General Category > Quadrajet Carb Talk and Tips

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Cliff Ruggles:
That will work just be aware that the source location has to be dead nuts on the money to work like it's supposed to.

Obtain a baseplate that is drilled for ported vacuum advance and make sure the hole that you drill is the exact same distance above the throttle plates.  The ideal location would be for the lower edge of the hole to be right in line with the leading edge of the throttle plate but not exposed to engine vacuum at idle. 

When done correctly the slightest movement of the throttle will provide FULL manifold vacuum to the port leading to the distributor.  It needs to be that way so the VA works exactly like a manifold vacuum source when driving the vehicle but no vacuum to the VA coasting and at idle speed.

I'd add that the factory used a "slot" (except for very early carbs) to make sure the VA worked like it was supposed to.  Here I drill a very small hole in the right location then "work" the bit back and forth to make a slot instead of a round hole......

novadude:
One potential "gotcha" when looking at "ported" sources on these carbs....

Sometimes there is a ported EGR source that is located a bit higher above the blades than the early carb ported ignition source.  Often these also have "bleed holes" as well.  This EGR source is not ideal for vac advance.  As cliff said, you want the port exposed as soon as the throttle moves off idle (like the early carbs).

Cliff Ruggles:
Correct, a high source location will NOT work for a distributor VA nor will a correctly located source with a bleed-off slot next to it.

Most Q-jets made in the EGR years will have one or the other.  The ones using a bleed off slot are easy to convert to full ported vacuum for the distributor by simply filling the slot with Marine Tex or the entire area behind it but keep the slot open to engine vacuum and to the lower left vacuum fitting on the baseplate. 

When I custom tune here in a lot of scenarios I prefer a CORRECTLY located ported vacuum source to the VA vs full manifold vacuum. 

There is a LOT of miss-information out there on the subject of VA, like ported was only done for emission reasons, and that MVA makes you engine run cooler, or you absolutely MUST use MVA or you are an idiot, etc, etc.

What I've found here is that following much of the miss-information on the NET folks try to use MVA and run the timing clear off the scale as a "crutch" fix for WAY too much camshaft for their application, LSA way too right, and more times than not insufficient idle fuel to the mixture screws at the relative "low" vacuum their engine build is producing.

IF you find your mixture screws unresponsive or very little control and you need to advance the timing clear off the scale to make your engine happy at idle speed simply "tip-in" the choke flap or gently place your hand over the front of the carb.  IF the engine speed increases idle is smooth you need to fix that issue BEFOE trying to run 20, 30 or even more timing at idle to correct your issues.

Once the carb is up to par you will find that the engine may not want, like, need or respond well to a butt-load of timing at idle. 

Of course none of this is going to apply to engine where very poor choices have been made for compression and cam selection.  For most well though out engine builds you will find initial timing around 8-14 degrees will work just fine and provide a smooth steady idle in and out of gear with very little RPM drop, and easy cranking on hot restarts........

mcx:
Thanks to all…I rechecked and no “bleed-off” EGR holes on this…I’ll carefully measure where the ported hole should be located and drill it…and fill the lower stock hole to discard.

Cliff Ruggles:
I do a lot of them here and made a very small fine point punch to punch a tiny spot to start the bit.  If not the bit is going to slide down some as you have to drill at a slight angle.....

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