engine troubles

[optiview]

I have a semi rough idling 440 with an avenger Holley 4 barrel carb and a mild compcam 255. when I put a vacuum gauge on it at idol I get the needle bouncing quickly and roughly between 15 and 17. I have adjusted the timing as best as possible but cannot improve vacuum. Im leaning to vacuum leak. Any suggestions?

 

[1badstroker]

Spray carb spray where the intake bolts to the heads and around the base of the carb, If the idle picks up you have a vac leak in that area.

 

[Manifold]

Pulled from Diamondback Engines' vacuum gauge white paper (http://www.diamondbackengines.com/technical-white-papers/vacuum-gauges/):

6)AT IDLE, Rapidly shaking needle (again a slight shake is normal with high overlap cams), becoming worse with higher rpm: ignition miss, damaged valve spring or blown head gasket. If it smooths out with increasing speed, it is likely worn valve guides. A head gasket will usually cause a prominent (10") or so regular drop.

 

[Redpolara]

Badly seated or burnt valves is also a possibility.

 

[optiview]

Im leaning tward burnt or bent valves also. Engine was rebuilt about 400 miles ago and always had this problem from day one. It was ignored for a while while trying to get rest of car completed. Now time to fix.
We are going to run a pressure test tomorrow to see whats up.

 

[optiview]

Well I had the compression checked and all cyl looked good between 140-155. Found 2 plugs fuel fouled. ( 2 and 4 ). So i'm going to replace them and try a new carb.

 

[Meep-Meep]

Since the compression is good you might check the float level in the carb if you see fuel fouling. A weak or overly rich mixture will cause a cylinder to misfire. Sounds like you were running on 6 cylinders to me. How about firing order? Worth a double check.

 

[mopar_71]

Also check the a/f mixture screws at idle and fuel pressure.
Did you change your fuel filter with the new carb?

 

[optiview]

discovered problem and resolution. Mechanic that had previously worked on the car had removed the carb and lowered it to the manifold removing the spacer. Well he put a plenum gasket on it instead of a ported and it caused wacky things to happen when the fuel air mixed.