I have some ideas. I hope they help. I have the same 2bbl/318 set-up on a '74 and I am having some idle/surging issues that I have been solving. I suspect you may have more than one problem.
First, I would do what has been suggested above: check for vacuum leaks.
- All hoses.
- If you have power brakes, do a bleed down test: shut off car and wait 15 minutes then actuate the brakes. You should still have power assist. If not, then the booster leaks.
- Check the few bolts that you can get to on the intake manifold to see if they are tight. If not tight, you need to tighten all of them.
- I usually check carb mounting nuts and carb cover for tightness. Since you have changed carbs, these may be tight.
- You PCV system should not leak and the valve should operate freely when shaken.
- You say no EGR. But if you mean that the vacuum hose has been disconnected, you can still have a problem. The valve may not be holding tight. Check by removing the valve and trying to blow (yuck!) through it. If air passes, that leak will cause idle problems.
Next, you may have an ignition problem: too much advance. Reason? You have backfire.
- The car may idle fine with no load. But when you put the car in drive, the rpm drops. This small drop may put you over the "timing edge" so that the too early ignition actually trys to run the engine backward.
- You have too much base advance. I would go back to 0 degrees until after I had diagnosed the problems.
- My car's vacuum advance runs off the port that you described so I believe that you may be connected to the correct port. Here's how to check:
- At idle, you should not have any "detectable" vacuum at the distributor vacuum advance or at the carb port. Remove the hose from the carb port when you are idling. If idle quality changes and you can feel more than a tiny amount of vacuum with your finger, you have a problem. (After a timing adjustment, when I reconnect my dist. vacuum hose, the timing does not change.)
- If you have this problem, reconnect the hose at the carb. Then remove and temporarily plug the hose at the vacuum advance and see if the problem goes away when you go into drive. If it does, you found your problem.
Once you are sure your ignition timing is correct, I recommend going on to the carb.
You may have a carburetor adjustment problem. Because you have such early timing, your throttle plates are probably too far closed with the car adjusted to idle specs. With the very high manifold vacuum, your idle mixture adjustment be at a none ideal place. Do a "best lean idle" adjustment.
Good luck. I have a leaking brake booster that I will be replacing this weekend. Then I am going to solve the "lean and retard" emissions control mentality: Overhaul the carb with the next bigger jet and lighter distributor advance springs.
http://www.forabodiesonly.com/mopar/showthread.php?t=162134
http://www.forabodiesonly.com/mopar/showthread.php?t=161055
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