New guy, Cadillac Q-Jet questions...

[Gator]

Hi, I'm Gator from Tennessee. I found this site while looking for info on the Q-Jet on my Caddy - I've ordered Cliff's book but have some questions while I wait.. I've rebuilt a few stock and performance carbs but never modified a Quadrajet.

I have a 1955 Cadillac Coupe DeVille, equipped with a stock 1970 Cadillac 472 / TH400 and the stock '55 Cady rear w/ 3:31 gears. The carb is the original Q-Jet, # 7040230 / 1310. The car runs and drives out fine, but I know there's room for improvement. My main concern is coaxing a little better MPG - I get 12-13 now, if I can get that number to 15-16 I'd be happy.

I'd originally planned on installing a 600 cfm Edelbrock I have here, but after much research I believe the smaller primaries on the Q-Jet might be a better option...

Anyway, from what I understand the book has 'recipes' for certain combos, I'm just curious what the experts will recommend for mine. Keep in mind these Caddy big blocks have a 4,400 RPM redline...

I've also read on other sites that since the BIG Caddys had such a low profile hood the intake plenum floor is actually BELOW the intake ports on the heads - therefore they respond very well to a 1" carb spacer and even better to the Edelbrock intake. I have plenty of room under the hood. What are your thoughts on using a spacer on this application?

 I also plan on swapping in a later Caddy rear with a 2:73 gear this winter.

Thanks up front...

[Cliff Ruggles]

Caddy carburetors are quite unique in several ways.  Like Old's, they continued to use the early float arrangement until 1975.

I would completely and correctly rebuild the original unit with one of our kits.  This will get it up to par for modern fuel.  Set everything to stock specs.  Most important is to effectively seal up the bottom plugs as most are leaking in those older units.

Make sure the mechanical and vacuum advance in the distributor are working correctly.  It may be in need of some help as well.

I would avoid aftermarket intakes, spacers, and carburetors.  The Edelbrock clones are junk, and that design was obsoleted in 1967 when it couldn't pass the emission standards put in place by the clean air act of 1968. 

The Q-jet on your engine was factory calibrated, and will be the best choice and most efficient.

Caddy engines are huge "tractor" engines.  It will run better with the 2.73's that 3.31's.  They don't need the additional rpms at cruise, and fuel economy will improve with the gear change.....Cliff