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- - Quadrajet tuning success (https://67-72chevytrucks.com/vboard/showthread.php?t=397124)

Quadrajet tuning success

I just wanted to give everyone some information about my attempts to get a quadrajet running on my 72 Chevy truck with 350 and 350 auto. First you should know that Cliff Ruggles has been helping me solve problems and has been very willing to email me with answers to all my questions. I have both his and Roe’s book but find his more helpful.

First thing I learned was to check the carb you are using for the original jets and rods. You can find this on several web sites if you have the carb number. The next and most overlooked is the size of the air bleeds and how many air bleeds the carb has. Large air bleed carbs require larger jets. I started this project with what I thought would be an easy bolt on and go. I found a Jet Performance Stage II carb in almost new condition. The guy had it on a Dodge pickup with a 318 and took it off to sell the truck. I don’t believe it ever ran well on the truck and that is why he took it off. The carb number showed it came off a 77 Corvette and should have 77 jets and 52 rods and had large air bleeds. It actually came from Jet with 73 jets and 43 rods (a pretty standard combination). I put it on the truck and stomped on it and it fell flat on its face. I tried a number of things but could not get it running right (not too much help from the Jet tech line). I finally switched to another carb I had but one I could not find the specs for. I ended up putting in 71 jets and 43 rods. By this time I got my Innovate LC-1 installed. After running it and checking the mixture on the computer I found it was running very rich. I changed to 64 jets with 43 rods and got it to lean out but now it was too lean so I went to 66 jets and have it running at about 13.5 at cruise, a little rich at idle (12.8) and 12.8 or so at WOT. It is leaning out a little when I get on it at cruise so I am going up to 68 jets. I have a carb with an APT that helps fine tune the mixture. If you are looking for a good tunable qjet get one from after 1975 but before they went electronic. These will have the APT for fine tuning the A/F.

The long and short of all this is that the truck has never run better. It had an Edlebrock 1406 on it when I got it and it ran fine. I just wasn’t satisfied with the power and the fuel economy. I can now spin the big tires from a stop and am getting 16+ mpg in a ¾ ton truck with highway gears and regular gas. Best mileage with the Edelbrock was 14 unless I was running premium gas and cruising on flat ground. I think I can still do a little better than 16 with the final jet change. My goal was 18. I can now tap the gas a little and it launches. Some have said a well tuned qjet is better than EFI and I can now say that I believe it. And there is no better sound than when the secondarys kick in (there goes the gas mileage).