I have a 250 OX66 on my Ranger that I purchased a couple months ago. It runs great on top end up to 6k, but does not want to get on plane. I have tried 3 props and it acted the same with all. When you trim down and give it full throttle it first acts like it wants to go for a second then sort of bogs for 5 second up to 10 seconds then it's just like all of the sudden takes off. It does it from first thing in the morning and all day. I have changed plugs, on board filter, filter on motor, vst filter, poppet valve, thermostats, and water pump all as routine maintenance since I don't know when it was all done last. There has been very little if any change. Took it to mechanic and had them adjust everything to factory specs like the throttle valves, oil pump, and tps. Went fishing this morning and same thing, although it does seem to smoke a bit at idle and by noon it didn't want to idle very well. Any ideas before I take it back to the shop? Maybe the switch that tells it to kick in the other two cylinders or something else? It's really starting to bug me. Thanks

The ox66 doesn't deactivate two cylinders like the hpdi. I would have them check low pressure fuel pumps and the o2 sensor.

Do yourself a favor and swap out the O2 sensor even if it tests good. I learned this very expensive lesson the hard way on my 2003 225 OX66. My boat would start and idle fine. When you'd get on plane it would run OK for a second then step down the RPMs until it would barely stay on plane even with the throttle all the way down. Those sensors can cause some strange issues. PM me if you want to get on the phone to discuss.

First thing I always do on a poor running Yamaha is replace the low pressure, manual fuel pump diaphragms.

Ok, so I just checked the low pressure fuel pumps and two leaked while I was squeezing the primer bulb. I am going to go ahead and replace all three. Hopefully my local shop has them in stock so I can replace them tomorrow. The only thing that doesn't make sense is it runs great at high rpms, but it doesn't want to get on plane. Usually low pressure pumps cause issues at high rpms because they are starving the vst. I would gladly just replace the o2 sensor, but I don't really have an extra $300 to throw at it just for the heck of it. If the lp pumps don't fix it I'm just going to take it to the shop. I don't have the time or money to just throw parts at it until it's fixed.

The O2 sensor can be cleaned by using a heat source like a torch and wiping with a soft cloth. You will be good after fixing the pumps.

You were right Gary Purdy, I replaced the pumps today and it seems to have solved my issue. I ran it some this evening and it gets up on plane well. I will run it more this weekend to make sure, but hopefully the issue has been solved. Thank you all for the help. And by the way I have already cleaned the o2 sensor.