I have a 440 that has a set of six pack carbs and if you set idle @ say 850 then rev it at the carb a few times then let go it will not go back to 850 usually 1000 to 1100 and the spring is bringing the linkage back to the stop.sometimes comes back to the 900's or so.

Is it possible that the throttle blades are hanging up just a very slight bit, not enough to be visually seen at the throttle stop position but enough to make the change in speed you are seeing? I'd measure the EXACT distance between a point on the throttle linkage and a fixed point on the carb body to confirm/elim. Your positive nothing is hanging up the throttle (choke/fast idle etc) are all off/not interfering in any way

Are you running a MP electronic ignition??

I will check all of those out soon thanks,I thought when I mounted carbs on intake I looked in there to make shure not hanging up at gasket.

Thanks

yes it is

It might be hanging up on the bore. Holler when you find something and welcome to Moparts

ok thanks I will hook up lite and make note of what happens and when and will be back to you,also just so you know I did have intake off and distributor out when we freshened up motor but not shure if it was doing this before or not car was sitting a long time, but this does not sem like a vacum leak issue to you ?

Vacuum leak could cause it but before getting to involved in looking for that throw a timing light at it....

Is the vacuum advance connected??
If yes, you may want to try disconnecting/plugging and see what happens...

Just my $0.02..

Front or rear carb opening slightly, throttle cable too tight, vac leak, distributor advance hanging up.......

The first thing I'd do is pull the throttle cable off and see if it is better. If that doesn't change it, I'd pull the linkage on the front and rear carbs (vacuum off, too - that is if I remember correctly how the vacuum secondaries work).

yes at one point I think I did that disconnected hose and plugged it but the rpm at idle did not change,also was told at one point to know if I have a vacum leak was to disconnect pcv line and see if I feel suction and if there is I have a vacum leak , i did it and did not feel any suction. when I hooked vacum gauge up had about 13 lbs @ idle but was bouncing around a bit say 12 3/4 to about 13 1/2 I do not know if that is normal.needle was not steady.

I will check that to thanks alot.

Every sixpack I have tuned leaves my house running better than when it arrived. I would try adjusting the front & rear carbs. There are adjustment screws in the front of the base plates. May have to pull out the lead plugs if original. I made a long thin screw driver type tool out of a piece of coat hanger & flatened the end like a screwdriver so I don't have to remove any of the three carbs to tune. This was the cure for mine.

Here is what I found after doing some checks. First I did disconnect throttle cable, when I set motor at about 780 rpm ( to low ) and hit the throttle it will come back to about that (bounces around about 20 revs either way. Then if I set rpm to 850 where I want it then hit throttle she will not come back to that stays about 1150 - 1250 rpm then I have to loosen idle screw to get it back down or when I unplug the vac line @ the distributor and plug with my finger then the car idles down. so in short with the vacum line plugged I can set idle @ 850 then rev it and it will return but still searches around give or take 20 rpm or so. some notes on what i am working with=
craftsman top of the line digital timing lite with the tach option.

at 800 rpm I have timing at about 10.5 car seems to start easy and run strong here. checking the vacum I am plugging in at rear of intake where brake boost hose goes.

at 800 rpm I have between 7-8 on the vacum,( bounces between the two ) do not know if this is sufficient or if the bouncing is normal ?
also noticed when i advance throttle slowly vacum will dip low to about 2 or so then takes off, went to 2200rpm and vacum went to 21 + also not shure if this is normal. any thoughts/ advice would be much appriciated,

Hope you guys all have a happy new year

Is your vacuum advance hooked up to manifold vacuum? It needs to be hooked up to a ported vacuum source.