Rough Idle

[Anonymous]

I have a 65 200 Mustang automatic with a Holley/Weber 2bbl and DSII distributor. The carb is great except for a very rough idle, no misfires just teeth jarring rough! The idle speed in drive is about 900. As soon as I touch the throttle the idle smooths out and the car runs fine. The idle improves slightly when the car get warm but it is never completely normal. The engine is freshly rebuilt and timing is good. Any ideas?

 

[Anonymous]

What do you mean by the timing is "good"? Are you talking about the initial timing being set to factory specs? You need to see what the timing is with the vacuum hose connected and also with it disconnected and plugged. I am guessing that the vacuum advance or total timing may not be sufficient at idle. You want something like 20 to 25 degrees of total advance at idle. Your idle speed sounds a bit high for a car in drive. Often, people compensate for insufficient ignition advance at idle by setting the throttle plates too far open at idle. The carb is then transitioning from the idle circuit and the idle is unstable. When you speed the engine up, you are getting past the transition phase and the centrifugal advance is kicking in additional timing to help smooth out the idle.
Doug