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440 Six Barrel Vacuum at Idle

MoparHonda

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What is the expected idle vacuum for a 440 six barrel? My car idles at 900 rpm, although the idle is rough.

I'm showing 7 lbs vacuum at idle, the vacuum jumps to the 18 range as soon as I open the throttle. It momentarily goes over 20 when I release the throttle.
 
You have a vacuum leak, OR about a half dozen other issues that could cause that.
 
Can you please suggest a place to start looking?

Would a performance cam cause a lower than normal vacuum reading?
 
idle vacuum is dependent on valve timing and ignition timing. lean or rich conditions also effect idle. 7" (inches) is low unless you have a lot of cam and slow timing.
 
I have timing set at 7.5 degrees BTC, the cam is supposed to be stock (I've never had it out of the car, the fellow I bought the car from 3.5 years ago says it was stock.)
 
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Vacuum leak- around carbs,intake around head/s
 
I know it's off topic, but I love that '62 Dodge, would like to get one. I have a '63 I got from a neighbor years ago.

How may I test/check for a vacuum leak? How would I check for one between the heads and intake?
 
Spray a little choke cleaner around all the seams,crevices,hoses.
 
Maaybe this will point you in the right direction..your readings will obviously be lower but this shows how different things affect the gauge behavior..
vacumm-reading-1.png
 
if the carbs are calibrated as new, then the circuts are too lean . the idle circuit is the worst culprit. the idle jet in the center carb is too small, and the idle air bleed is too large. enlarging the center carb idle jet may not be easy for the average guy but the air bleed can be dealt with. the old cure was to tamp a piece of lead shot into the idle air bleed well and re-drill to .046". go to the end carbs and remove the lead caps that cover the idle fuel mix adjusting screws. back them out about 3/4 of a turn. the center carb mixture screws will need to be out about 1 1/2 turns. the main jet in the center carb needs to be #64. this should make a noticeable difference.
 
Thank you, LewTot164, I'll check those jets, I'll have to get the carbs off the car and apart.

I did have all three of the carburetors rebuilt, and I will have to say the car ran a lot better with them rebuilt, but the vacuum is low, which is why I posted this thread.
 
another thing that will screw the idle up is having the linkage that connects the three carbs too tight. if the linkage is tight it will cause the end carbs to crack open and screw up the idle mix. proper adjustment is the linkage slightly loose at curb idle. you should be able to take your finger and wiggle it up and down a little. the linkage's function is to open and close the carbs in unison. the throttle shafts on the end carbs are located off the centerline of the bore. this keeps the throttle blades closed at high vacuum and helps close the carbs when the throttle is lifted. there is a tang on the diaphragm side of the carbs that is the throttle stop. this should be bent to where the throttle blades can never go past center at wide open throttle. in fact it's best to adjust the tang to where the blades open a little less than 90 degrees to the throttle plate.
 
The linkage is correct, the end carburetors are closed at idle, there's no air going thru them at idle.

There is apparently a vacuum leak somewhere, which is why the vacuum at idle is low. Agreed, the jetting is probably a little too lean.
 
I guess the easiest thing to try is to advance the timing more. My six pack likes a lot of initial timing and I went through the same thing. Not saying it will cure everything but will likely improve you vacuum reading and you don't need to pull the carbs, yet.
 
Why have timing at 7? Move it up to 15 and see how the engine likes it, then you can move it up more from there after you figure out if there is a vacuum leak and carb adjustments. It'll start way better also.
 
adding additional initial timing without knowing how many centrifugal degrees are in the distributor can lead to problems. find out what the distributor has in it and go from there.
 
Agreed on the distributor, advancing the timing may help with the vaccum.

What figures should I expect at what RPM from the mechanical advance with the vacuum advance plugged? What figures should I expect at what RPM from the mechanical and vacuum advance combined?
 
Probably over 50* with vac&mech. 20-26*mech only, and plus ur initial. IMO.
 
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