General Category > Quadrajet Carb Talk and Tips

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mcx:
Hello Cliff...I picked up this carb which is supposed to  be from a 1978 454 application...it looks almost identical to my 1970 and 73 Chevrolet versions with huge  MABs and divorced choke...this carb has no ported vacuum advance nipple on the drivers side? I would of thought all Qjets would of had it by the end of the 70s?? Did some applications not need it?

Cliff Ruggles:
Most Q-jets have a port in the main castings just to the right of the fuel filter housing (as viewed from the front) for the vacuum advance.  Most of the time this will be ported vacuum, and apply FULL vacuum right off idle.

There were exceptions as the factory was trying all sorts of different methods to control spark advance for emissions.  So some carbs will not have that port available......Cliff

mcx:
Cliff,  Thanks for the reply...I'll  drill the required holes to add it.

Cliff Ruggles:
It's pretty easy to drill the casting. 

If you are going to add ported vacuum the supply hole MUST be very precisely located to provide full vacuum right off idle but nothing at idle speed.

The factory knew this and cut a "slot" so that to supply the distributor vacuum advance with FULL manifold vacuum with very low throttle angles, but no vacuum at idle speed.

To duplicate that deal it's best to look at a carb that already has the slot drilled and very carefully measure where it is and drill the hole accordingly.  I've modified quite a few of them and use a very small bit to drill the initial hole, then go side to side with it to open it up some on a horizontal line vs drilling a bigger hole........Cliff

mcx:
Cliff...since replicating the "slot" is impossible (for me), what is a acceptable size hole to drill ? 
...btw, Happy Bday!!

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