I have a 2000 Yamaha ox 66 and its surging on the top end. If the water is smooth and running wide open its surging from 66-67 miles an hour to 61-62 then back up to 66-67. It probably doing it rough water but you can't seem to feel it.

Anyone know specifically what is causing it? Or the correct steps in order to start to diagnose the problem? My mechanic thinks it's a injector ? Anyone know what it could be? He basically going to start with an o2 sensor clean then test , vst filter test and fuel pressure then test . Then clean the injectors test. Does this seem correct?

Sounds like the correct approach. What ever you do, don't continue to run it until it is fixed. Leaning out a cylinder will cause you to blow it. Ask me how I know.

Fuel pressure should be checked first. Possible manual fuel pump diaphragms.

02 sensor cleaned. Ran it yesterday and it's still surging. I am taking it back to him and his going to work on the fuel pressure test. Gary are you talking about the lower pressure fuel pumps?

Yes

Let us know how it's going with the trouble shooting / results Thanks.

The guy is having trouble full pressure at wide open throttle. Hard to test wide open throttle at the ramp or tank. He is going to clean the vast tank and retest.

Sorry, I'm not a mechanic. However I hade the same problem on my 225. dealt with it for about a year and many different shops trying to figure it out. New fuel pumps, filters, coil packs, cleaning the screens, everything to never seem to solve the issue. I did get it fixed at the Skeeter owners tournament when I asked the Yamaha rep if since no one else could fix it, could he? All I know is that he hooked up a volt meter to something and had me work the throttle while he adjusted something with a screw driver. In about 15 min. I was back on the water and it ran great and never had another issue with it. He told me what he did but it was several years ago and I cant remember what he told me. Hope this helps in some way even though I don't have details.

He adjusted the TPS

Thanks Scram. I am now 560 in and still have gotten fixed. We have cleaned 02 sensor, new plugs, cleaned the vst tank, removed the screen from the pick up tubes. Replaced 10 micron filter,. Still having the same issue.

Gary What is the TPS?

Throttle position sensor. I had some surging at low speed. Sent the intectors off for a clean and calibrate which cleared that up. I had top end surging that cleared up when I cleaned the O2 sensor and tube. If the low pressure pumps haven't been replaced, replace those too. They are a maintenance item.

Basically what I did when I got mine was to completely go through all the fuel system maintenance items including sending the injectors off. Once I did all that, it ran like a top and still does.

One more thing, has the timing been checked? I had a 200 Yamaha some years ago doing this and found the timing was too fast. Someone thought that would make it run faster. Set it back to specs, no more problems.