I'm tuning for altitude. If EGR makes the air/fuel mix leaner it would still influence my choice of a primary idle jet on a progressive 32-36 DGV carb because it is still in play at part throttle, just not exclusively? And there's some wiggle room within the idle mix screw. I plan to install a new O2 sensor and a readable air-fuel guage. Where I typically drive, the elevation varies over 5,000 ft.
Again it doesn't make the mixture leaner. The mixture stays the same just slightly less of it and the difference is made up of inert exhaust gas that does nothing.
I was assuming that the pressure from the exhaust manifold, via EGR valve, would offset intake manifold vacuum, and with less being pulled through the carb venturis less gas would make it to the motor.
I thought that EGR was a means of getting less fresh-air/fuel to the motor when the need is to simply fill combustion chambers rather than supply power needed?
Manifold vacuum would remain the same. But there would be gas/air mixture and exhaust when the EGR is working, rather than just gas/air mixture. The EGR is regulated by the VVT valve which monitors exhaust 'back pressure' and adjusts the vacuum signal from the carb going to the EGR valve. Thus the engine load self regulates the amount of EGR delivered to the engine. More throttle = more back pressure = more EGR.
Yes, slightly less gas and air reach the cylinder but mixed with a small portion of exhaust that just takes up space and fill the cylinder. EGR isn't a huge amount, just enough to reduce peak cylinder temperatures and pressures when nitrogen combines with oxygen.