Should I have vacuum at hot idle from the port on the carb that is meant for the distributor vacuum advance ? Mine was originally routed thru the OSAC valve, but I nixed that years ago. At hot idle, the nipple shows me 12" of vacuum and goes upwards of 20-21" at higher speeds. I thought that this nipple wasn't suppose to show any vacuum until you were moving faster than idle. Am I incorrect here ? Thanks in advance...

If you lower your idle so the throttle plates are closed, it will go to zero at the ported vacuum fitting. The point is to get the plates closed down so it isn't pulling vacuum. You may have to modify your idle mixture adjustment to be sure it stays running.

It is a common tuning error.

If you can't get it to go to zero and idle well, then there are other problems.

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Aggressive Ted

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74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

No. Your distributor should be connected to a port on the carb that has no vacuum at idle and has vacuum when the throttle opens.

The correct answer on '75 and later carbs, including the 1945, is that it depends on the transmission type (automatic, manual) and emissions package (passenger car, light-duty truck, heavy-duty truck; Federal, California, Canada) the particular carburetor is for. I'll scan the relevant pages from the relevant service bulletin when I get a minute, but in a nutshell, manifold vacuum was applied to the spark advance port on certain carburetors for cars with automatic transmissions and certain emissions packages.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for the info Ted, Reed and Dan. I suspected something was up w/ this carb. Dan, I will await to hear back from you on the final answer. Ted & Reed, your answers do make sense to me.

Wow, learn something new every day. I thought it was always the ported nipple for the distributor. Never knew Mopar used full vacuum on certain applications.

Well at this point I am gonna do what Ted says and see if that drops the vacuum at the port to 0. Thanks to member matv91, I was able to identify that my 1945 carb is probably a pieced together rebuilt.I would still love to hear from SSD if there were any 1975 or later 1945 carbs that had vacuum from this port all the time....

...? You already did hear from me that there are '75 and later 1945 carbs with manifold (full time) vacuum to the distributor advance port.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Oh OK I apoligize. Thought that previous post of yours was a "I'm not sure- I'm going to check" and not a "I'm quite sure and these are the facts"

My misunderstanding. Thank you for the education..

Ah, yeah, it was "I'm sure, and I'll post proof when I can."

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

OK then let me ask, on these engines (1975 and newer) that have full manifold vacuum at idle, how is the vacuum advance can set up? If vacuum at idle is say, 12" would the vacuum advance can be adjusted so it wouldn't engage until 13" or 14" ? Can the advance can be adjusted that high up ? (Please be patient I'm learning)

No, the idea is that the vacuum advance is active (advancing) at idle on those carbs that have manifold vacuum to the spark advance port.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

OK, but would it be desirable to have that much advance at say 1000 rpm ? (right off idle) By 1200 rpm I would assume the vacuum advance would be fully engaged and not there for the higher cruise speeds ?

By 1000rpm there'd be no real difference in the vacuum level at the advance pod regardless of whether the carb provides vacuum at idle or not. The difference is at and just off idle.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Your last post has me a little confused (not uncommon for me )LOL. Should your vacuum advance start to come in right off of idle ? I would assume it would until the centrifugal advance starts to kick in ? At higher RPM when centrifugal advance really comes in, is the vacuum advance still advancing the timing in addition to centrifugal or is in inactive by then ?
(again thank you for the education here)