GatorDog72 Posted September 10, 2021 Share Posted September 10, 2021 I just installed a brand new Holley 750 double pumper on my father's car. It runs pretty well right out of the box. It's installed on an F-word 351c with a 4 speed. I know.... it's not a Chevy, but were gonna pretend it is 😆. When I get the idle circuit dialed in where its happy, it will sneeze and backfire out of the carb when you give the throttle a good whack. (too lean) It goes away completely if I intentionally dial the idle circuit too rich, but then its blowing black clouds and smells like gas. I'm thinking my pump nozzles are way too small, it has a 28 in the front and a 31 in the rear. I'm trying not to guess in the dark, the whole nozzle tuning kit from Holley is almost $200. I'm thinking step up to a 31 nozzle in the front, 35 in the back. Any thoughts? Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted September 10, 2021 Share Posted September 10, 2021 2bbl or 4bbl heads? Since you don't have bigger pump nozzles, temporarily swap them. The bigger 31 in front will show you if you are heading in the right direction. Obviously with the 28 in the rear, if driving, when you fully open it up it may still do what you are describing. You can also try a different pump cam adjustment making the shot longer. Right now you are trying to see if it helps the initial hit. 2 Quote Link to comment Share on other sites More sharing options...
GatorDog72 Posted September 17, 2021 Author Share Posted September 17, 2021 On 9/10/2021 at 12:31 AM, MC1of80 said: 2bbl or 4bbl heads? Since you don't have bigger pump nozzles, temporarily swap them. The bigger 31 in front will show you if you are heading in the right direction. Obviously with the 28 in the rear, if driving, when you fully open it up it may still do what you are describing. You can also try a different pump cam adjustment making the shot longer. Right now you are trying to see if it helps the initial hit. It's got the 4v closed chamber heads. 11:1 compression. I gave this a try and put a 31 in the front. I used an extra one from the previous carb. It did exactly what I expected it to do, so I'm on the right track. I was able to lean out the idle circuit and it cleaned up the initial hit, but opening the secondaries still causes a lean bog. That's with 31's front and rear. I'm gonna put the 35 in the rear and see what it does. 1 Quote Link to comment Share on other sites More sharing options...
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