O2 sensor.

less than 1

I initially thought you meant the CO level, but if you meant the stoichiometric ratio is 12.8 then that is way too rich, as I am certain you know that 14.7 is right and more is too lean.

It's an SD1, did they ever run lean ? :-)

Before cats, the ratio did vary by quite a bit. But I can't find out from what to what. A cat can be damaged by the wrong ratio - which is what killed off lean burn engines.

The injection ones were notorious for it. Presumably to give the best cruising MPG.

The spec for that is 1.5 +/- 0.5% CO.

"Dave Plowman (News)" gurgled happily, sounding much like they were saying:

I thought it was high NOx levels which did for lean-burn, but that's another story...

All I can find here is 14.35:1 for part throttle cruise, 11:1 for full open, so 12.8 seems a little rich, but the rolling road is surely beckoning?

A bigger issue was that cats just don't work with the any ratio other than stoichiometric. Almost every engine ever built before then ran on the rich side.

No point since I'm changing to the MegaSquirt - this is the readings with the standard injection. However, I fitted a new pump at the same time and haven't checked the pressure. I have an adjustable regulator and it could have been set to compensate for a clapped pump.

They certainly had a quite wide spread. Most leaned off considerably at cruise.

In message , "Dave Plowman (News)" writes

Think it was more like a cat won't work with a lean burn engine, you need a certain amount of unburnt fuel and oxygen in the exhaust gas to keep a cat lit up and working efficiently. Lean burn engines produce more NOx which is a nastier pollutant than CO.

NOX comes down again when it's truely "lean of peak" like a Wright R-3350 was taken by the flight engineers 60 years ago to get a Lockheed Constellation across the Atlantic - normal fuel burn would have seen them ditching dry and empty. The technique was developed by Charles Lindbergh in P38's during WWII, low revs, high boost, running lean nearly doubled the range. Temps go up as it's leaned out from

Found one figure - seems 12.5:1 is ideal for maximum power.

yebut you always want to be rich rather than weak for max power, a bit too rich will only lose a tiny bit of power, while the same amount too weak loses lots. Hence a richer mixture for power, cruising can be much leaner as you will only be using a very small amount of power.