Looking for suggestions to solve a problem with my brothers 1969 455 that he recently purchased. Carb is correct carb. for engine according to part #. Carb. has been disassembled, cleaned, new float(large one), fuel filter, choke pull-off, carb. kit installed. Performed all adjustments as per shop manual specs. Checked fuel pump pressure. 6 psi, 7psi with return line pinched. New spark plugs,breaker pts., rotor, dist. cap and plug wires. Dwell and timing set to specs. The engine runs great under all situations except when accelerating. The last portion of throttle opening, as you approach full throttle, the engine stops accelerating/pulling and starts to sag/flatten out. If you let up on the throttle slightly, it begins to accelerate/pull again. Rechecked float level-OK. Ran vehicle with fuel return line pinched off-same condition. Decided to try another carb. Had a 750 AFB style Edelbrock that was not in use, installed that w/adapter plate. Engine accelerates normally, continues to pull until 4500 or so. Put the Q-jet back on, same condition. Anyone had a problem like this? Suggestions? Thanks in advance. Gerry
Did you replace the secondary airvalve tension spring? Sounds like the secondary airvalve is opening too soon, which it will do with a weak spring. Both the secondary airvalve spring and cam SHOULD be replaced when rebuilding a Q-Jet. Jon.
Re: Q-jet Carbking, I did not replace the spring or the cam. However, I did try adjusting the air valve spring at various positions, 1/2, 3/4, 1, 1 1/2 turns after contact. Did not have any effect on the problem. Spring/tension did not seem abnormal. What affect does a worn cam have? Where would one locate a replacement?
Gerry - the secondary air valve cam controlls the rate of lift and the actual lift of the secondary metering rods; thus in effect controlls the secondary fuel. Wear on the cam will delay the timing of the secondary fuel causing a lean bog as the secondary side kicks in, and excessive wear may cause a lean secondary at WOT. Jon.
Re: Q-jet Carbking, That explanation sounds like a cure to the problem. Thank you. Now, where can one find a replacement part? Are all secondary cams the same or does it depend on year/application?
Gerry - as far as I am aware, all secondary cams are the same; but we do not work on computer-controlled Q-Jets so am not making a blanket statement. The same is true for the secondary airvalve spring. Both the cam and the spring are contained in the better mail order carburetor repair kits. While these items may be available "ala carte" I am unaware of such a source. Jon.
Try these guys out for the secondary cam: http://www.carburetion.com/quadrajet.asp Part number 85-104. JP 73' Centurion Convert
Does it bog when you first hit the secondaries? From your description I do not think it is spring or cam ralated. Unless the cam has a flat spot at WOT... Any spring issue would show up the instant you hit the secandaries, not at steady WOT. Sounds more like running out of fuel. Maybe related to the "big" float you put in...
RE: Q-jet Problem solved. It turned out to be the new choke pull-off. It was extremely slow to release, took about 7-8 seconds. When I disconnected it and drove the car, the problem disappeared. Thanks to all who replied.
