Yamaha Ox66 Power Issues Please Help!!

More likely low oil in engine resrvoir causing engine to throttle back to 1500.
Check if the oil pump in the remote tank is working.

I have an Ox66 and last summer I had the same problem, replacing the O2 sensor fixed the problem.

I would bet on the low pressure pumps. Easy and cheap fix (3@$35) plus 1 hour or less. If oil pump on reservoir, the one on the engine would run low.

My dad had similar problems with his 150 ox66 and it has turned out both times to be the vst filter, it gets plugged up with small particles from the ethanol in the gasoline..

cgmiller said:

My dad had similar problems with his 150 ox66 and it has turned out both times to be the vst filter, it gets plugged up with small particles from the ethanol in the gasoline..

Click to expand...

To be correct; ethanol does not plug the filter. Crap in the tank or fuel delivery system, may get dissolved by the alcohol and get pumped into the motor

nitrox32 said:

I have an Ox66 and last summer I had the same problem, replacing the O2 sensor fixed the problem.

Click to expand...

Honestly, what else was done to the motor? I wouldn't expect a bad O2 sensor would create the symptoms described. When the sensor gets plugged or the barbell gets fouled, the motor will run richer and perhaps put out more smoke usually black. That may lead to plug fouling after a while but won't result in what seems to be safe mode operation.

The problem I am referring to is the power reduction occurring several minutes after being at cruising speed the engine acted as if the throttle was brought back to neutral if I did nothing the engine would die, If I would retard the throttle the engine would not stall then I could advance the throttle and it would run ok, this would happen only once or twice a trip.

The 1500 rpm thing sounds like the low oil sensor is tripped because the oil tank in the engine cowling is not full enough.

Ok, yes I replaced the VST filter, fuel filter, fuel water seperator, low pressure fuel pumps, spark plugs, checked out fuel pump, but the problem still occurred.

Replaced the O2 sensor and the problem went away. Cost for O2 sensor $300, O2 sensor drawtube $34, O2 sensor gasket set $17....parts ~ $350. http://www.simyamaha.com/category_s/1071.htm

You can clean the parts but some have reported that is a waste of time here is a link on how to clean the parts : http://www.theburningreel.com/forums/bo ... aning.html

And yes I always use RingFree, and use ethanol Stabil in the gas.

It's a combination of LP pumps and o2. When LP pumps fail they flood the engine with excess fuel at low rpm fouling the o2. The fouled o2 causes the cpu to defaults to a full rich condition which also sucks enormous amounts of fuel at higher rpms. With one or two bad LP pumps the engine will idle but the HP pump can't get enough fuel under load at high rpm causing the engine to "throttle back." You will have enough fuel in neutral to Rev the engine, but not under load. Been there, done that.
As for the low oil in the onboard tank, I have no experience.

Definately the low pressure pumps. Had the same thing happen. I would change those before doing anything else.

Unbolt the pumps leaving the hoses attached. Pump the bulb and see if they leak from the back.

Your symptoms sound very similar to the problems I had with my OX66 250, I did EVERYTHING people are suggesting, the stuff I did improved the motor alot, no doubt there, but never truly fixed it when finally I tried a simple step and found a clogged fuel screen on the fuel pressure regulator (sits atop the VST canister) and sure enough, that was my problem.

If you are having hesitation at mid to upper RPM's, stalls, comes back, stalls and boggs down etc, but idles and runs fine at low speeds, I would ABSOLUTELY check that. The problem was severe enough for me to finally commit to just selling the motors, granted I fixed it.....but I was just so fed up with working on them, I let them go with hopes of repowering later.

I think I put a post on here about it a while back, I had a long post about it on THT, (Im in Bahrain right now, so some sites are working, some not so i cant get access to THT to post the link) Do a search on THT for authors "walden75" thats me, you'll see my long read and pics etc on it from the May/June time frame this year.

When I say I did everything, here is a list:
- EVERY filter you can think of (spin on, cotton engine mounted, VST (twice))
-changed all LP pumps
- changed fuel lines
- cleaned, then replaced 02 sensor
- pressure tested and cleaned injectors
- changed injector pre-filters
- new plugs
- fuel rail pressure test
- TPS voltage check
- cleaned and replaced every fuse/connection i could get my hands on (theres a lot under there!!)
- decarbed like nobodys business
Im sure the list could go on.

