Carter Thermoquads ~ 'More General Information'

we hear that TQ's after 1974 are leaner, then from another source TQ's after 1975 are leaner, and also from a third source that they're all good up to 1977, etc.

well the truth is always very hard to come by, but is a good thing and very enlightening.
so today I compared a 1972 #6090 TQ 850cfm, to a later model 1977 #9103 TQ 850cfm
the only difference in the description is the later model is labelled "HP" in the Carter Thermo-Quad Guide online. both are for automatic transmission. the drill bit pin hole test and dial calipers don't lie. they may be off a thou now and then, but still a murky image becomes a clear picture.

what I found was, both carb top lids are identical in every way. every single air bleed is the same, even the secondary spray bar discharge holes are drilled exactly the same way. and BTW there's more holes in one side spray bar than the other from the factory, and many different sized holes side to side. they are not the same side to side on secondaries. and they are the same offset on both carbs.

the ONLY difference I can find, is the idle jet tube. the '72 6090 has a .036" idle jet, and the '77 9103 carb has a .034" idle jet. not much but that is noticeably leaner on an idling car.

now the puzzling, perplexing part. we always think the later the carb, the leaner it got. not in this case, the later '77 carb is jetted WAY richer:
'72 6090 had 95 primary/137 secondary jets
'77 9123 had 98 primary/143 secondary jets

here's where the biggest difference was, that leans out the late model '77 carb at midrange/part throttle, the primary metering rods:
'72 6090 has 1937 rods, they are 70-46-40 in thou/inch
'77 9103 has 2211 rods, they are 71.5-64-40 in thou/inch

what's it boil down to ?
the '77 400 HP had .0035" leaner idle mixture, with the fatter rod/smaller idle jet. it also had much leaner by .018" midrange throttle mixture metering area due to the wider rod middle step. just going by rod/jet sizes. not doing the math for actual area.

now the kicker...at wide open throttle, both power tips are .040"- dead nuts even.
the '77 carb has much larger primary/secondary jets, .003" bigger primary, .006" bigger secondary. 3 more jet size primary is a lot bigger. 6 more jet sizes higher on the secondaries, is a real lot more. that's like running a power valve that opens at WOT on a Holley.

and so much for the later carbs having more emissions or being leaner. only a tad leaner at idle, and at midrange/part throttle. if you simply drilled out the idle jet on the '77 carb with a long .036" drill bit, you basically HAVE the '72 carb, and then some- the '77 carb will fuel a larger engine at WOT, with the richer jets, same power tip size metering rods.

the secondary main metering air bleed tubes were identical on both carbs, .040" tubes, drilled through on the bottom, showing two .034" emulsion holes, being drilled through once in and out of the tube horizontally. the diminishing idle air bleeds, primary discharge nozzles, and air bleeds in the nozzle tips are identical.

yes the later carbs are leaner- but only a tad, and only at idle, off idle, part throttle.
at WOT the later carbs very well can be, and are, richer.
so...take the lid from the 6090, with its attached metering rods, and plop it on 9103. you really only need the lid from the early carb.