Idle tuning problem

My idle started acting weird recently and I can't figure it out. When I rev it up and drop it to idle, it idles at 1400 for a minute or so and then will drop to 900. I can't get that 500 rpm jump to clear out. I've plugged the vacuum lines, sprayed for vacuum leaks, replaced the throttle plate, removed the power valves, balanced the idle screws, reset the floats and ensured the timing IS NOT increasing at the higher idle. Also replace the plugs and wires and the compression and valve lash are perfect. I'm running a LARGE solid cam and have very low vacuum at idle but it was idling fine before now. Any ideas?

Two things if the timing is going back to the base
timing... make sure the blades are closing to the right
point and if so blow out the mixture screw holes..
but normally the timing is slow returning to base..
clean and lube the dist weights

The blades are returning to the stops. I also put on a new throttle plate with no change. I also force them back to the stops and it doesn't change the idle any. I put heavy springs on the distributor and checked with a timing light so the timing doesn't' start to increase until past 2k rpm and I've watched the timing while it is at high idle and it is not moving off of base idle setting. The idle screws seem to work but I will try blowing them out.

what carb? what power valve?

It's a 4781 850 Holley. I was running a 2.5 power valve but it still does the same thing after I removed it and put in a block-off plug.

Vacuum advance hooked up?

No vacuum advance.

What dist because I know the msd with the light spring after awhile the springs loose there tension.

I've ruled out timing because I am watching it with a timing light while this is happening and the timing is not moving off of base. When I rev it the timing will come up but drops back down to base right away even though the idle will not drop back down to 900 for a minute or so.

Also, high float levels will cause this

What are the cam and motor specs?


I'm wondering if it needs more initial advance to carry the idle. twist dizzy up 5 to 10 degree to test idle at that spot (not drive) just to see what gives a better idle adjustability.

I use mopar springs in my MSD to give quicker advance and thus more idle advance.

it needs a 6.5 power valve in front.

http://www.allcarbs.com/tech.php?art=21
6.5 pv front and rear

a holley carb must run rich, put those 6.5 pv's back in there and square jet it somewhere in the 80 range of jets.

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Also, high float levels will cause this

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That may be it. I have them adjusted way down but it seems like they seep out of the holes more than they should. I wrote it off as from engine vibration but now I think maybe the needle and seat may be leaking a little.