Rough idle? 318

[TopBanana72]

1972 Dodge Charger. 318 w/ 4bbl avs2 and performer manifold. Engine rebuilt 12k miles ago with KB167 pistons and 302 heads, mild 340ish cam. It has been running perfect until now. Now it has a very rough idle that I have not been able to pin down. It is fine at highway speeds and over 20-25 MPH, but misses like crazy below that. So far, I have basically thrown the parts cannon at it, plus some tests:

1) New plugs
2) New Plug wires
3) New cap and rotor
4) New fuel filter

Swapped coil to my spare - no difference (both coils tested good, but my spare seemed a little better)
Swapped ballast resister to my spare - no difference
Swapped ECU to my spare - no difference
Checked for vacuum leaks - none found
Checked timing - set to what it's always been (about 34 deg. total, 10 deg. initial), timing mark rock steady under the light.
Adjusted idle mixture screws to highest vacuum - still rough idle
Checked the new plugs after about 1000 miles of use (took car to Carlisle with this rough idle condition) - all plugs perfect read
Checked compression - all cylinders even at about 135 (I used a thick head gasket on the rebuild and stock springs so I know it could/should be higher pressure, but it's been running strong since rebuild.)
Checked PCV valve - it's functional

I'm in need of the B body brain trust! Thanks in advance for any suggestions.

 

[Rusty knuckles]

I'd bet on a vacuum leak. You said you checked but I'm pretty sure this is the culprit.

 

[R413]

Do you have power brakes? Disconnect booster and plug line.

 

[TopBanana72]

I'd bet on a vacuum leak. You said you checked but I'm pretty sure this is the culprit.

Sure sounds like it, I'll triple-check

 

[TopBanana72]

Do you have power brakes? Disconnect booster and plug line.

Yes I do. I have not tried that yet... next on my list.

 

[davek]

Vac leak,had that problem on my 318 and chased it thinking it was something else.

 

[TopBanana72]

Guys! You are the best. Finally found it! It was indeed a vacuum leak at the brake booster...

 

[davek]

Guys! You are the best. Finally found it! It was indeed a vacuum leak at the brake booster...

View attachment 1891853

Yep that could do it.

 

[WileE]

What is your vacuum gauge readings ? That's the first thing I'd do when you brought your car into my garage. { Edit: sorry, didn't read all the posts, only the first 2-3. But you did post in #1 that you checked for vacuum leaks. This is one of the first things I learned as a kid 55+ years ago in the garage. }

 

[TopBanana72]

I kept saying to myself this feels like a vacuum leek. And indeed it was, I just did not check that booster connection like the other guys said. I even think I know what happened. The last thing I did before the problem started was check the master cylinder. I probably knocked that plug off when I was wrestling the master cylinder top clip thing off. Vacuum at idle was 15 inches.