APT Adjustment

[byf41]

I recently purchased a NOS #7041201 Quadrajet for my 1971 BB Chevelle . I disassembled It to see how It was set up and found the APT or power piston set too low . I was advised to bend the tang under the piston In the base plate up to bring the piston height up where It needs to be and not mess with the APT plug and/or screw . I got the piston height I need but In doing so , the tang (piston stop ) Is now raised up off of the adjuster screw . So when you depress the piston It has a fairly positive stop when It contacts the tang , but If you depress It with further effort It will still go down because the tang Is not physically contacting the adjustment screw as It was originally . Question Is , will engine vacuum pull the piston lower than my setting being as It doesn't have the solid stop It originally had ? My gut feeling Is the vacuum would not be strong enough to to bend the tang but now Im concerned . Is there a way to apply vacuum to the piston with carb apart to see what It does ? Suggestions or comments appreciated .     

[Cliff Ruggles]

"I was advised to bend the tang under the piston"

Why would you want to bend the APT into position rather that using the APT screw?

I would recommend to remove the cover over the APT screw and try to remove it.  I'm about 95 percent successful at getting the APT screws out of the baseplate, then we replace them with a custom APT screw we machine in the lathe and put a spring on it.  This give the tuner FULL control of part throttle A/F ratios just like the later APT carburetors.

It is an excellent system, and I recommend using it in lieu of trying to bend parts with next to zero precision for the final position of the metering rods in the jets.

I would also ask how you know the power piston was "too low"?  The factory set each one of those precisely where they needed to be per carb number and application.  You would NOT know if it was too low or too high without testing the carb first to determine if it was within range for A/F at part throttle.....FWIW......Cliff

[byf41]

I got the screw out ok and will order youre custom apt screw and carb kit . Should I make any other changes to the stock calibrations ? Its a .030 over LS5 454 with Crane commander hydraulic cam and forged flat top pistons with slightly higher compression than stock and manual trans . Stock 1971 LS5 Intake and exhaust manifolds . The carb has #77 primary jets with 49B rods , BG secondary rods with M hanger . Thanks for youre help . 

[Cliff Ruggles]

We'll make a slight change on the primary side, the secondary metering rods are fine......Cliff