I can't get her to ilde smooth (well as smooth as an LT4 HOT cam will allow). I've got a 383 with the computer disconnected. Edelbrock 750 Performer carb, non cc HEI distributor and an MSD. I have replaced my plugs and wires and checked my cap and rotor. I hooked up an Air/Fuel ratio guage to try and tune the idle mix on the carb. My old plugs were fouled (I think she was running rich and my A/F guage showed rich) so I tried my best to get the A/F closer to ideal and got pretty close, but it still idles rough. It doesn't seem to consistently stumble in any pattern (to indicate one cylinder missfiring) at ilde either. My vacuum guage needle vibrates between 13 and 14". Idle speed is about 800 rpm and base timing is set to 8*. At higher rpms it smooths out like about 1000 or so. One thing I did notice is when it stumbles at idle my A/F guage would read REALLY lean at that instant. I've also checked for vacuum leaks with a can of carb cleaner. Any ideas? My best guess from that A/F guage is it's fuel related. The carb hasn't seen much use in a year and a half. Could old gas cause this? I'm stumped.

Any ideas would be greatly appreciated, thanks,

 

Mechanical fuel pump? Put a pressure reg on it.
Non CC HEI you say? Vaccuum advance eh, you sure you have the vac hooked to the carb in the right spot, that is a common mistake, the vac advance should not have full vac all the time.

 

I have an inline fuel pressure guage and it reads 4 PSI at idle. Is that too low? My vacuum advance is hooked to the timed vacuum port on my carb. Any other ideas?

Thanks

 

Well, one of my Haynes books (f-body) says these symptoms are either:

Vacuum leak
Leaking egr (which you probably don't have, skip)
Cogged air filter (I'm sure you've ran without during this trial, so skip)
Fuel pump not delivering enough fuel Carb out of adjustment (double Hmmm)
or Leaking head gasket. (Yikes! Let's hope not)

Now I remember back in my Holley days, when my engine idled, my fuel pressure gague read between 6 and 7 at idle (I also had a reg on it set between 6 and 7, with out it, it bounced allover). I can't remember if low pressure idle is normal or bad, been into fuel injection too long I guess.

 

Too tight of a Valve lash also has been a culprit for rough idle. Just a thought.

 

Should run 5-7lbs.Max at 9

 

Thanks for the replies. Hmm...well I'll look into a new fuel pump, sucker has almost no miles it. Any other possibilities?

 

In addition to the aforementioned possibilities:

Lean idle mixture (various causes);

Narrow plug gap;

Secondary ignition problem (coil, wires, cap, rotor);

PCV connected to an individual runner instead of the carb base;

Sticky centrifugal advance mechanism;

Timing chain slack;

Sticking valve(s).