The secondaries open up when your foot pushes too hard on the pedal next to
the foot brake. If you do not like that flushing sound of your hard earned
dollars being forced through the intake tract, lighten up on the pressure a
bit. Those secondaries make a holley four barrel double pumper seem like a
fuel saver by comparison. At road speed on flat country, they normally do
not open, but, anytime the vacuum dips below about 7" of water column, the
metering rods lift and the upper plate opens. Dumps fuel right out of the
float bowl through the main jets. $$$$$.

thanks for clarifying. interesting. I have very little experience with Q-jets, webers are what Im most familiar with and would prefer but the floor is the limit here.

FI is great but all in all, if youre familiar with carb tuning, the reliability and the amount of spare parts (both in cost and weight) to carry along and then be able to use effectively, is much less.

the E.Quad-Jet does seem like a perfect compromise between carb or FI for these though but definitely it would take some work to have something better than just a regular Q-jet.

ESC and a stock Q-jet seems to be a sound logical step up without adding too much unneeded complexity and cost. I'll keep looking into this..

the vapor lock is always tricky but Im sure it can be eliminated while keeping the Q-jet, thats not really directly related to the carb but the fuel supply system, right?


Edit to add: a GMC site about FI just came up in my 'ESC' searching : http://www.gmcws.org/Tech/efi/efi.html

its from '00 and has this tidbit "Since our distributors do not have the ability to respond to ESC, we can use something like the manual MSD spark adjustment and the MSD knock sensor. (MSD devices are used to permit electronic spark advance and retard as a means of providing appropriate timing for engine loads, altitude and similar conditions where it is desirable to be able to adjust the timing without manually rotating the distributor.)"

is this still the case? and brings me to this question, do the chevy distributors interchange? (fit into the Olds 455)

Sounds like you really want to try 80s electronic controls on your GMC. FYI the Olds V8 continued in 307 cu in displacement through 1990 in some full size GM cars. These later years all have electronic quadrajets and HEI. I don't know if they had knock sensor control.
For what it is worth I'd stick with the OE quadrajet. They are incredibly tunable and if you are familiar with other carbs you can learn quadrajets as there are a few good books devoted to the subject such Doug Roe's book. The later years (76-78) have adjustable part throttle (APT) metering. Between primary jetting, primary rods (the GMCs are a special triple taper), secondary rods, primary power piston springs and the APT the Q-jet can be dialed in quite satisfactorily. It just takes a linear lambda gauge, an assortment of jets and rods and careful note taking. The only downside relative to fuel injection is the lack of altitude compensation. I may get some argument over this last point, but there are many here who will agree that the q-jet is actually pretty darn good in terms of performance and economy.

Well, I thought Lean Burn represented the worst of both worlds. No loss at all, and a good way to ruin a Carter carburetor.

--johnny

I got one of those carbs-they call it a dual capacity accelerator pump that the shop I bought my 76 GMC from
put on when they built the engine. The builder left out the solenoid and has a pug in the top where the wiring
would go. These carbs had no (MC) mixture control solenoid or (TPS) throttle position sensor. From what
I have been able to find out so far is they were on 85-86 non computer controlled trucks 305 on up. I am looking
for the jets and rods that were used on this carb to have some place to start tuning.