Hey everyone, I am thinking about going back to a Q Jet. I have had it with my 750 Double Pumper Holley, at least for now. My question is - how many mods are generally needed for a early like 1970 Q jet on a hot small block? The car is a stick, so idle vacuum is good at around 13 inches. Would it be reasonable to expect just a jet / rod / hanger change?
Unless the idle and off-idle calibration is correct for the conmbination, you could end up tearing your hair out over seemingly insurmountable metering problems. The rod, hanger and jet conmditions typically do not matter until moderately heavy throttle maneuvers are in question. It is very likely that a late '60's carb will be better for you rather than a '72 or later unit. This is due to the fact that the idle and off-idle metering was richer in those units. It might also be wise to consider using a big block carb instead of a small block unit. A lot will depend on the engine combination, and the carb chosen. Most of the in-town driving is done on the idle, off-idle metering. Ignition timing will greately affect the carb metering as well, as it will directly affect the signals getting to the carb. I am not sure of the problems you are encountering with the Holley, but these problems may be adversely affecting the Holley as well. One thing to never overlook is the condition of the ignition system. Ray
Thanks Ray, The Holley had been very good for me, but first it started leaking from the front bowl (3rd time) and after I fixed that, now it is mysteriously running rich to the point of flooding. It will only idle with the PCV vacuum wide open to lean things out. Maybe I got dirt in one of the metering block passages (?) I have an Old 1970 Q Jet for a 350" Buick auto. I put 70 Primary jets with 41B rods, with CE secondary rods and a good hanger. I had the stuff lying around. The car runs pretty well, but doesn't hit as hard as my Holley when I jump on it. The secondary plastic air valve diaphram seems to be weak. When I am thinking about mods, I am wondering about modding the power piston spring, drilling emulsion tubes, or modifying circuits. Again, the Holley 04779-2, 750 double pumper worked well for years, but now it's acting up. I would rather have a Q jet if I can get the same performance. Ignition system is good. Hot HEI with a recurve kit.
Somewhere online there's a terrific write-up of where to start tuning a Q-jet so that you don't yank all your hair out... Maybe look at Smartin's website? I think it's linked to from there.... I've gotta run, I'll try and look later on. Go Q-jet! :TU: Worth the effort.
I assume that the hitting hard is during the use of the secondaries. Two things control the application of secondary fuel. The first involves the secondary metering rod hanger, which can be replaced with one allowing more fuel, earlier. Use one of a lower letter value for more fuel. Another thing controlling the secondary fuel is more of a mixture control, and it will allow the secondary air valves to open earlier. It involves adjusting the secondary spring closing rate. The less tension it has, the quicker it will open. Ask if you want instructions. Ray
I will double check the letter on the hanger. I do remember that the hole for the rods is "high" meaning that it lifts the rods early. Also, the rods have long "power tips". The secondary butterfly spring has been adjusted to very loose, maybe too loose(?) I wonder if the secondaries are opening a little too early?
Can't you bend any secondary rod hanger to get the height you want? At least you have to check/bend them when rebuilding to make sure they're in spec. On the secondary windup spring, I think it's 1/2 turn past contact as a baseline, and then you can go looser until the engine bogs, then go back 1 notch. Better to be too tight than too loose.
quadrajet problems I have an original rebuilt quadrajet for my 68 nova ss 396.the problem i am having is once the car gets operating temp and you shut the car off ;it has trouble cranking up or willnot crank at all;unless you remove the air breather or open the back barrel to let the gas vaporize .it has the vent valve which is at the open position ;when the car is shutoff.the floats are sat at a quarter inch. no nozzle drip either once the car is shutoff .any help out there ;cannot figure this one out.chris
