The idle fuel is controlled by how the idle system is set-up. In theory it really wouldn't matter if the power piston was up or down as the starting point for the idle fuel is at the tiny hole in the bottom of the idle tubes at the bottom of the carburetor.
On very early units the upper idle bleeds were tiny and there to break the siphon effect as the fuel bowl would drain empty after shut-down.
Tiny bleeds didn't require very big idle tubes either. As emission standards tightened we started to see the factory come in with much larger idle bleeds to add more air to the fuel that made it to the idle mixture screws and transfer slots.
As they continued to increase air supply to the idle system most Q-jets were no longer able to function well when the user installed them on engines that made poor vacuum at idle speed.
This very quickly gave the Q-jet a bad reputation as a "high performance" carburetor even though the rest of the design is excellent for high performance work.
Between the HOPELESS 1965-66 (and some 67 models) plunger type bypass fuel valves, the early poorly designed hinge pin float arrangement, and leaning them up for emissions, Q-jet got an early bad reputation for high performance work.
Some specific models had a good reputation for high performance, the Chevy 7029207's for example were, and still are sought after by folks who race in Stock and Super Stock Classes. In reality the 207's are no different anyplace compared to the "grocery getter" and truck carbs from the same period, they just had really good idle calibrations and work well with "hefty" camshafts and lower vacuum at idle speed.
Not really trying to put a history lesson up here, but in reality the 1976 and later APT models are far superior to any of the early and highly coveted "high performance" factory Q-jets. They have an excellent float/hinge pin set-up, APT, convert to electric choke, and most are bigger CFM.
Edelbrock even patterned their "famous" 1910 850 cfm units from 1980-81 Chevy truck carburetors. They gave them a little more idle fuel, .149" fuel inlet seat, Marine length accl pump, larger primary jets, increased the open angle of the secondary airflaps, and added a couple of holes and "goofy" notches/bent tabs to the flaps.
They called them "850" cfm models and they were and are very popular to this day. As good as the 1910's are, they are identical to most 1979-86 truck SMAB Q-jets just have a few simple mods made to them.......Cliff
