Always good to rule out the simple things…like vacuum leaks “somewhere!”
Just disconnect the minor vacuum hoses from the rear of the engine, cap the nipples and tune away!
As Royce suggested, check your timing advance vacuum mechanism on the distributor…with a piece of vacuum hose connected to it…BLOW into it…don’t suck…don’t ask why…as you’d likely figure it out shortly! IF you can blow in it, well that’s a different problem, not your idle.
Once you get your low idle carb adjustments made with initial timing set to somewhere between 8 and 14 degrees BTDC, THEN you can check for maximum advance! (Presuming your vacuum advance canister on the distributor is GOOD!!!) With the vac advance connected (none of the other possible leaking bits… Rev the engine up and read your timing. Should be in the 34-36 range. Fords seem to like this area… If you were tuning this on a dyno you could play around with this number…but you’re not…so this is a good starting point for good running.
If the amount is tooooo much, you can either dial back the vacuum advance mechanism (allen screw inside via the vacuum hose nipple hole) OR simply adjust the distributor timing down just a hair until you get the 34-36 range. Ignore the wild swing on the timing marks when you back off the throttle as that is when you get a moment of big vacuum and it advances the distributor fully…
Adjust your engine idle via carb idle screw.
Use a vacuum gauge connected to an engine vacuum port and tune your idle screws for maximum vacuum, then back in an 1/8th of a turn, adjust idle screw on carb for idle speed…check vacuum again… Some minor back and forth here is possible…
This should get you idling smoothly and not gassing you out when you pull up to a stop light and your exhaust catches up to you…
Along with fastmerc stuff , after getting rid of all the vac leaks , put a vac gauge on manifold vac and avance dist until vacuum stopps rising…lock distributor down . Shut off car for 10 min…try to start…if it bogs , retard dist a couple degrees. Keep doing this until it starts after heat soak . This timming method works on all engines that have incorrect timming pointers or bad balancers
So i’ve spent the afternoon tweaking here and there.
I eventually got the timing back at 12 BTDC and an Idle at about 850/900 RPM (my exhaust manifold leak really stands out now lol.)
Now the car idles but i can hear some regular drops (vacuum gauge is showing said drops).
I plugged the headlights vac hoses and began spraying brake clean here and there.
Spoiler alert : vacuum leak at the base plate underneath the throttle linkage. Could this be the cause of these drops ?
Thank ya’ll for your help btw !
Yes it could be…
Check gasket , many aftermarket holley type gaskets are cut too small in the front.
Also , you can provent paint damage by using a propane torch with flame off instead of brake clean or quick start
By the way : I have about 18 psig of vacuum at idle on the distributor port that’s on the metering block… i was told it’s not normal… so muche trouble arrgh !
Manifold vacuum is tapped from the intake side of the carburetor butterfly (throttle) valve. It is created by the pistons sucking air against a near closed butterfly valve at idle. Ported vacuum is tapped from inside the carb above the butterfly valve. Ported vacuum increases as you open the throttle creating a vacuum in the barrel of the carb. It may not always be 0 at idle, but close enough that it should not advance the distributor timing at idle. If you are seeing 18" vacuum at the distributor vacuum advance port at idle, you more likely have manifold vacuum routed to it than ported vacuum. Some people run this way on purpose, but that was not the original intent of vacuum advance.
The distributor advances at idle. That’s why the factory shop manual has you plug the hose before trying to set initial distributor advance. While ported vacuum is not manifold vacuum it still does what it does.
The internet warriors say that it is supposed to be zero - they are totally wrong.
You do always have to disconnect and plug the manifold vacuum retard on dual advance distributors. But I’ve never seen enough ported vacuum present at 600 RPM idle to advance timing on 69 and 70’s. You must be right in some cases though, because they do tell you to do it even on single advance distributors. Interesting.
I don’t know the number but it isn’t zero. Typically I am setting the vacuum advance on a totally stock 390, 427 or 428 CJ. All of them have vacuum that moves the advance.
It does make sense that ported vacuum is not zero at idle. But I agree ported vacuum would never be 18" at idle. That has to be manifold vacuum you are measuring. Ported vacuum would come from a nipple on the carburetor above the butterfly.
I swear I measured it at the side of the primary metering block… now maybe my vacuum leak at the base of the carb could do that. Im gonna rebuild it anyway
Not sure what you are referring to as the “metering block”? I would go directly to the source and measure ported vacuum at the carb port, and manifold vacuum at the manifold port.
The vacuum block on the intake manifold just distributes the same manifold vacuum to multiple hoses. Ported vacuum that is used to advance the distributor timing comes from a port on the side of the carburetor.
