Ported vrs Maniffold vacuum

[72CampSpecial]

So-- ive done tons of reading and im still not sure how to proceed. here is where im at.

'72 C20 w/350 3/4 ton.
I Rebuilt the Quadrajet (w/in last year)
Dropped a HEI in about 6 onths ago... from a '75 GMC.(dont have model #/serial #..i need to go take a pic)

eveything works.. i had the vacuum line from the HEI hooked to a manifold vacuum source for the last few months, but the truck seemed like it was working hard. Ran smooth, shifts great, but no umph.

I moved the vacuum line to a ported vacuum source today and the truck seems seems to have more "pep"

I set the idle to 8 on the balancer timing mark with the ported vauum source pluged then hooked up the vacuum line from the vacuum advance on the HEI..idle jumps up to 14-16 (since that is past the marks on the balancer its a guess)...and like i mentioned i cant bring the idle back down with the set screw since its all the way out..


Issue: i have the idle set screw all the way out and the truck is still idling high (after kicking down once its warmed up not on the High Idle setting). its not too teribly high...just higher than i think it should be.

All i have is a basic timing light and a vacuum gauge so i cant talk in specific RPM #'s.. my guess its idling around 900-1000ish in park and drops down to a good idle in drive 700 maybe??.

Am i doing something wrong?

[Molberg]

If your idle mixture screws are cranked in too far.. It can give you idle issues at varying engine temps as well in my experience. (Can't get idle down) Turn and a half out is a good starting point on a q-jet I've found.

[Wrenchbender Ret]

You probably have worn out bore for the throttle shaft. it lets vacuum in & doesn't let the throttle plates close good. You need to rebuild the carb & put bushings in the holes for the shaft. To get around this sometimes you can put an aux. return spring on the upper part of the throttle lever & hook it to a bracket towards the front of the engine. Hook the vacuum advance to the ported oulet on the carb so you won't get advance till the throttle is opened.

[72CampSpecial]

Originally Posted by Molberg

If your idle mixture screws are cranked in too far.. It can give you idle issues at varying engine temps as well in my experience. (Can't get idle down) Turn and a half out is a good starting point on a q-jet I've found.

Molberg: ive set the idle mix screws to pull max vacuum at idle (gauge shows 20..not sure if that is true) and then turned them in 1/4 turn.

Id have to turn them all the way back in and then back out to know how close to turn and a half out.. should i start over??

[72CampSpecial]

Originally Posted by Wrenchbender Ret

You probably have worn out bore for the throttle shaft. it lets vacuum in & doesn't let the throttle plates close good. You need to rebuild the carb & put bushings in the holes for the shaft. To get around this sometimes you can put an aux. return spring on the upper part of the throttle lever & hook it to a bracket towards the front of the engine. Hook the vacuum advance to the ported oulet on the carb so you won't get advance till the throttle is opened.

Wrenchbender...the 1 (!) thing i didnt do when i rebuilt the carb was putting in new bushings in the throttle plate... and its bugged me ever since.. think this might be happening just to eliminate it!

Also, there is (and has always been since i got the truck) a spring on the upper part of the throttle hooked to a bracket toward the front of the engine)..so someone else already did that..

thanks!

[Molberg]

Wrenchbender is right on his comment as well. You could count the revolutions while turning them in with the engine off. Then just turn back out to get to where you are when done.

[LONGHAIR]

As a side note, your '75 GMC was a great candidate for a donor. That was one of the early ones which should be before the "ported vacuum" debacle....assuming that it wasn't changed over the years.
There are calibration differences with the units intended for ported vacuum and you can't just go swapping the vacuum line around

[72CampSpecial]

Originally Posted by LONGHAIR

As a side note, your '75 GMC was a great candidate for a donor. That was one of the early ones which should be before the "ported vacuum" debacle....assuming that it wasn't changed over the years.
There are calibration differences with the units intended for ported vacuum and you can't just go swapping the vacuum line around

Longhair, Thanks.. can you point me to where i might go to figure that out.. if i get a model # of the HEI where can i look to see how its properly hooked up?

[myyamr7]

I had a similar issue with my idle and it was the choke rod to the heat riser not working properly. Keep in mind, you have an idle screw when the choke is off, and a high idle screw when the choke is on.
As for the vacuum advance, it should be hooked to the carb, not the intake. Vacuum from an intake port goes down with throttle, the advance to the HEI wants increased vacuum with increased throttle...which I believe is the left front port on the carb.
I have the timing light with the spin dial on it to set the mark on 0, and the dial tells you what the timing is. I've checked mine at idle with the vacuum plugged in, and with it unplugged, it is always the same at idle. As soon as you give it throttle, it should then advance.
In the past, I would never set the timing at idle, I set it at total advance at about 2500-3,000rpm. I want to say 36 total was what I set mine at, and at idle it's end up around 12-14

[72CampSpecial]

Thanks for the isight guys.. i did some more tuning today and i think I'm getting closer!