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I'm contemplating an evo-type airbox mod - it's Pete's fault. There's an opinion (at least as far as Futuras are concerned) that this can make the engine run lean, with a possible risk of damage to valves, if one doesn't also fit a PCIII or Tuneboy. Does anyone here have any knowledge to impart on this subject?

I'd rather not bother with a PCIII right now, because of the cost

Well, I suppose it could do - after all, what you're trying to do with the airbox is to flow more air through the engine and assuming the FutFut's engine management is open-loop like the Falco's, it's not going to know to add more fuel - so the effect would be to lean the mixture.
Whether it gets to a point where it's dangerously lean is a question I can't answer.
An old-fashioned plug-chop would be a crude way to get a handle on it.

I still don't like Evo airboxes - I don't have the faith that the seal around the underside of the tank is 100% guaranteed never to leak.
Others with greater faith are happy though.

There are so many of our engines running about with a plethora of air box modifications and I don't recall anyone finding there being a leanness problem to the extent that valves have been damaged.

Do it and to hell with the consequences.

After all it' not my engine

Sorry, forgot to mention that Futs can't be chipped; their ECU is, as it were, a sealed unit