holley question

Hey all, new to the forums and I hope you can help me.
I have a 350 that I will be replacing in the next year or so and I just want to keep the current one running until then. My problem is the carb, it is a Holley spreadbore, mechanical secondaries, 650 cfm. It runs, but the idle will fluctuate by 500 rpm's or more. For instance, I can run it for 15 minutes with the idle set at 1000 and then shut it off for 10 minutes, restart and the idle is at 1500-1700 rpm. If I set it back down, it will eventually not run because it tries to idle at 300-500 rpm. It's not a choke problem, because it is disconnected. It will also slowly creep up or down as I am driving. Any help is much appreciated!
Scott

I guess I should mention it's a '69 Corvette.

Check the flow from your feul pump. If it surges at highway speeds, it could certainly be the problem.

No, it runs fine with another carb I borrowed. It only seems to affect the idle, driving around town or on the highway is no problem, it is only when idleing that it is noticeable. Thanks for the suggestion tho!
Scott

Likely the throttle shafts are worn out. The rods that the throttle blades pivot on wear and get loose in their bores, allowing air to leak into the intake manifold, when the vaccuum is at its highest, such as at idle. This extra air increases the idle speed and leans out the mixture, causing surges etc.
Try moving the throttle shafts. If there is any detectable movement they are worn. Also try spraying WD 40 around where the shafts are, on the outside of the carb. If the idles speed goes down, thats where your air leak is.
A good carb shop can put in new bushings for about $100-150. Higher mileage Holleys are notorious for this.
Less likely is the carb base gasket is leaking. WD 40 can help you find this out too.