stock 750 quadrajet...no rods or jets have been changed. bottom end is a stock SP 350. compression is probably down around 9.5:1 due to a head change and the use of composite head gaskets. headers. comp cams camshaft, i believe it's 218/218* on the duration and .469/.469 on the exhaust. stock valves and springs, cast iron intake, and a 1" phenolic spacer. pertronix ignition. at WOT it'll due it'll stumble and stutter like it's gasping for fuel, and even once it catches, the power isn't there. by looking at the base ring, the plugs read slightly lean...a little too hot as well. anyone know the primary jet size on my carb? i can't take it off/apart to look atm, so it would be help if anyone knows what i have, so i can go ahead and order some new parts. i'm guessing a 68 or 69 jet stock, but i know it changed every year. what would you guys look to move to? maybe a 73? with i would guess slightly bigger rods to back it? (43-44?) do i need to bother with the secondary side at all? i know i should for max tune, but isn't it the nature of a vaccuum secondary carb to be able to adapt fairlu well to most applications?
guess no one has any info. should've expected that, since i'm running a qjet, which a rare an unused thing... :laugh:
David I'm sorry if I missed it but what year carb is it and do you have the carb number? Your jetting will really depend on where you are in altitude, your total set up including rear gear ratio. Most of the 70 to 72 Q-jets came with 68 and 69 primary jets. If that is what you have and you jump to a 73 you will probably be dissapointed. Give us the carb number and we can go from there.
7040244 is the carb number, it's original to the '70 350 in my car. 200r4 tranny and 3.42 posi rear. nashville is probably barely above sea level...the plateau is to the east, and we're in a basin. no idea of the actual number, though.
stock 750 quadrajet David, According to the TN map (US Geological Survey) in my office, Nashville is between 400-600 ft above sea level. Also, if I remember correctly your carb should have .069 jets and 45 rods. Hope this helps.
Re: stock 750 quadrajet Good memory Butch! Got my 70 Chassis manual, 7040244 has .069 jets with .045 rods, and BS secondary rods. David, the BS rods are lean(.0947 power tip) You can richen up the secondary side of the carb very easily by going with some thinner rods like BB's or BG's. That should make quite a difference.
thanks for the numbers, guy. at WOT, isn't fuel mixture primarily dependant upon the primary jets, though? maybe step it up to some 71 or 72 jets, keep the primary rods, and get some richer secondary rods? how about the hangar?
David, I just had a 750 carb jetted for my friends 69 GS 400. The motor has a mild performance cam and headers with the stock cast iron intake. We used 73 jets and 44B rods with some AU secondary rods. I think we really hit the nail on the head with the jetting. The car does everything right, idles smooth, pulls nice, and starts with a twist of the key from outside the car. It also gets phenomenal gas mileage( although the 2.93 rear has something to do with that) I think your motor might like that jetting. I would certainly look around for some richer secondary rods, but you are right, a given percentage change in secondary metering has 1/2 as much effect on the overall A/F ratio as the same percentage change in primary metering.
is there chart or list with the rod diameters on the secondaries? i say all these letters tossed around, but most are hard to find becuase most places only carry 5 or so secondary rods at most. that carb setup sounds good larry, i bet it would work pretty good on my 350...any idea of the diameter of the AU rods? and someone mentioned earlier that rear end gearing matters...with the steeper gears in my car, how will it affect jetting?
got it http://www.hioutput.com/tech/qjetrod.html au are .0527. i can get CK's from jegs for under 8 bucks. same rods. i've heard a lot of guys recommend the various .0567's as good all-around rods. i'm leanin more towards those becuase i don't like the fact that the AU/CK rods have the long power tips, which means they reach full rich when the air doors are barely over half open...o No: atm i'm looking at: .073 jets stock .045 primary rods .0567 secondary rods with a B or G hangar...i'm guessing the B will make the fuel curve a little meatier and might be a better choice.
David, here is an article which taught me a lot about tuning a Quadrajet. http://www.dapa.org/tech_articles/rfrazier-qjet.htm These are also some "must read' articles... Performance Tune Your Q-jet Secondaries Tuning Q-jet Secondaries Part 2 Q-jet Tuning For A Better Idle My Q-jet Tuning Experience
