For sure, (my favorite F1 phrase), under load, on wide open throttle, a very low AFR indicates incomplete combustion. I used this fact to home into the best advance for a particular rev band on wide open throttle...ie the best advance setting gives you a higher AFR than a retarded one or indeed one that it too advanced...hard to believe I know, but during this particular method both too advanced AND too retarded give lower AFRs than the optimum advanced setting (regardless of the fueling ...within reason). I got this AFR method to set the advance curve off the net BTW...it worked for me.
The nearer to 14.7 you get, then the more efficient the combustion...but you NEVER want an AFR of 14.7 (or higher
) on WOT because of the extreme temperature of the burn. At WOT, full power, with good torque, is best found on AFRs of 12.5 to 13.4. My engine feels torquier though on nearer 13.2 at WOT than nearer 12.5, that's for sure.
On part throttle you can get away with a 14.7 AFR but when you go beyond 15 it'll start to misfire especially as the load increases.
SO if you have no misfire on part throttle, or under gentle acceleration on part throttle then there's not much to worry about there.
Obviously you know already that when reading the plugs you need to distinguish between the effect of weakness on part throttle and weakness under full load. I know from my WinLog logs that the amount of time spent on WOT is extremely low even when driving fast on the road but that depends on the state of your engine and gearing of course ...a low powered engine on std gears will be driven on WOT much more often than a high powered one on a CR box.
The other thing to consider is separating the effect of the mechanical advance from the vacuum advance. I you want to be sure the full power WOT advance is right then disconnect the vacuum and do a test drive...if the plugs look weak after that then it's more important you sort that out than when the vacuum is connected (OK it'll run shit on part throttle withOUT the vac advance but you know what I'm getting at.)
Crikey, I've probably gone off track and not answered your original question....but it might be in there somewhere.