O2 sensors - jetting tools

Ok, I admit it - I'm jetting challenged. Or maybe I just need an excuse to buy more crap...

Have any of you used the rich/lean indicators made by K&N, MSD, Edelbrock, etc? If so, do you still use them? where did you mount the O2 sensor boss?


Due to having a pumper ciruit in the carb its hard to jet by one of these sensors as if you open the throttle 1/2 open and keep it there the sensor will read rich than ok for a sec or 2 then lean.

This is due to the pumper poking heaps of fuel into the engine, then dying off to the end of the stroke and not adding extra fuel after its finished the stroke.

Usable to an extent on engines with no pumper circuit in the carb but not really for us.



**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Rubber Chicken Racing

Uh, I'm a little confused now...

I'll buy that there is a rich condition when the throttle is suddenly opened - that's the purpose of accelerator pump. But I'm not sure that the accelerator pump 'injects' any more fuel into the air stream after the initial squirt - unless the throttle is opened/closed to 'activate' the accelerator pump.

Most automotive 4bbl carbs have an accelerator pump circuit and the O2 sensors seem to work okay on them.

What about using an EGT probe - similar to the pyrometer on my diesel?


your accelerator jet takes quite a while to unload, similar to me i'm sorry to say!

it's not revs aware. it's similar to listening to the air leaving a pumped up tube in operation. that's why if you accelerate initially & then go to an even throttle it is still squirting in (rich) then it gradually dies off.

i don't know about it running lean but remember it's a competition machine & it should always be running slightly rich. when rich they pull still, when lean they do silly things like glitch, cough & hunt. again rather like me!

when very rich they "eight stroke" (ever left the choke on too long?). several people have talked of plug colour. my advice is never to go by plug colour. pull the exhaust apart after 2 or 3 runs on a mile long road. kill the engine with a WOT. pull the exhaust apart at the mid point. a very milky tea is what your looking for-not literally! know what i mean though?

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