GREG K wrote:When an engine is idling there is a lot of vacuum that sucks in whatever there is to suck in.
If you lean the idle fuel out and advance the timing,then the exhaust is not full of fuel anymore so it can suck in O2,where as before it couldnt with rich retarded settings.
The only test you need to do is grab the dizzy,turn it and lean the mixture screws.now your engine runs smooth and rpms go right up.keep going if you want,2,000--3,000---4,000 rpm sometimes.
You havent opened the carb,its shut,so you know that you are sucking in O2.
If the engine can get up to 4000 RPM without changing the throttle angle, and just leaning and advancing, it means the initial point was way too rich or way too retarded and needed a very open throttle just to idle or just wasn't pulling fuel in well. It's not getting O2 from the atmosphere through the exhuast, unless there is a leak within a few feet of the head, or if you count misfiring. Getting more O2 from the exhaust isn't a good thing, either, unless you're already rich. O2 doesn't burn. O2 + vaporized fuel burns. If you're leaner than ~lambda 1, more O2 cannot help make RPM higher.
AFR and Spark affect power, such that at idle you need say 8hp to keep that speed. You can get 8hp by many ways - stinking rich lambda .65 + open throttle + late spark; or lambda .9 + much less throttle + near MBT timing; or Lambda 1.2 + open throttle + near MBT timing; or lambda .8 + a little open throttle + near MBT timing; and so on.
The power required to keep idle speed is not affected, but the efficiency is. If you do something to change only one part of the equation, the RPM will change, without getting O2 from another source. If you change 2 things both towards better efficiency, of course the RPM will increase, but doesn't mean it's from the exhaust.