct procedure is to stop filling when you first hear the "gurgle" sound.

 

[gradywhite248]

Excellent detective work - and thanks for posting back!

The first thought that comes to mind is that the vent hose is not vertical enough. It may be running too close to horizontal, or even have a slight dip in it.

Now, in terms of "topping off" - if you end up filling the fill hose, even partially, then you're putting too much fuel in. In that case, you're also filling up the vent hose. The correct procedure is to stop filling when you first hear the "gurgle" sound.

Thanks for the follow up. That is a good point. We do most often over fill. I guess Im the issue.....

 

[DennisG01]

Thanks for the follow up. That is a good point. We do most often over fill. I guess Im the issue.....

Hey, at least it's easily fixable! If you have trouble hearing the gurgle, install a "fuel whistle" (google for that).

 

[SkunkBoat]

read thru this OX66 225 "stumbling" when running over 3500 rpm
and this OX66 -fuel injector man

 

[seasick]

Who wudda thunk?
If the vent were blocked completely or to a point, I would expect the primer bulb to get soft or even collapse when running. I don't know if that was checked.
If it was a case of too much back pressure due to a somewhat blocked vent, then the vent hose must have a section that sits lower than the top of the gas tank, in effect a downward loop. That loop can fill with gas and create more back pressure than the LP pumps can overcome. For the surging to cycle on and off while underway, it would seem the the blocked vent hose emptied and refilled.
If the fuel fill is a combo fill/vent model. it is possible that the vent in the cap is not working correctly. Usually a blocked vent will result in the motor stalling due to loss of fuel flow.

 

[Andrew93]

When you say surging is it hard to keep a constant RPM? I had this issue once on my 272 with twin OX66's. One motor would hold rock solid and the other would always be drifting at cruise. Ended up being the O2 sensor. It would otherwise run fine at lower RPM's, it was at cruise it was work to try and keep motor RPM's matched.

Another OX66 I had sounds more similar to your problem. When testing out a new to us 250hp at WOT the RPM's would drop 1000 and then surge. There was also a noticeable ticking sound from the motor, almost like a bad lifter in a car. Both of those are signs of a clogged O2 sensor sniffer tube. Took it apart and it was clogged solid. Cleaned out with a drill bit and some cleaner and she ran great following that.

Low Pressure fuel pumps are cheap and if not replaced recently its not a bad idea to do them no matter what.

If you choose to take the O2 sensor sniffer tube out you don't need the O2 socket if you don't have one, an open end wrench does just fine once you take the block it screws into out I found. There is a gasket kit to go with removing and replacing if you go that route.