I need some advice on how to adjust/set the air/fuel mixture on a Dualjet carb. I rebuilt the carb. last year and it ran fine. A few months ago I replaced some vacuum lines and the ported vacuum switch on the right side head.(Pontiac 301, building a 455 for it.) After the repairs the car ran like crap, poor power off the start, no pick up, and around town driving gas mileage was horrible, about 100 to 130 miles a tank, 24 gal tank. Driving the car at highway speeds no problem, good on gas, and has power. The last thing is at idle if you rev the engine a little it will back fire , pop from the carb. The plugs, wires, cap, rotor, coil,module, all replaced last year. The RPM at idle is around 850-900. I even ran a compression test, the numbers are fine. Any help or advice would be great. Thanks,Chris.
Computer-controlled DualJet? Or non-computer? Do you mean "setting the idle mixture"? Idle mixture screws are just like a Q-Jet--often sealed behind tamper-resistant hardened steel plugs. The "clean" way is to grind a small hole through the hardened cap, insert a sheetmetal screw, and connect a small slide-hammer to the screw to pull the cap out of the casting. Most guys just cut the casting, and knock the cap out. Looks ugly when finished, but works just fine--and the ugly is hidden by the base gasket when reinstalled on the vehicle. If it ran good before screwing with the vacuum system, and ran crappy afterwards...start by looking at the vacuum system. In this case, I'd especially verify that the vacuum advance (if used) works properly.
If it ran fine until you did these repairs and all you did was replace the switch and vacuum lines, sounds like you have some vacuum lines crossed or the new switch may be defective.
I agree with Shurkey and Jim,,,,, start checking out what you did and pay close attention to the vacume advance at idle...sounds like you are not getting any advance at all which will do exactly what you have described....obviously it is relative to something that you did .....
Hi,Shurkey, Jim & Doc, thanks for the advice I will work on it a little today and tomorrow and post my results. It's not a computer controlled unit, and I have 2 screws that stick out from the front of the carb one on each side of my fuel filter inlet I was referring to them as the idle mixture. As far as the vacuum lines I replaced one at a time. I will check the vacuum advance and make sure it works correctly, If it was not working the right way, would that cause the back fire pop sound from the carb? Thanks again.
Hey guys, I did some work on her today, first thing I checked the timing, it was way off. I set it to factory spec 12 degrees, while the vacuum advanced was disconnected I tested it, all was good. I put the line back on the vacuum advance, the timing mark went off the tab, not even near a number. I unplugged the advance and it went back to 12. I changed the vacuum line on the advance to a ported vacuum line instead of the factory manifold port and it ran fine, good idle and power. My Chilton's manual shows the 77 & 78 as a manifold vacuum and the 79 as a ported vacuum, not sure why.
That is the very thing that the advance is supposed to do.... nothing wrong with it.... the difference between the straight vacume advance and the ported vac advance is pollution regs....I run my cars on straight vacume....
