Missing Vacuum Port? what do i do?

[72Campcust]

The round hole inside the bore just aft of the slot that provides ported vacuum.....Cliff

Cliff-- the hole that im pointing at now (down in the bore) is the only hole in the bore..is that the right hole to plug? (there are no other holes down in the bore that ive got the pencil down inside)

Or am i totally off the mark?? Sorry for so many questions.. i just dont want to plug the wrong hole..

[Cliff Ruggles]

You're getting closer.....INSIDE the bore, just aft of the slot that provides vacuum.  As the throttle angle increases, the round hole is uncovered by the throttle plate, and kills the vacuum signal by bleeding off the source back to the main bore.....hope that helps some?.....Cliff

[72Campcust]

You're getting closer.....INSIDE the bore, just aft of the slot that provides vacuum.  As the throttle angle increases, the round hole is uncovered by the throttle plate, and kills the vacuum signal by bleeding off the source back to the main bore.....hope that helps some?.....Cliff

guess my reference points are off..

Pic #1: Inside the bore there is this hole, which is below the throttle plate..


Pic #2: and these two holes are above the throttle plate..Pencil is pointing at the sloted hole, screwdriver is pointing at the round hole..

[72Campcust]

Bump...

[omaha]

I think it would be the "hole" that the screwdriver is pointing at. That would make sense as it is common with the slot, just at a different position in the throttle bore. I guess as the throttle opens, it provides vacuum to the slot but because the throttle blade is not past the hole yet, it is not a strong vacuum to the slot because of the hole being on top of the blade is bleeding off the vacuum(to the top off the throttle blade) once the blade passes the hole, then, vacuum signal would be returned, BUT, by the time your throttle get that far, the vacuum signal is not as strong any way. (unless your just at light cruise or slowing down from heavy throttle.) That is wild how just one carefully placed hole can have such an effect. like that.  Good luck!!

[72Campcust]

Thanks, appreciate the feedback.. the wife is getting anxiuos to get the truck out of the garage..

[Cliff Ruggles]

Correct, the bleed-off hole is behind the slot, and uncovered as the throttle plates open.  The first photo is idle bypass air, under the throttle plates....Cliff

[72Campcust]

So-- wanted to provide a wrap up for this; plugged the hole as suggested by Cliff.. truck runs great now! I have had to redo all my Idle Screws/APT settings now that the idle has changed but definitely have noticed a difference in performance..power/acceleration at all speeds...now just need to dial in the settings and i think ill be good! 

On another note, while i had the carb apart i changed the heater Core (it was leaking BAD), painted the valve covers and repaired some cab rust (since the carpet was out...)

Thanks for all the help Cliff/Everyone-- and patience with me finding the right hole to plug!

[Cliff Ruggles]

Good news!  Strong ported vacuum works great to apply the advance so you get maximum efficiency at cruise, but no additional timing coasting and at idle speed.

Some engines just don't need all that timing at idle, and it's nearly impossible to get them thru emission testing as well, if you are using MVA to the distributor.....Cliff