Trying to solve rough idle

[Alabama65stang]

:
Still trying to solve a rough idle. 800 miles on fresh rebuild, Comp 260 cam, Holley Weber carb, DuraSpark II with Blaster coil, C-4 tranny. Idle is erratic, vacuum moving between 13-14". I have sprayed everything with WD-40 , no indications of a vacuum leak. It is not a dead miss and pulling the plug wires one at a time does not indicate a single cylinder causing the problem.

I did notice today with the breather off at idle, gasoline drips out into the venturi onto the top of the butterfly. I backed off the idle throttle screw and the gasoline flow seemed to have to stopped but still had the rough idle. Can't leave it there because it gets real rough in gear. Idle mixture screw is about 3 turns out, a little richer than lean best idle.

Spark is about 25^ initial advance with the vacuum can hooked up to the manifold.

Everything runs really good above about 1000 RPM.

I still have the stock 1 3/4" exhaust system and muffler, could this, in combination with the cam be a suspect? Back pressure does not seem to be a problem at higher speeds and it runs well up to 80 MPH.

Anybody got any ideas?

 

[CobraSix]

25* initial advance? Way too high. I would take it down to 10-18*.

Slade

 

[Anonymous]

Alabama;
If I understand you correctly, you mention 25 degrees advance with the vacuum can connected - what is the advance without the vacuum? That's more important to know. Without vacuum, and running at LESS than 600 RPM, it should be 8-10 degrees (as a starting point). By the time it reaches 1000 RPM, it will already be advancing mechanically inside the distributor, which makes it hard to determine the setting.

One thing I ran into with mine, following my rebuild, was a bad distributor cap. Three of the 6 contacts were 'clocked late' by being manufactured wrong. They were 2 distributor degrees (4 crank degrees) late on the 3 cylinders in a row that were physically on the side toward the block. It took me a year to find it! I tried EVERYTHING to get my idle to settle down, then accidentally found this. Since they make these caps several thousand in a batch, I'm sure there are LOTS of these bad ones out there.

To check yours, look inside the cap and VERY CAREFULLY measure the distance between all of the spark contacts. If it's like mine was, you'll find 3 in a group that are the same distance apart, then 2 sets that are farther apart. You can also tell by looking at the contact's location in the 'boss' where they are inserted: they're off center.

This erratic timing made my carb act lean at low idle speeds, yet it would 'spit gas', much like your Holley/Weber. Mine, however, is the Holley 1946.

A new cap solved the problem.

Hope this helps.