General Category > Quadrajet Carb Talk and Tips

Performance improvement

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Big Blue:
 If I follow the instructions in Cliff's book about modifying my Rochester Quadra jet for my 500 in.³ Eldorado, how much of an improvement should I see in the performance and the gas mileage of the vehicle ? Also, is this far too difficult of a task for a newbie to attempt ?  Thanks, just looking for an answer or something before I get in over my head.

Cliff Ruggles:
What has been done to the engine?

All of the later q-jets are emission calibrated, so pretty lean.  With this new fuel and ethanol it ALWAYS helps to custom tune them exactly for the application.  The later carbs have APT, so it's much easier than with the early units.....Cliff

Marx3:
For your information: I once rebuild a 1780215 for a hot 383 stroker, but never got to using it on that engine. Later on I bought a 1975 Coupe De Ville. The car and engine was completely stock ( 500/th400 with 2.73 rear. )
I put the 17080215 on it and the engine couldn't have run much better.
With base timing bumped up to about 20 in idle ( giving a total about 30 ), it pinged a little on part load. My wide band read 17:1 at cruise. It had 73 jets/44 rods in it. I upped the jetting to 74/ 45 if I recall correctly. This put the afr at  about 15.5:1 and cured the pinging.

The rest of the set up was:
All air bleeds: .070
Idle tubes: .038
DCR: .054
Can't remember if it had idle bypass. Don't think it had.
Secondary rods were Cliff's DA's. Can't remember what hanger, but wot afr was dead on at 12.8:1

The car ran perfect. Startet even better. Even when it was half cold, it would jump to life just by looking at the ignition key and WITHOUT touching the gas pedal :-)
The stock untouched Cadillacs, from the late 70's,  I have had my afr on, all showed near optimal afr's with the original Qjet.
Feel free to copy my setup. After all it WILL make your caddy run perfectly and should also give the best mileage possible.
Just my 2 cents.

Marx3:
Edit: just found out that the 17080215 did have idle bypass air at approximately .110

Big Blue:
Marx3,
Thanks for all your information. As a newbie, it is going to take me some time to digest all the information you passed along.
I may just take you up on using your set up.
I don't think anything has been done to my '76 Eldorado. It has 32,000 miles on it and needs an exhaust, so I'll be taking off the catalytic converter off when the new exhaust goes on.
My carb is pretty clean, but the casting wholes have been filled with epoxy. At Cliff's suggestion I am going to pressure test them before I tear the carb completely apart.
My unit is a : 17056230, 1976, Quadrajet 4 barrel-Federal Standards, Cadillac, Automatic Transmission, if. My decoding is correct and it should be, because it matches the car then.
Thanks again.

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