Using a 71 Qjet on a 68 GTO with five port TVS

[dataway]

So I have this 71 Qjet on my 68 GTO (stock YS400, auto-trans). On the TVS I've looped the carb retard port and the dist. retard port.

The three left are dist. advance (no problem), manifold vac (no problem) ... but the "carb advance" port .. is supposed to go to a ported vacuum source. I don't think the 71 QJet has one (7041264). I've been advised I can drill one. But I'd need pretty detailed instructions before I drill into this NOS 71 carb.

Any advice?
Thanks

[Cliff Ruggles]

A correct ported vacuum port can be added to any Q-jet.  The main casting will need to be drilled in the appropriate place to cross connect the vacuum supply to the baseplate.  The baseplate will also require a very small/precisely drilled hole to provide FULL MANIFOLD vacuum right off idle, but no vacuum at idle speed.

That is the hardest part and usually where we see folks mess things up with this modification......Cliff

[dataway]

How necessary is it to have the ported vacuum source. From what I gather it is only used at idle for the most part, and is of questionable value, something that was used along with the TVS for reduced emission at idle.

For the sake of half assed visual correctness I was thinking of running both TVS ports to manifold vacuum instead of one to manifold and one to ported. This would be the same as running straight manifold vacuum all the time correct?

I have to review the TVS schematic to see at what temps it is switching between ported and manifold but as of yet, I'm not understanding any big reason it even needs the ported source. Am I wrong?

[Cliff Ruggles]

Ported supplies nothing at idle, but full manifold vacuum right off idle. 

Starting in 1970 they played around with systems to limit vacuum advance under certain conditions.

For the most part none of those efforts were all that successful as they started lower compression, retarding camshafts, hotter thermostats, EGR and other items thrown into the mix.

Took them about 20 years but they finally learned that engine efficiency starts with higher compression and improved combustion efficiency.  At least at that point you don't have nearly the PPM of pollutants to deal with in the first place, so LESS efforts are required to clean things up in the exhaust stream........Cliff