Bit of a struggle today. My engine has been running really rich - literally makes-my-eyes-water rich - reminds me of watching top fuel! It also runs like s**t over 1500rpm, popping and banging. So today was spent outside trying to figure this all out. I'd tried the mixture screws before.

It's a SBC with Holley 4150 650cfm double pumper. Davis Unified Ignition (DUI) distributor+coil. And I'll preface all this with the fact that although I've run this combo a few times, I'm far from an expert with the inner workings of a 4150. Carbs are complicated

I set the float levels to the bottom of the sight holes. While I was checking it all out I found a broken vacuum plug, so hoped that was the root cause.

I put a vacuum gauge on the manifold, and got 18 inches, and then tuned the mixture (to now about 3/4 out) to get 20 inches. I figure that means there are no more vacuum leaks.(valid?) The exhaust is instantly better, now invisible where it was colored when super-rich. When revving it gets grey/black again, like it's still rich at revs. I set the secondary mixture to the same turns as primary (not sure how to set secondary mixture).

I put a timing light on, set to zero so I can read the numbers painted on the balancer (0-40 is marked). I'm at 14-16 at idle, around 35 total. But to get it to 35 is painful. The motor is revving but backfiring regularly, popping and banging like it's at a rev limiter (which I don't have btw).

But the timing numbers seem close, so I'm moving on. The leads are new, and everything is connected ok - correct firing order on the distributor, and going to the correct plugs.

I check the spark plugs (only a couple miles go-karting on them) and they are black of course (see richness above) but not oily or wet. First thought, can plugs get damaged by running very rich? They are Accel 576S, as recommended by.. well I have no idea now, but they seem to be suitable for this DUI HEI. Gaps are all 55, as DUI recommend.

Last thing I check, I get temperatures from each exhaust - ~650º on all 4 outers (1,2,7,8) and ~750º on all 4 inners (3,4,5,6), so I took that as proof there is nothing terribly bad inside motor; e.g. stuck valves, etc. (?)

Note that there is no "horrible" noise when revving, like a bent pushrod, etc. No tapping, no pinging, no metallic anything. To my novice ears it sounds rough and backfiring - so much so that it's hard to drive. But still smooth at idle.

I have a call out to a couple of SBC folks who have been doing this since the sixties, so we'll see what they suggest.

My gut feeling at this point is that it's the distributor, but it's entirely possible I've done something dumb...