Stock 68 Quadrajet... In drive my idle is normal say 750 rpm. In neutral or park it is jumping up to 1500 rpm sometimes. I disconnected the vacuum advance after it jumped last night and it dropped back to normal idle. I checked the distributor port on the carb at idle and it's shows 10-12 degrees vacuum??? Why is there vacuum at idle? It used to work properly...no vacuum at the carb port at idle. Any ideas what to look for? Thanks. Andy
Did you make any changes recently? The ported Vacumm should only show vac. when the primary throttle blades are moved past the ported vacumm slots. (Just alittle above "normal" primary throttle blade position) If your Primary throttle blades are opened excessivly at idle you could get some vacumm at idle. If you have a large cam and no idle bypass or holes drilled in primary throttle blades to allow additional air in, you may of just turned in the idle screw to get the car to idle. But this method can and will cause other issues.
If you have the proper Q-jet with the timing set up correctly, the RPM drop from Park to Drive will be 200 RPM or so.
The car was in the shop and they did a compression test. I think they messed with the idle adjustment too. I backed it off, reset it, and checked the timing, and all is well. I think the idle set was so far in that it may have opened the primaries just enough to tip the vacuum can. I am running on ported vacuum with a more aggressive cam. Maybe it's time to tune with manifold vacuum. Thanks for the input. Took me a few days before I could get back into it.
Anytime the engine receives a different cam from the original, the combination will require timing and idle changes. I suggest either advancing the timing, or allowing manifold vacuum to the cannister at idle. Why? A different cam changes the compression pressures the engine sees at idle, either due to cylinder filling, or effective compression, or due to changes in overlap, which directly affect mixture density. You are likely to require a richer mixture as well, due to the same dilution from the exhaust. All of this depends on the cam chosen, but always expect to make changes to the idle adjustments, and possibly off-idle fueling.
