Recently, we high jacked a poor guy's thread ("Ignition timing") who was just trying to hook up his new carb. The issue became whether Stude carbs have ported or un-ported vacuum slits, for vacuum to the distributor's vacuum advance (VA). The technical difference is whether the horizontal slit is located, above or below the butterflies (at idle); ported is above, and un-ported is below.
I said un-ported since, though I have not ran a OEM carb for decades, I recalled well, when unplugging the VA hose from the carb nipple, there was a strong vacuum leak. That leak, if not plugged quickly (i.e. finger over it) would result in idle speedup a few hundred RPMs initially, then stumbling. If plugged, the RPM would drop down so slow it would seem about to die.
Others said ported, and recalled none of the above; said nothing changed whether the VA hose was connected or not.
I don't trust anyone's memory, esp. my own, so I looked at an old WCFB, which appeared to be non-ported. Several others ignored that observation, and suggested other ways to determine. Then we all took our marbles and went home. Later, a demi-god spoke.
This morning, I looked very, very, very closely at the 1956 WCFB again, also a 1970s AFB, a 1980s Holley, and new Edlebrock. I used a scribe to trace the closed butterfly position, in relation to the vacuum slit. I also used a strong pen-lite to see down inside the venturi.
After looking at the carbs, then refreshing my memory on "timed vacuum" in a 1980s Holley manual I'd had for years, I now have a possible explanation for the above: There is ported vacuum, and there is ported vacuum.
The WCFB's slit is even with the butterfly, BUT the edge of the butterfly is angled upward & away from the slit, so the lower edge is effectively all that is below the slit. So technically, it is PORTED. The AFB slit is maybe 1/64" above where the WCFB's is, in relation to the butterfly, so it too is PORTED. The Edlebrock & Holley's slits are near 1/16" above the butterfly, so definitely PORTED.
The difference is in timing; how far off idle, before the slits become un-ported. For the WCFB or AFB to stay ported, they must idle very slow, say around 550-600 RPM. Much above that and they become un-ported. The Edlebrock & Holley would probably still be ported at anything less than around 1200 RPM. Further, the OEM carb's transition to un-ported is likely much quicker due to the position of the butterfly, in relation to the slit when it passes it.
The 59-64 Shop Manual says time the V8s at 600 RPM then readjust the idle to 550. That accounts for the different memories. I have set my Studes to idle at 750-800 is cool weather, and 850-900 in AC weather for as long as I can recall. At those idle speeds, the vacuum exists as I recall above. But for others, who set their idle around 550, there is likely little or no vacuum, as y'all recall.
However, in light of the above, I MUST CONCEDE, technically speaking, all Stude carbs appear to be PORTED. (Though I am unsure whether a 56J with WCFB will idle with the butterflies locked in the bores.)
I said un-ported since, though I have not ran a OEM carb for decades, I recalled well, when unplugging the VA hose from the carb nipple, there was a strong vacuum leak. That leak, if not plugged quickly (i.e. finger over it) would result in idle speedup a few hundred RPMs initially, then stumbling. If plugged, the RPM would drop down so slow it would seem about to die.
Others said ported, and recalled none of the above; said nothing changed whether the VA hose was connected or not.
I don't trust anyone's memory, esp. my own, so I looked at an old WCFB, which appeared to be non-ported. Several others ignored that observation, and suggested other ways to determine. Then we all took our marbles and went home. Later, a demi-god spoke.
This morning, I looked very, very, very closely at the 1956 WCFB again, also a 1970s AFB, a 1980s Holley, and new Edlebrock. I used a scribe to trace the closed butterfly position, in relation to the vacuum slit. I also used a strong pen-lite to see down inside the venturi.
After looking at the carbs, then refreshing my memory on "timed vacuum" in a 1980s Holley manual I'd had for years, I now have a possible explanation for the above: There is ported vacuum, and there is ported vacuum.
The WCFB's slit is even with the butterfly, BUT the edge of the butterfly is angled upward & away from the slit, so the lower edge is effectively all that is below the slit. So technically, it is PORTED. The AFB slit is maybe 1/64" above where the WCFB's is, in relation to the butterfly, so it too is PORTED. The Edlebrock & Holley's slits are near 1/16" above the butterfly, so definitely PORTED.
The difference is in timing; how far off idle, before the slits become un-ported. For the WCFB or AFB to stay ported, they must idle very slow, say around 550-600 RPM. Much above that and they become un-ported. The Edlebrock & Holley would probably still be ported at anything less than around 1200 RPM. Further, the OEM carb's transition to un-ported is likely much quicker due to the position of the butterfly, in relation to the slit when it passes it.
The 59-64 Shop Manual says time the V8s at 600 RPM then readjust the idle to 550. That accounts for the different memories. I have set my Studes to idle at 750-800 is cool weather, and 850-900 in AC weather for as long as I can recall. At those idle speeds, the vacuum exists as I recall above. But for others, who set their idle around 550, there is likely little or no vacuum, as y'all recall.
However, in light of the above, I MUST CONCEDE, technically speaking, all Stude carbs appear to be PORTED. (Though I am unsure whether a 56J with WCFB will idle with the butterflies locked in the bores.)
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