#1 cylinder leaned out cause?

[Bangstick310]

Hello, can anyone tell me what would cause #1 cylinder to lean out on a 2001 Yamaha ox66 250. Cylinder 1 is the cylinder that has the 02 sensor in it, it happened twice and second time on a fresh rebuild, I put All new low pressure pumps. Had the injectors done and flow tested, I’m scratching my head, thank you.

[boscoe99]

Leaned out how? Stopped injecting gasoline?

Partially clogged so as to not deliver the amount of gasoline that it should be delivering?

Has the injector been sent off and tested to confirm that it is not flowing the amount of gasoline that it should be flowing?

[Bangstick310]

I honestly have no idea, I can attach a picture of the plug, the spark plug looks a lot dryer then the other 5 and same with the piston when I look through the plug hole On that cylinder, doesn’t seem to be getting oil, as far as injectors yes, it has been sent off and tested , cleaned , I got the before and after report. Can something cause the injector to not flow? ECM computer etc? What do you think of the plug by looking at it. Thanks

[Bangstick310]

Leaned out how? Stopped injecting gasoline?

Partially clogged so as to not deliver the amount of gasoline that it should be delivering?

Has the injector been sent off and tested to confirm that it is not flowing the amount of gasoline that it should be flowing?

This is a pug with maybe 30 min of run time. All other plugs where clean and fresh wet oil

[99yam40]

that plug does not look like a lean cylinder plug to me.
too dark like it was too rich

[Bangstick310]

that plug does not look like a lean cylinder plug to me.
too dark like it was too rich

I thought that too, but, it’s dry no oil, the other plugs wet and even the piston was wet when looking through the spark plug hole.

[Bangstick310]

Can you tell me if a bad O2 sensor would cause a issue like this? Or if any issue?

[99yam40]

Can you tell me if a bad O2 sensor would cause a issue like this? Or if any issue?

I do not have the experience to answer that question properly.
But I do not understand how the o2 sensor could affect only #1

[Bangstick310]

I understand, I’m scratching my head, I wouldn’t think the O2 sensor can only effect the number one cylinder. I would think all. I’m going to get a picture of the other plugs. Just to compare. Also I don’t know if this makes sense but the plug almost looks lean like Dry carbon covered, Remember Thai is a fresh build with literally 30 min of run time, that’s why I assume it’s lean, def did not look like any of the other cylinders, cylinder 2 through 6 was exactly the same while pulling the plug to inspect.