17058212

[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!!

[Cliff Ruggles]

I would use a very fine punch and precisely locate the spot to drill.

Then drill it with a very small bit in a pin vise, around .030-.036".  Make another hole just to the right or left of it and drill another hole.  The factory actually did this with some baseplates when they slot wasn't used. 

If you are unsure where to drill find a baseplate already set up for ported vacuum advance and keep in mind that you are drilling at a slight angle and have to compensate slightly for it, so measure twice and drill once........Cliff

[mcx]

...I think i can handle that...thx for the tip.   

[Cliff Ruggles]

I think you'll be fine, a quick check with a vacuum gauge after it's placed in service will let you know how you've done.  It should have very little if any vacuum at idle, and FULL manifold vacuum with a very slightly movement of the throttle.

This is important as it basically provides the same strong vacuum signal to the advance that using manifold vacuum does, but without adding it at idle and coasting. 

Many engines don't like, want or need all that timing at idle, despite what you might read on the NET about that sort of thing......Cliff