Don Terrill’s Speed-Talk

I recently put a 400 sbc in my 71 C20. It's a stock freshened short block with Vortec heads and a small Crane cam, H-272-2. The compression is 8.8:1. It was running really good, too good for belief, and after installing an AFR gauge it showed a ratio in the 12s. I bottomed out the APT in the Quadrajet, and re-adjusted the idle mixture and managed to get the AFR in the 14-14.5 range, and it will tow the boat and race car at that same value. I can step up a size on my primary needle for better economy, but I am unsure how lean I should be for towing. These days the truck spends most of it's time with a trailer behind it. Thanks for looking, Rick.

Looks to me like you have it set pretty well for a steady state cruise. What does the A/F ratio do when you are pulling a grade or accelerating?

I'd one size bigger maybe 2 to get it into the 15 - 15.5 range. If it doesn't work out you can use APT to lift em a bit. At what vacuum does it go to rich?

It never really goes rich, maybe 12.5, until you get to WOT. Slowly rolling the secondary's open doesn't change the number much. Occasionally at 1500 cruise it will get richer, close to 13. My car does this also. Maybe a QJ thing. Both carbs are late 70's. The vacuum towing is phenomenal. On level ground at 60 mph it is 15" or more. Un-loaded vacuum is 18-20. I have given consideration to larger needles to allow me some more adjustment. I'm just not sure how lean is safe under load. The car on the trailer weighs 5000, and the boat close to 4000. I don't do many hills, and towing the car is 75 percent highway.

As long as it fattens up to around 13 under heavy load I'd say it's fine.

One of the cool things about the QJ is that the primary needle tips are all the same size. I can install a larger needle to lean out the part throttle and not lose fuel at WOT. Unfortunately, I didn't record the rod number when the carb was apart last. I don't really know if I have a pair a couple sizes larger.

Late ones have .036 tips early have the .026 tips. They are different length too.

I guess I should have said within each length, they have the same tip. I'm sure there exceptions to the rule. Every application had it's own recipe.

Best cruise AFR is generally ~ 16:1; typical US V-8s start to develop a lean surge (especially noticeable with a manual) around that point. I crank the APT down until I can just feet a surge, then richen it up a 1/2 or so. Going for the last ratio lean can result in a 'stretchy' throttle response, requiring a stiffer power enrichment spring to bring it in sooner.

I had to make a quick trip down I-5 to my parents with the truck last week. Pretty much all freeway. Speeds were pretty much 60-70+ mph, fairly aggressive with the gas pedal when ever it seemed like the thing to do. I got 12 mpg, pretty good for a brick with big towing mirrors. Yesterday in search of more tune-ability and mpg, I replaced the 46 primary rods with 52s, the rods that were stock in the original application. The afr was really odd, and a good indication the air horn gasket was a leaker. When I was replacing the gasket I removed a jet, #77, to be sure that's what they were. It dropped off the end of my screw holder at the front of the radiator never to be seen again. Nothing even close in the big QJ pile. 3 hours till time to go to the race track. I found a carb I had run once before, one of the 800 cfm models, gave it a pump and a new air horn gasket, and back running again. Without touching a thing, afr came out in the mid 15s. Now it runs better than before, and towed just fine. We all need a happy accident once in a while.

It's hanging out down there next to your 5/16 and 10 mm