I was curious what you guys have your Q-Jet tuned. Let's see your basic car combination along with your Q-Jet set-up. Here's mine: CAR - 3950 lbs (`72 Skylark 455 with TH-350) - SP-1 single plane intake - 234/244 cam duration @ .050" with .500" lift both int/exh, 106 degrees intake centerline - Ported small valve heads 260/175 @ .5" and 28" - 10.0:1 compression, zero deck, 26cc piston, 70cc chambers - 3250 stall converter, 3.42:1 posi, 26.5" diameter rear tires - M/T Scavenger headers, 3" Dr. Gas X-Pipe exhaust, Large case mufflers - HEI distributor w/MSD coil, Total timing 34, in by 1500, initial timing ~25 degrees. Current Q-Jet tuning: Jets - 76 Primary Rods - 46B Primary Jets - .076" Secondary Rods - had .040" but have since changed to .031" - Vacuum gauge not used yet, but both idle mixture screws are 2 turns out (this was best setting with previous 350-4) General thought, I'm sure someone told me this at some time or another: - Rods sit in jets, typical set-up is .030" more than rod (.076 - .046 = .030) I'd like suggestions on what to try at the strip or dyno to get things fine tuned. As it stands it runs very good, but wish idle was a little better when in drive. I feel confident with the current set-up (w/std 255/60-15 radials) the car should run 12.6's. First track visit within two weeks :TU: (if all continues to go well). I'm fairly close to sea-level (Fayetteville, NC track). Share those secrets
Wow Craig - I forgot you had a car to go with that engine!!! I still need to sort out my Q-Jet, it takes time. I'm running 74 jets with 44 primary rods. Power piston spring is cut off, I have an assortment of new ones to try when I get around to it. I'm pulling 15" of vacuum at 800 rpm so I'm thinking the spring is probably too weak to give me much enrichment. I have three or four sets of secondary rods, I use some thick ones for cruising and swap out the thinner ones at the track, watching mph to see which ones are working the best on any given day. There's a big difference between the street cruisers and the "race" ones, sometimes as much as 4-5 mph. I have less cam, convertor, and gears as well as the stock dual plane intake. My best run this year was a 13.31 on street radials, so your 12.60 is certainly reasonable. Not surprised that you have a rough idle with that cam and the close lobe centerline. Healthy (20 degrees) of initial advance and careful tuning of the idle mixture will give you the best idle. mine will idle down to 600 or so with a 230-245 cam on a 116 lobe centerline. I have 1/16" holes in my primary blades to help get them closed at idle. Make sure you are idling without getting into the transition slots - the timing really helps this. You really need to try a vacuum gage for tuning the idle screws - nothing like it. Set your idle as low as you can and still keep the car running, then tune the screws for highest vacuum, then slow the idle again, keep going until you get the best idle that you can at the lowest speed, then set it back up to your desired idle speed. Good luck at the track.
Craig, After a fair bit of testing with a mild combo (edelbrock, stock heads, ported & hogged exhaust manifolds) I ran 75 jets and 47 rods. The smaller size and smaller metering area are due to the Calgary Altitude and a desire to stay on the rich side. After a cam swapout the idle was rough, and there was an evil bog as the secondaries opened. The suggestions in Doug Roe's book, were right on! Drilling out the idle metering screws, idle metering passage, and the secondary well tubes took about 2 hours. Also swapped to a lower power piston spring. All this fixed the idle problem, and gave a better transition to secondaries. I also put on a new secondary pulloff and got the damping rate adjusted to just under a second. When tuned the Q-jet was good at the track, but had a great idle and part throttle response. Very pleased with it.
Thanks guys! Anyone running a combination close to mine with a single plane? When I dyno'ed mine back in Oct. the operator thought I should richen the primaries some. Is this done be increasing the get and rod on the primary side. I assume you need to take the top of the carb. off. I do have a Q-Jet book but it is slim with performance applications tuning. Anyone ever heard of offsetting the carb. to the rear (slightly) to increase fuel distribution to the front cylinders? The angle of flow could help more of the fuel to get to the front runners.
Craig, Doug Roe's book on the Quadrajet is ideal. It discusses intake selection, carb sizing, performance modifications, idle modifications, and the variety of performance options for a Q. It also has a modification for increasing distribution, by increasing the final angle of the throttle plates. Changing the primaries isn't too difficult, you do need to take the top off the carb. It's not a difficult procedure, just remove the pump lever arm first (knock the pin out using a drillbit) and then remove the choke, choke lever, and the top screws. Takes about 30 minutes when you're practiced.
to "richen the primaries" there are multiple choices....up the jet size to richen for all conditions. go smaller on the primary metering rods to richen at part throttle. the Spring below the power piston will alter at what vacuum the part throttle richening comes in. Good Luck Fred
ok heres mine, 70' block,sm valve(untouched heads) block uncut,#413 cam straight up-no degree,stock timing set,performer with 73' 800 q-jet with 48 rods/73 jets. autolite #25 plugs gapped at .035, AX secondary rods, not sure of hanger, st 1 pump with elect. carter in rear(run all the time) timing is 17-18 initial with a total(in at 2500) of 32-33deg with vac advance hooked up. 1 7/8" headers(T/A) with 2 1/2" flowmaster exhaust with summit fully welded muffs to bumper. TH400 with a 11" JW 2600 stall/3.55 rear car will pull over 5500 rpm's but is shifted at 5200 anything else just let me know...........
