John,
This diagram helped me when I ran into this situation on the Lister COOX motor replacement project. Formerly we had a Holley HP DP on the prior 377 motor with a moderate cam, it worked okay but had a mild stumble at about 1500rpms. With the new 400ci dart motor and slightly larger cam we knew the former Holley HP DP 750 was not going to cut it, and did not want to risk running the new motor too lean. The thought at that time was a QFT 750 of the same size was "much more adjustable" so we tried it. Ugh. The lean stumble was there again on the 2nd 400ci Dart motor even with a stock Quickfuel 750Q carb stumbling and bumbling at 2100-2500rpms. Originally, Quickfuel said we could make a 750 work (being conservative), but it would take some jetting and metering plate jet changes to go with larger the idle feed restricters. In the end, we got it to work. However, reflecting back, they oringally recommended an 850 and I could have gone with that. We eventually got the 750Q to work well. However, if I had it to do again, I should have taken their first recommendation which was the 850. This was with a 400ci 10.25 compression motor with .248 cam at .050 and free flowing sidepipes just like the Cobras. Originally I was afraid the QFT 850 would be too much but later learned the whole carb looks similar to Holley but it is a different design in terms of sizing, metering, and adjustability. The only reason I'm typing this is we spent a lot of time on this same situation with two motors. Having the right carb and setup day-1 would have really helped avoid all of this.
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If it helps anyone who goes through this in the future, in this diagram you can see where the idle circuit and transition circuit are shared. Even on the new 750Q we had to go with larger and larger idle feed restrictuers to richen the "transition" part of the shared circuit below. On the QFT carbs these are simple replacement jets. On the Holley HP DP 750 we had, you have to drill them out so we did not go there. My next carb will be a fully adjustable QFT on my Cobra.
