One of the main issues with poor a/f and combustion is the tendency to cause hot spots, then detonation. Detonation may not be audible, and may not show up on plugs as you may think, assuming you were even looking for it.
With varying degrees of poor a/f ratio, atomization, fuel distribution, adequate % of atomized fuel vs. liquid flowing into the cylinder, etc., etc., there can be both rich pockets and lean pockets within the combustion chamber space, simultaneously. The area which ends up with a lean spot once, will likely end up with a lean spot over and over again. Depending on conditions, it may burn there, and may run localized temperatures up, and we're not even talking about an overall lean condition.
Now with a lean condition, there will be hot spots. Once a hot spot is created, how long does it take to cool off once the a/f ratio is optimal? How long does it take for the a/f ratio to become optimal? Does it ever become optimal before the end of the run?
To add some additional complexity to this, when a hot spot is created and (low level) detonatin begins and affects combustion efficiency by upsetting flame propagation, combustion efficiency falls off to some degree (how much?), then the exhaust valve opens and you have less combustion pressure to release, this changes the rate and strength of the exhaust pulse affecting resonant tuning of the exhaust system (at a given rpm) outside or differing from design (intended) parameters as well as the effectiveness of inertial scavenging.
The exhaust system isn't working to aide cylinder filing as you intended and tuned for - perhaps under more ideal conditions, so the carburetor sees less signal strength - drawing less fuel and effectively leaning the a/f mixture delivered to the cylinder. You already had a hot spot due to momentary leanness, now you've allowed the engine to lean further - all everything I just explained gets worse and the engine has only turned another 20 or 30 reveloutions. The engine has only climbed a few hundred rpm - you've (no one in particular) got another 2000 rpm before you shift or let off the throttle.
The fact you've relied on a cam with excessively tight lobe centers to maximize the benefits of inertial and resonant tuning exaggerates the whole dilema because it relies more heavily and is more sensitive to what's happening in the exhaust system.
What helps to reduce or even eliminate these undesireable conditions is a great carburetor tuned properly. Fuel injection - as stevek provocatively jabbed - just kidding - requires tuning, whereas a carburetor may be deemed good enough out of the box - whether it is or not. The fact that it is accepted fuel injectin needs some tuning to function, and carbs may not is without examing the facts an advantage credited to fuel injection, often unfairly.
Ed's right, the best way to pull out power is a retard box, and a/f meters aren't all that effective and may give a readying for lean, for example both for a lean condition and for a rich condition causing poor combustion / scavenging etc., as I have explained - it's happened to me. They are too slow. Data acquisition systems will likely record the data at a faster rate than you meter (be it lights or whatever) can register and read them, never mind your eyeballs seeing and your brain registering them while your driving and applying throttle.
I found meters to be distracting (requiring me to take my eyes off where I was speeding) and required significant understanding of what's going on in the engine and with a/f ratio in the first place to properly interpret and apply, and data acquisition systems to slow with regard to downloading / reviewing, interpreting, etc., after the fact as I had already developed my own methods any way. If I had more money at the beginning, I probably would have purchased a data acquisition systems and become dependent on it, or not.
Troy Patterson TMPCarbs.net TMP Carbs