Thread: Q-jet question

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My question is what would make a Q-jet carb not make the secondary open under full throttle? engine is in top notch shape and runs great. There seems to be no sticking of linkage and secondary throttle plate can be opened manualy.Is there an adjustment for the tesion on the butterfly for the enrichment circut. Thanks for any and all info. Tracy

Maybe this will help. First check the spring tension on the secondary upper blades by pressing them down with you fingers. They should have enough tension on them to open then when you release them snap closed. If they don't snap closed and stay open this will creat a bog. The adjustment for the tension spring is on the left rear corner of the carb underneath the casting as your standing in front of the vehicle. Now if tension is ok open your thottle blades wide open by hand (engine not running of course) and look just below the choke housing there will be a Z type or S type stop which falls off when thottle is off open. the Choke fast idle cam controls this in the cold mode so secondarys don't open, as the engine warms up the Z or S stop is allowed to fall away and thus secondary blades open. The top blades are controlled be the vacum of bottom blades as they open. Hope this helps you. Bestbbq, No. Cal

Thanks for the info. The s or z bracket is out of the way and the spring tension is about what you have told me. I looked up the carb numbers and it is a 795 cfm on a SBC 350 is`nt that to big? does the engine make enough power to open the secondaries?

Hi Tracy - sounds like your doing your homework. That's a good thing. The 795 cfm's were installed on alot of small blocks and other GM motors. Here is another test to check your secondarys. Start the motor and bring up to operating temp. This going to check the spring tension on the top blades. When up to operating temp bring the engine up to about 1500 rpm then rap the thottle to WOP (wide open position) you should see the upper blades partially open. If they don't your spring tension is to tight or your secondary metering rods and or the cam that they are attached to are hanging up. To lighten the tension of the blades. Remove the carb (you can do this with the carb on the engine but it requires the use of a small mirror) and on the left rear of the carb as your standing in front of the vehicle turn upside down and you will see a small allen screw stop and a spring also on the side of the air horn castin where the screw and spring are located there is a another straight slot screw this screw applies or takes away the spring tension. unloosen the allen screw and turn the flat head screw clockwise to increase of counterclockwise to decrease it. One other thing make sure you put a vacum gauge on the manifold side of the engine which a port that is blow the thottle blades. You should have 18 - 20 inches of vacum. Then put it on the port side of the Carb. This port(s) are above the thottle blades. Normally to distributor vacum advance. You should have 0 vacum at idle and when you rap the thottle vacum should increase and climb. Your on the right track don't give up. Hope this will help. Signed GM dealership tech retired after 32 years. J.B.

Hi Tracy with all due respects to all the knowledgeable people here, I was taught a different way. This aint a magig bullet but it works real good for my street cars. Say a 350 cubic will turn a max of 5000 rpms 350x5000=1750000. If u have 85% engine efficiency then 1750000x85=14875000 divide that by 3456 which is unknown to me that =4304.1087-or rounding off to 430 cfm for an engine with 350 cubes @max 5000 rpms 85 % efficiency would be a nearly new rebuilt engine. Now if u have variables-rpm-hi lift cam- hedders- hi compression-NOs-or engine wear then u have to start race tuning. But this is a good place to start from, more is sometimes less. Hope this helps