I cant explain how frustrating this experience was for me, everytime i thought i had it, it would come back, I wasted thousands of dollars on this and (for my motor) it was such an easy fix, just one of those filters you never know is there and not one of the "hot" filters constantly mentioned on the web.

I hope that gets it for you, if not I would certianly check LP pumps, they are an easy test, a 10mm wrench to losen 2 bolts each, prime bulb, see if fuel leaks out, if so, its bad.

The VST is an easy change, 10 or 12 phillips head screws to loosen the tank, VST turns to come off, put a new one one and click your done. As noted, the VST gasket is very skinny and hard to put BACK on, new is always prefered, but putting it in the freeze once you get it off, will help it shrink up for re-install.

I just found my post about this on this site: viewtopic.php?f=3&t=18327&p=115656#p115656

Thats the little SOB piece of crap that ruined my 2011 offshore season.....I think its fuel line deterioration from the return line of the HP fuel rail. That line stays at 36psi all the time when the motor is running, so after enough years of use, heat, Ethanol gas, Im sure its breaking down inside.

Good Luck!!!

SN,
Do you have any more detail or photos of where to find the screen. I could not find mine on our 225 when I last worked on the engine.

I wouldn't get near the VST without a shop manual. The pressure regulator is on the upper rear of the VST. It has a vaccum line T'd into it. You can see how the screen mounts by getting on boats.net web site and finding your motor through the queries. Don't recall if there is a gasket where it bolts onto the VST. But I would be real slow when pulling it off. Be sure you don't pull off any vacuum lines when reinstalling it.

The pressure regulator is on the inboard/backside of the VST, its on the top of the tank, its small, has some strange looking fixed bent tubing coming off it with as Tucker said, vapor lines as well, very brittle if ten years old, as mine were.

If you can see the attached image Im going to post, the filter Im speaking about is part # 13, the entire fuel regulator sits inside an almost "compression" style fitting, it is bolted to the VST top, but the fitting itself is compressed on to a flange nipple coming off the VST top, not to tight to move, but I was expecting it to just come right off when I removed the bolts, when in fact it has to be wiggled off.

Once its off, the filter is plain as day to see, small, but right there (what a PIA too.....Yamaha really promoted Mechinics some job security with this one, even though 3 mechanics I took it too never found it)

Its easily cleaned and reinstalled, gaskets are large and easy to replace on it. Now, I did take my VST top off to get to this, If I remember correctly, you have to, to get access to the regulator.

Hope this helps, hope you find it, if you do, take some pics of the screen and debris like I did, Im curious how much yours has compared to mine.

OK, tried to attach the Boats.net file, but says the "Board Attachment Quota has been reached".....so, go to this link: http://www.boats.net/parts/search/Yamah ... parts.html

Good Luck!!

I would suggest that if you are going to change or clean the VST screen, that you do only that task. Although all the maintenance items mentioned have their place, doing a bunch of things can make it really difficult to troubleshoot if something gets done incorrectly.
With respect to the VST job, there are lots of posts on what to do but here is my two cents:
Do the job on land
Drain the tank using the lower drain plug, a container and a rag. If the motor was recently run, there may be pressure in the tank, watch your eyes.
Stuff rags in the lower cowling to catch the screws and washers that you will be dropping
Make a note of the order of the mounting bolts/washers/spacers.
Those washers and spacers are not physically connected to the VST, they are just stuck there and usually some or all will fall off when you pull the bolts out. That is why I advise noting the order of the parts before pulling the bolts. When the parts fall , the rags in the lower cowling will stop them from sliding into the mid housing, something you want to avoid.
Do no cut/unscrew/unclip ANY hoses or lines. It is not necessary to swing the tank out
The screen needs to be twisted to remove. It twists ( to me) in the opposite direction I would expect.
be careful of the pump. Do not try to remove it or play with the float.
The screen may look clean even when it's plugged. If you look through the hole in and through the mesh, you should be able to make out shadows. If it is opaque, it is plugged.

The gasket (rubber ring) often won't fit back in the grove. Freezing it may help, it may not. If it is damaged or kinked, you need a new one.
To keep the gasket aligned, some folks use Vaseline to hold it in place. I use marine grease. Some use RTV adhesive but that makes the next job more difficult.

Pump the primer bulb after the tank is reassembled and mounted to get fuel into it. Do not turn on the ignition with no fuel in the tank. It will take a lot of pumping. Running the pump dry can damage it