70-74 Cadillac Qjets

[mcx]

Cliff...I didnt read any mention in your book(excellent) about the early 70s Qjets on the huge Caddy motors...those were 472 to 500ci behemoths....are they a decent starting point simiilar to the same era Buicks qjet???  Thx, Mike

[Cliff Ruggles]

No, the Caddy carbs thru 1974 are the smaller castings and still used the early hinge pin and float arrangement same as the Olds carbs thru 1974.

In and after 1975 they used the later design and larger castings, same as Olds, Pontiac and Buick did until 1980.......Cliff

[mcx]

Cliff , thanks again..so it's a standard 750cfm design and less desirable float(damn)..my neighbor offered it up,  it has the thicker air horn for the primary POE and no one has fussed with the APT  plug in the base plate....I was hoping those features can be swapped onto my Buick 800 qjet,  if everything matches up....time to disassemble it and see.

[Cliff Ruggles]

The early float arrangement is OK, but I don't use or recommend using it for "big" power.  Fine for most street cars to at least 450hp without much trouble.......Cliff

[mcx]

Cliff...this Caddy Qjet has notched out sections on the secondary air flaps but not in the location usually seen on Pontiac versions...it's not by the POE holes
 but  on the rear towards outside....what was the purpose for this location?

...I was also looking over the idle air bypass path and its differant from side to side?..one side has the air horn cut out and the other side has a drilled hole in the main body.

...I see what you mean about the  early style float pivot, much differant than the Buick.

....I tried reading the primary jets size but its stamping is impossible  ...the primary rods are 41B...I forgot to write down the secondary rods  but they are skinnier than what is in the Buick.

[Cliff Ruggles]

The Caddy carb will have very small MAB's and small jets.  Those big engines weren't all that efficient so the carbs used on them are designed to pull in a lot of fuel with a much weaker "signal" from the engine.

Conflicting information on the rear located holes in the flaps, they were used on some Marine units as well.  For sure it would be more advantageous to have POE holes or notches much closer to the exit holes.

The big Caddy carbs used very small secondary metering rods for the most part, once again those big engines weren't overly efficient so love a lot of fuel coming down from the carb.......Cliff

[mcx]

...it's official carb # 7043234
Primary jet 68 (I think ?)
Primary rod 41B
Secondary rod DF
Hanger N
...today I attempted removal of the APT in the base plate...I used your advice heating it up with Mapp gas and used transmission oil to soak in as it got really hot...used a snug screwdriver and tapped with hammer and loosened and tightened and repeat...it came out!!...I installed a mixture screw to test it out and it works ok for now...I partially assembled the carb just to see the range of adjustment of the primary rods in the jets  and am impressed what it can do.

[429bbf]

for what its worth i had a 76 eldorado  472 or 500 i don't remember that car got excellent fuel milage . it would do 18 all day long and climb any hill in wyoming with out any effort . my neighbor had the same car with a 350  it was a gas guzzler he got about 11 tops . we used to race stoplight to stoplight won a lot of races and destroyed a brand new set of bridgestones in less than 2000 miles . man them were the old days .

[mcx]

...I believe a 472 Eldo might get 18 miles per tank..LOL...cool cars.

....I spent the morning recording measurements as best as I could , wish I had a better set of bits.
Air horn:
Main air bleeds .05ish ..guestimate as the closest i had was .047"
Primary POE discharge shooter holes... .033"
Idle air bypass hole (passenger side  primary bore)... .1094"
Idle air bypass in driver side primary bore was usual notched out section.

Main body:
Upper idle air bleed... .05ish, as close i i could get.
Lower idle air bleed... .067"
Idle down channel.... .048"
Idle tubes....still have to pull.
Secondary POA well holes.. .039"
Idle air bypass  .1719"

Base plate:
Idle mixture screw holes. .0625"
Idle air bypass hole.. .0938"

[mcx]

Cliff...another question regarding those secondary Air horn flaps with the unusual rear cutouts,  do you think those are designed to open slower? I would think there is less leverage  on the longer side of door to open quickly.

[Cliff Ruggles]

They would allow a bigger gulp of air than a solid flap the instant the throttle plates opened.  Not sure about how quickly they would employ vs solid ones.  They are pretty rare right to start with, showing up once in a while on a Marine Q-jet.

I never spent any time testing them and use and prefer solid flaps over any other type......Cliff

[mcx]

Cliff, thanks again.....I'll swap the flaps out  with a cheap eBay parts core.

[Cliff Ruggles]

As noted in my book solid flaps are preferred with a hole under the flaps. 

Second best is a large hole directly in line with solid flaps as used on the 1975-79 Pontiac 350/400 carburetors.

Third is making a very small "notch" directly in line with holes just above the flaps.  An alternative to a notch is to allow just a tiny bit of slack in the linkage so the flaps "tip" back every so slightly with the pull-off fully applied so the fuel can go into the engine the instant the huge secondary throttle plates open.

The worst set-up is solid flaps with holes located high above them.......Cliff

[mcx]

Thx Cliff...I think this Caddy is the worst set up..5th place LOL

I disassembled the 1973  Buick 800cfm and was matching it up to this 1973 Cadillac....the base plate is similar enough to the Cadillac...they both have matching lower gaskets(3 holes on one side and 5 on the other)..the passages take slightly different routes but seem to go to the correct place....the key thing here being I can get the Cad base plate with APT back in the game on the Buick, which is great..
..I was measuring the height adjustment by turning the APT screw and it seems to give 4/32" rise and fall...is that enough to function correctly?

[Cliff Ruggles]

I would NOT change the baseplate as it's never a good idea to start mixing/matching parts.  There are small differences between different baseplates even if the bolt in place and look pretty much the same at a glance......Cliff