Not quite...
HANK B said:
so all q jets are the same right?
These are a few differences off the top of my head. In
no way is this
all the differences:
Generally, there were the ‘small’ and ‘large’ casting carb bodies. These were 750 and 800 cfm, nominal- other cfm requirements were met by tailoring the air valve and/or the secondary throttle stop. Then there was the rare (one year-‘71) Pontiac HO carb that used a smaller booster to get even more flow than the “normal” large casting, 800 cfm carb.
There were straight or 90° fuel inlets, w/short and long filter housings.
There were at least 4 different choke arrangements.
There were two different needle and seat arrangements regarding the type, and several different floats (size and material) used throughout the years.
Later carbs (~’76-up) have APT (aka “adjustable part throttle).
Some have idle air bypass, others don’t.
Most had only two primary rods, some had three- one hooked to an aneroid. Sort of like 2 power pistons.
Various carbs had different bowl vents, some had a hot air compensator. Some had secondary fuel booster outlets (passive accelerator pump) that were located above the air valve, others below. Some secondary air valves were slotted, others not.
Accelerator pump pistons varied by length and spring rates, power piston spring rates varied, obviously jets and metering rods varied- there are literally thousands- likely
millions- of different combos possible between all the different primary and secondary rods, secondary hangars and jets.
Throttle linkages varied greatly- some were made for manual trans apps, these can be used w/a TH400 if the kickdown switch is relocated (or is at the throttle pedal), some were only for a TH400 apps w/carb-mounted kickdown switch, some were used w/the TH350 (has the ability to "pull" a detent cable), some for the Powerglide trans. Some have cruise control studs, some not. Early carbs used throttle linkage, later used a cable.
There were front choke break, rear choke break, both front AND rear choke break carbs. Vacuum port size, number and locations were all over the place.
There are two different length primary rods, there are stepped and tapered rods.
Secondary rods can be slotted or not. They are hung from one of almost a hundred different hangars.
All said, you really need to pay attention to a few important things.
- The fuel inlet (straight or 90º), so it doesn't interfere w/any of the accessories or water neck, etc.
- Throttle arm- whether or not it can accommodate the trans you're using. This is important if using a TH350 that needs a "below-the-shaft" hook up point to pull the detent cable.
- 1976-up carbs are preferred (by me, anyway), like DV said, there were improvements throughout the run of Q-jets, newer is usually better. These carbs will also have APT.
- Choke type. Most prefer an electric. A hot air type choke can be converted to electric easily.
Beyond that, you just work w/what you have. The Q-jet is very versatile and will work fine on any number of engines.
EDIT- Interestingly, one of the best/easiest way to find a good carb to use is to look for one off of a truck or van.
Up until '86, these carbs were still non feedback (not an "electronic" carb) in some applications and they were nearly all 800 cfm units- even those found on 4.3L V6 engines! These engines can also have a large cap, non feedback HEI distributors.
Cars from '81-up are going to have feedback carbs. No good for anything other than computer use (CCC), or possibly as an all-out race carb where it is set up to run w/o primary metering rods.
any help would greatly appriciated :thumbup:
