I was curious as to if there is a proper way to adjust my 800 CFM Q-jet that is in my 1972 Riviera? I get a real bad bog or stall when I try to punch the gas. Should I turn the passenger side screw clockwise until the engine is about to stall then let off or what? I have noticed that I can push my air valve "butterflies" in when the engine is running. The vacuum breaks look in really good shape. Any ideas or tips? Thanks, Nick
Those vacuum breaks need to be checked with a vacuum pump. The one that operates the secondary air valve needs to hold vacuum. Also, with the motor off, look down the primaries while opening the throttle. A steady stream of fuel must squirt out into the bores or else you will have a stumble. A weak drip won't work, you need a steady squirt of gas. :bglasses:
To answer your question, it is possible to operate the secondary air valves with the engine stopped. With it running, it should take considerably more force. With respect to the throttle blades, they operate independently of the air valves. They operate in concert with the throttle itself, as long as the throttle choke stop is off. This stop will inhibit secondary THROTTLE opening if the choke is more than 1/8 on. The bog, hesitation etc is typically caused by accelerator pump problems. Either the passages are restricted by dirt or the pump itself is worn. If the pump is worn, fuel will bypass the diaphragm, significantly reducing the "pump shot." The external linkage can be troubleshot using a calibrated pair of eyes. With the air claener off, you should be able to look down the primary venturis and see the shot on both sides when the throttle is opened suddenly. The mixture screws are different. They only set idle mixtures, and off-idle fuel delivery indirectly. These screws should be set evenly to a lean best idle point. This means that the slightest leaning of the screws, clockwise, makes the idle speed drop slightly. They should be set to maximum idle speed. They will have no effect on acceleration hesitations. Further questions may be addressed to me either here or privately. Ray
Accelerator Pump? I'm pretty new to the old classic cars and have never heard or seen an "accelerator pump". Do you mean a fuel pump? Is this a part I can get at NAPA and is it fairly simple to install? I might just take the carburetor in to my local carburetor and ignition place to have them take a look at it. If it needs to be rebuilt then I will just replace it with a newer holley or edelbrock equivalent but keep the original. So with the engine OFF I should push the throttle blades in and see if fuel squirts from both side? The front idle blade IS considerable more difficult to push in when the engine is running and I get a "hiss" sound when I do push that blade in causing the engine idle to change. Let me know if I'm doing this correctly. I'm a pretty big newbie when it comes to 1972 800 CFM GM carburetors. Thanks again, Nick
Hi, Nick. If you don't know too much about carb, ignition etc, I would go for the local carb & ignition shop. Let them do a good complete tune-up, (including compresion test). Than you will know "were you stand". By time, and as you are getting more familiar with the car and the systems you can start adjusting things yourself. When it comes to carburetor adjustments, the compresion AND the ignition have to be set correctly. You can make things worse by "fiddling around in the dark". Do a search on this forum, or at yahoo. There is plenty good stuff out there, (including pics), that will help you out. BTW, I would rebuild the Q-jet, it can't cost that much? Per. :beer :beer
Re: Accelerator Pump? No, not the fuel pump. The accelerator pump is a plunger inside the carb. The stem sticks up through the top of the driver's side front of the carb and is connected to the linkage. When you move the throttle you can see the lever move the pump up and down.:bglasses:
I strongly recommend against "having a tune up done." I say this because very few people in these shops have even seen a carb, never mind having set one up. I also recommend haveing the carb rebuilt by an experienced individual, such as the Carb Shop. I do my own simply because I have the experience, and because I know about the kind of work I do. You might try doing a tune up yourself; they are not difficult. The most demanding part is the points adjustments. I can walk you through one. As previously stated, the accelerator pump is inside the carb. It is intended to create fuel flow until the correct passages in the carb fill with fuel and begin to feed. Air is lighter than fuel, and carburetors operate using a venturi pull fuel through passages. The air must be moving before the fuel moves. Since the fuel is heavier, it flows AFTER the air, causing a temporary lean out. The fuel pump feeds fuel until the correct passages are full of fuel and can feed. This pump is a rod with a diaphragm on the end, which expands when pushed down, causing fuel to be pumped through "squirters" in the primary venturis. As this diaphragm wears, less and less fuel is moved, causing a lean spot, or a hesitation. In the worst cases, the fuel flow temporarily stops completely, causing the engine to die. I recommend against replacements as these carbs are very reliable, and when properly set, will allow the best operating conditions, including performance, reliability and economy. Ray
Can you do it? Ray, it seems you know a lot about these carbs! Since you recommend against having a tune up done at most of these shops, could you possibly do it for me? I would pay you of course. I live in Minden just south of you and could even drop the car off if you wanted me to. Or I could try to figure out how to take the carb off and take it up to you. I'm not sure which one you would recommend as you would probably have to tune it again once the carb is put back on. I would pay for all the necessary parts you think I would need and if I were to get an Edelbrock Performer intake manifold later would the carb have to be re-adjusted? I'm gonna call TA Performance and see if they know if an SP1 or SPX intake would even fit under my hood as I have 2.5" of clearance from the top of the wingnut on the air cleaner to the inside of the hood when it is closed. Let me know if you are interested. Thanks, Nick
Grendel, I would be happy to do the work. I have 30+ years experience in the area of tune-up and engine performance alone. I suggest some time in June. I also suggest obtaining a carb kit for the carb you have. The seven digit number is on the side of the carb, in your case, starting, I believe with 702. I also recommend purchasing the spark plugs, either AC or Rapidfires (not Splitfires), and a good set of points and condensor. In what condition are the plug wires, cap and rotor? Have they been recently serviced? If necessary, we can check them on my analyzer, (you did know that I take things seriously right?). I can be reached at (775) 882-6600 ext 330. Ray
