[SCHOON]

I have a 650 Holly DB and it will not hold an even idle.
Sometimes when the engine is warm it idles at 6-800rpms and other times 1000-1100rpms
What gives?

 

[OAKLYSS]

things to check: power valve settings, metering block gaskets, fuel bowl screws, vacuum leaks, distributor vacuum advance and springs

 

[zdld17]

And throttle plate openings against transfer slots.

 

[Vintage Musclecar]

How old is the carburetor? Check to see if the primary throttle shaft is loose in the baseplate (i.e. if the baseplate might need bushed)

I second the notion to check the distributor advance springs as well. Sometimes light/worn springs can allow the advance to hang up a little and cause weird idle problems.

Eric

 

[Straight-line-69]

Are you running vacuum advance on your distributor? Ported or manifold vacuum source?

 

[dubs68camaro]

I know that I had a similar problem. I called Holley and they said to back off the tiny screw that is on the bottom side of the secondary throttle shaft. Back it off flush and they turn it about a 1/8 to 1/4 turn. What happens is the secondary will pull just enough vacuum to cause the rear discharge nozzles to drip just a bit, causing a rough idle. I'm not saying this is your problem, but it never hurts to eliminate it.