I have a "troublesome 73 Qjet for 350 that I have been troubleshooting. It has a consistent problem that I am having trouble diagnosing. Maybe you can help. The carb, when freshly reassembled and installed on the car, will start right up. I tune in the usual way, achieving 14-15 inHg (normal) and a decent idle (750RPM) When I stab the throttle for a rev, it will not return to a normal idle....I lose vacuum - it flutters 10-13 inHg. This to me seemed like the secondary plates hanging. So I checked it out, found nothing, and even installed another (correct) baseplate. this resulted in no improvement. When I disassemble, inspect, reassemble and reinstall, it repeats the above. Good at first then loses vacuum after a strong rev. I am certain the internals of the carb are correctly sized, I went thru it all. No base gasket leaks. vacuum cans are good. Baseplate has bushings and plates are indexed correctly. Its specific to this carb, my other works fine any thoughts? does this problem have the characteristic of an internal vacuum leak? What does it sound like to you?
the best thing would be to try a known good carb. a couple questions, is there gas dripping out of the primary venturi while it's idling? you may need a flashlight to verify this. also do you have mixture screw response (forward two screws), from both screws? if not the carb may require more cleaning and blowing compressed air through the idle tubes or replacement. possible gasket mismatch can cause fits also. good luck
If it wasn't for the vacuum drop, I would ask if the springs in the distributor were too light, and the timing was not returning to initial settings when you let go of the throttle.
Check the pri. throttle plates and shaft to housing clearence. It could be that the throttle plates are not centered in the bores and not returning smoothly. Also worn or too tight throttle shaft housing could cause same issue. Could also be warped housing causing the throttle shafts to bind. You need to 'play' with carb after 'stabbing' throttle to determine what has changed, and what corrects it.
I have had this problem in the past which turned out to be ignition related. The advance springs were too light causing erratic or inconsistent mechanical advance and return after releasing the throttle. If the springs are too light the mechanical advance will not consistently return to the base position. Are you using the stock distributor & have you modified the centrifugal advance/installed a curve kit? What I did/do is set the timing at idle with the stock weights & springs. Then I install the weights & springs from the curve kit and check the timing at idle. If the timing changed, the springs are to light creating too much advance at idle. I change the springs until I have no additional advance at idle. Then I open the throttle slowly to check when advance starts and when total advance is achieved to tailer the advance curve to the engine.
I have had this problem in the past which turned out to be ignition related. The advance springs were too light causing erratic or inconsistent mechanical advance and return after releasing the throttle. If the springs are too light the mechanical advance will not consistently return to the base position. Are you using the stock distributor & have you modified the centrifugal advance/installed a curve kit? What I did/do is set the timing at idle with the stock weights & springs. Then I install the weights & springs from the curve kit and check the timing at idle. If the timing changed, the springs are to light creating too much advance at idle. I change the springs until I have no additional advance at idle. Then I open the throttle slowly to check when advance starts and when total advance is achieved to tailer the advance curve to the engine.
First, let's be sure that the carb is the issue. Many of the emission controls, being vacuum operated, can have leaks, and when actuated, then begin leaking. You might try disconnecting ALL of the vacuum hoses, and plugging them. I have seen, on many occasions, EGR valves sticking open when actuated, causing the mixture to go lean due to dilution. Chevrolets were known for this more than others, though none were immune. It is possible for the valve, having a weak return spring, may open without vacuum activation, or not close completely due to exhaust deposits or seat or pintle corrosion. If the issue is indeed mixture related, it is necessary to determine whether the mixture is rich or lean, and it is easy to get confused, since both will cause the exhaust to stink. Something to consider is debris in one of the passages, which, when the engine is revved, is pulled deeper into the passage, causing a lean condition. It is easy to determine which passage is involved on a dual-plane intake. Pulling plug wires of of the affect cylinders will cause a reduced effect. On a dual plane intake, the driver's side of the carb supplies cylinders 1,4,6,7, and the other side affects the other cylinders. It is also possible, when the engine acts up, to spray a small amount of carb cleaner into one side of the carb, then the other. The side most reactive is likely the side not feeding sufficient fuel . Let us know... Ray
Thanks for the input guys. Looks like the Air horn gasket was the trouble. I doubled up the air horn gasket and installed the carb on the car. Idles 14 inHg @750RPM (perfect) and does NOT lose vacuum after revs now. I took it out for a good cruise, doing some errands, and it seems the vacuum issue has been solved. I did several good long hard pulls from a stop and from a rolling start.....several heavy downshifts.....everything I could think of to shake things up. It now returns to idle fine. The carb needs a little fine tuning, but that is another matter. Anyone know who sells the THICK air horn gaskets?
We have thick main body to airhorn gaskets for that carburetor.....Cliff http://www.cliffshighperformance.com/
