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On my 1982 E350 4x4, 460, C6, I'm having a nightmare... Supposed to leave GA for MI 3 days ago and 5 days ago, started having some trouble.

Barn find/survivor with 32k original on it. Carb was leaking so I rebuilt it. I've rebuilt several carbs, mostly Holleys, but this was my first 4180. Had some issues after rebuild which included sticking needles, and I had installed wrong vacuum diaphragm on secondary vacuum pod, so got those resolved. Thing sounded great, ran well, I loaded it for my trip. This, of course, included installing the doghouse again, which is a pain. When driving home from local old-school carb guy who looked it all over and said it was ready, I had some minor, intermittent backfire. Seemed like it did this after engine was hot, at highway speeds, usually when under stress (starting up a hill), but not ALWAYS. It can do it going down the road when road is level, but I think I notice the difference when starting to climb more often. I can feel a minor hesitation, drop in power just before it starts to do this. When it does it, I can usually hit the throttle and it will go away/override it.

At first, this happened rarely-ish: every 10 or 20 miles. Now it is every 2-10 miles. Starts fine, idles fine, runs fine EXCEPT when it hits the backfire cycle.

Had some ignition problems about 4 months/1000 miles ago. Replaced ICM (?), Coil, dist, cap, rotor, plugs, wires, fuel pump, and probably a few other items. Ran great for last 1000 miles until it started flooding some and I noticed carb gaskets leaking.

After rebuilding carb, had problems with stuck needles, float levels, etc. Changed them out, reset floats and that seemed better. Did not backfire at that time during all this process, though minor after fire out exhaust after turning off at times.. At one point, when almost completely resolved carb issues, I tried switching onto other tank (we had suspected some bad/dirty gas, but after all this work I wanted to see if it was some of the other problems that had made us think that), and after a few miles it started spitting and coughing and minor backfire. Switched back to other tank, it got better, but still intermittently did the same. I thought it was just burning off minor amounts of bad fuel in the lines and carb bowls. Then started the backfiring.

I've now changed the spark plugs, which they say to do after a carb rebuild anyway. Still backfires. I've checked timing. Very close to factory 8 TDC with dizzy vacuum disconnected and plugged.

It is still an intermittent backfire, but it's a considerable backfire. It seems to be getting worse/more frequent. Air filter is clean and recent (6 months, 1200 miles or so). Fuel filters in last 1000 miles. I went ahead and got new cap, rotor, and wires, but obviously, they are new enough it SHOULDN'T be them. Feels more like coil, dist, or module, but those are also new and, again, it SHOULDN'T be them.

Anything anyone can see that I'm missing? Thanks! I don't have the money or time, tools, etc, to switch to fi right now. Just need to get this thing running well enough to get on the road. Any help is appreciated!
 
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