Yamaha ox66 issues

[striper56]

I have a 2001 Yamaha 250 hp ox66. When I run the motor between 2000 and 2400n rpm the motor will lose rpms and sometimes almost come to a complete stop. This happens while increase or decreasing the rpms. The motor runs great from idle to 2000 rpm and above 2400 rpm to WOT.
So far I have done the following

replaced all fuel filter
replace fuel line from tank to the motor
replaced vsp filter
cleaned o2 sensor
replaced plugs
bypassed the flow scan

Any help would be appreciated.

Thanks

[bumpdraft]

I used to have a 2001 225 pos66. About every 20 hours, I would have to put a torch to the O2 sensor and it would run better. It would also shudder when getting up on plane and switching to six cylinders. I don't remember at what rpm that was.
I happened to be at my mechanic's place when the factory rep was there and he offered to go on a test ride. He determined that I would benefit from a prop of less pitch (19" to 17") and he swapped it out for me. It helped, but I never did like the way it idled and at slow speed.

[Old'sCool]

Most of time it's fuel related. Although it's usually a little lower rpm range than what you describe, they run on all 6 cyl in neutral, "drop" two when put in fwd, then pick those two back up around 1800 or so. Sounds like you're trying to run close to the switch-over rpm. (Assuming the LP pumps are good and fuel pressure is good)

[erebus]

We have a customer at my boatyard who has a 2003 Pursuit with twin OX66 250's, and when she got launched last month she would not turn up over 2200 reliably.
Sometimes she would run OK up to 4600 RPM's and then fall on her face, other times she just wouldn't get up and go.
Only on one motor, other ran fine.
We did all the usual stuff, checked the fuel pumps (low and hi pressure) cleaned the VST tank, cleaned the O2 sensor, etc. etc.
Nothing helped. Eventually we sent out the injectors for cleaning and that fixed it right up.
Sea trialed her today, and she ran like a champ. Well, until I spun a hub. Doh!
Heres where we send our injectors for cleaning. Super fast and reasonably priced. Located in Bow NH.
https://www.dieselsfuelinjection.com/
FYI-OX66 Injectors are kind of a pain to remove though.

[afterhours]

Striper56,

I've owned an OX66 250hp for several years now. Sounds like a fuel supply issue. If you haven't replaced your 3 low pressure fuel pumps recently, it is easy to check if they are leaking. You won't know if they are leaking because they are bolted to the manifold and use vacuum and positive pressure from the manifold to operate. The fuel just leaks into the manifold, causing your engine to run rich at idle and starve the engine at higher RPM's. Surging is a classic symptom. The engine will run fine until the vst tank doesn't get enough fuel from the lp pumps. RPM's will rise and fall as the vst tank fills and empties. You may not notice this at higher rpms, except that WOT might be lower than expected.

Simply loosen the two bolts holding each pump to the manifold and squeeze your fuel bulb. Do this with your engine off and cool and check one pump at a time.

If the diaphragm is shot, fuel will dribble from behind the pump. If it leaks replace it, or for that matter, replace all 3 as they are only about $30 each. Don't forget to replace the $3 gasket behind each pumps. You can also rebuild the pumps, but for $30 why bother.

[striper56]

Thanks for the replies Olds cool, erebus, and vancouver.
I will check the fuel pressure and LP pumps first. If that doesn't work I will dig into the injectors. I did send out the injectors three years ago, 2 needed to be replaced and the other 4 needed a little cleaning.
I will report back the results. Thanks again.

[striper56]

Changed the LP fuel pumps and took it for a hide. Same issue except it is now cutting out SOME of the time when pulling back the throttle from WOT down to 2400. I run it up to say 4000 + RPMS then pull the throttle back 10 times and it will only cut out once or twice. Injectors next?
Thanks

[fredbrad]

check your gas tank for a anti-siphon devices,sometimes are inside of the pickup tube similar to a little wire mesh (like on my case) or little steel ball inside the elbow fuel pickup

[striper56]

Here is the update on the still not running correctly motor. I installed 3 new LP pumps. The motor sounds better but still cutting out when increasing or decreasing the rpms from 3000 rpm down to 2000. I took a 6 gallon tank with me and connected it to the LP pump. Still didn't run correctly.
One time I shut the motor off and could not get it to start. It sounded like the kill switch was disconnected but was not. Tried a few more times still nada. Went back pumped the primer ball switched the battery to both and it fired right up and ran fine.
Great website VancouverPotter23. I will add it my favorites.

Thanks

[striper56]

This afternoon I was testing the TPS the motor would not start. I checked the primer ball it was soft more like empty. Pumped the ball for a few minutes until the ball was hard and it fired right up.