Timing question

[cam.man67]

Ok, so I've got the new motor in, and the timing was supposedly set, but it wasn't fine-tuned. At idle, off-throttle, and low rpm driving, the engine stumbles and bucks some, very much like an out of time engine would. But when throttle is on, it drives just fine. My question is twofold:

1. I'm thinking this means the timing needs to be advanced, which I've already done to the tune of about 5 degrees. The problem is still persisting, but when I advance it any further, my idle rpm drops. Is this normal?

2. Regarding the two vacuum ports on the carb (Edelbrock 1405), is there any reason that I shouldn't go from the alternating vacuum port to the constant one to run the vacuum advance?

[68gmsee]

Re. your first question. Yes, it can start to slow down on you when advanced too much. At least it does it on mine. Also check your total advance..

And the full vacuum port would be fine. I've tried it both ways on my trucks and always end up with full vacuum advance.

Now as for the idling rough. Check your carb idle screw setting, all the vacuum lines for any leaks and also gaskets and any other vacuum area like the brake booster. Maybe start by disconnectng and plugging all the vacuum lines one at a time to eliminate them.

Also, check carefully for flooding (dark gray to black smoke). If all of that
checks out, you may be looking at internal engine problems like valves or cam.