Auto Racing: secondaries on carb, barrel carbs, sarasota florida

Question
My name is Michael.I have a question about a 600 cfm vaccum secondary carb model number is 6919.the choke is dissconected. I recently changed the secondary spring to a lighter spring.what happens now is,after i drive the car hard i notice that when i come to a stop the idle rpm is increased by a few hundred rpms,after a while of driving around or if i turn the engine off and then restart it, it goes back to its normal idling rpm.It did not do this prior to the spring change.I also checked the diaphram and it doesnt have any tears.I tested the diaphram when i push the rod in and then hold the vaccum pasage the rod does not move it stays.i also noticed that when i manully push the rod in and out with out holding the vaccum passage on the diaphram(i did this while the diaphram was completely assembled) i noticed that the rod allows me to push it in all the way but when i let go of the rod iT very slowly releases and goes back to its normal state almost as if it were kinda sticking.Im not sure if it suppose to do that or not.my question is , do you know whats causing the rpm to increase after a full throttle run? Your input would be appreciated.Thank you very much."

Answer
Hi Michael,

I know absolutely NOTHING about 4 barrel carbs, but in this case you answered your own question.  The lighter secondary spring is allowing the INTERNAL vacuum of the carburetor to hold the secondaries open after the hard run.

I would bet that if you were to change back to the proper secondary spring your problem would disappear.

If not. well - - -I'm afraid I'm the wrong guy to ask., I mostly work on them "funny little furrin cars" ..

Best of luck

Dan Liddy
Sarasota, Florida