Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time.
Holley carbs, VS Edelbrock, that is a deep subject, I could write 20 pages on that one, LOL. You mentioned backfiring and Holley's. Anyone that tells you that doesn’t have a clue about Holley’s. They use a rubber diaphragm in the power valve, if it is ancient, and rotted, a backfire can rupture it, but these diaphragms are tough. Think of the rubber diaphragm in an old mechanical fuel pump, same basic thing. Normally when you hear horror story’s about Holley’s, it is an old Holley, that has been taken apart 5 times by people who don’t know a lot about what they are doing. Or a Holley that has been sit up, had a low quality kit installed in, or otherwise screwed with, like bad modifications.
Ebelbrock carburetors are copy’s of the Carter carburetor designed in the 60's and used on production vehicles. They were designed to be cheap smooth, and didn’t need to be highly tunable, or precise. The big down side to the design is the fact that the front and back barrels have venturis and other characteristics that are non symmetrical, it makes it almost impossible to make a super precise mixture especially wide open. They are at their worse wide open.
Holley’s were designed from the ground up for performance. Most designs have virtually identical barrels, and are almost infinitely tunable. They were not designed to be the least expensive to produce carb like the Carter design, but to be the best. Holley’s produce a more precise mixture, and even wide open still produce precise mixtures. Holley’s offer tunable vacuum secondary’s, or mechanical secondary’s with secondary accelerator pumps, the carter design is light years behind in this area. What this Holley design selection does is to allow a specific type carburetor, IE vacuum or mechanical secondary’s, and a huge range of sizes to give an engine what it needs. That and providing precise mixtures is the key to their success/power. Carter/edelbrock carbs are crude in comparison, and less expensive to produce, they are great on grocery getters, there is a small range of sizes, you basically get what they give you secondary actuation, non precise wide open mixture, and more. Carters do what they were designed to do, and they do it well, but they have severe limitations by design. If you go to a drag strip and look at winning competitive race cars they all use Holley’s, or Holley clones, because they are superior by design.
There is a lot of opinion when dealing with carbs, but if you stick with the facts you are better off. I can give you the name of 3 shops that claim quadrajet carbs are the best performance carbs when they build them, but you don’t see them winning any races. The street isn’t a race track, but the performance of a Holley still shines through. Holley’s are great on the street. Chevy chose them is the late 60's, on all of their hottest factory performance engines, for good reason. One last thing is I have taken Edelbrock carbs off that were tuned well, and bolted on Holley’s and have seen HUGE power increases. It isn’t that Edelbrock carbs are bad, they just don’t have the advantages of a carb like a Holley. OK, that is my sermon for this Sunday, let us all pray for the Edelbrock carbs. LOL, seriously If you are trying to make power Holley’s do it, are rock solid, and do it right. If you had a bad experience, buy a new one it should/will destroy any Edelbrock craving. I do love Edelbrock intakes, on their carbs they wanted to be different, and they are. I had better get out of here now.
Oh, you can pump through your old pump if you disconnect the wires. It will freewheel, and create a slight restriction. I don’t recommend it, but I have seen it done over and over. As the old pump turns it will produce electricity, the wires have to be taped. If you connect the wires it will virtually lock the pump and create an extreme restriction, keep them separate, and tape or heat shrink them. If you leave the old pump in use a strong electric pump mounted as near the fuel tank, and as low as possible. You asked about the feed line, it should be larger in diameter. Open both lines any turn on the key to a second to confirm, use a piece of hose on the line and put the end in a can, so you wont have gas everywhere. If you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.
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