I have a 600 VS Holley on my 350. It was set up really well with no stumbles, flat spots or other poor behaviour but suddenly I have a crook idle. It is not bad enough to make it die but it is rougher than normal. The just off idle throttle performance is poor until you move the throttle about 10%, then it's fine.

I don't think it's the accel pump because that should not affect the idle. I'm thinking blocked idle bleed or something around there. WDYR?

There doesn't appear to be any vac leaks and there is no black smoke at idle.

 

lol..crook idle. I havn't heard someone say something was crook since I was so hung over in australia I couldn't function. Everyone kept saying I was crook...at the time I had no clue what they meant.

anyway check 3 things.
1. Make SURE there are no vacuum leaks. Remove and plug every hose coming off the carb..see if it fixes it.

2. take out the idle screws, noting thier setting first, and blow out the passages good with carb cleaner.

3. Check/change the PV...it might have blown.

One of those should fix it.

 

Does it respond to adjustment?

If not, it's probably some "foreign matter" stuck in a metering passage. A typical cause of sudden mystery perofrmance changes with a carb.