My car has what i feel is a lean off idle stumble. Once it hits 2500 rpm, it begins to really haul @ss, but if i punch it from a dead stop i cant even chirp the tires anymore. When i powerbrake it stumbles for a second then comes clouds of rubber smoke. My car had a good amount of power, but now with this issue i cant do anything better then a mid 15 in the 1/4. It all happened when i rebuilt some of the top half of the carb. Maybe i messed up the accelerator pump, or maybe its just gone bad. I need to know all there is to know about these dumb things, thanks in advance

Could be the accelerator pump seal...

Did you adjust the air valves on the secondaries? If they open too quickly they will lean out the mixture.

Have you checked the pump operation by holding the choke open and looking down in the primaries while you work the throttle? (with the car not running). Should be a good stream of fuel.

Did you remove the accelerator pump baffle when you had the carb apart and did it get put in correctly?

The stumble is usually due to too much air in the transition of the vacuum signal from idle and off idle ports to the main venturies. The vacuum signal stops pulling fuel from the idle circuits but airflow is too slow to pull from the main venturies. That is what the accelerator pump is suppose to make up for. That, and too richen up the mixture to accelerate the engine.

Best of luck.

accelerator pump baffle? i remember trying to get the spring off of the pump to replace something or other and not being able to get the spring off and just slapping it all back together. im going to check the fuel flow when it gets a little warmer outside, thanks

Accelerator pump baffle.... a little "spacer" that divides the main fuel well from the accelerator fuel well. There is a small slot in one end that lets fuel into the accel well. If it was put in upside down then it would seriously restrict flow to the pump and reduce the volume of the pump shot. Oddly enough, when I pulled mine apart, it did not have a baffle.

Which spring? The big one in the well or the small one on the pump? Did you also look at the pump seal. The seal is held in place by a spring shaped like a Cheerio! (cereal)

Hope all works out!

yeah it was the spring with that cheerio shaped thing at the end. well, i just remembered that the whole reason i rebuilt the top half of the carb was becuase i drilled the rivets out of the iab valve and dropped a peice of drill bit into the carb, which stuck inbetween the metering rods, which caused the car to run like crap. after i rebuilt it, i never adjusted the iab valve, so i messed with it today, and turned it one turn richer. it had a worse stumble off idle, so i turned it two turns out, which leaned it out a little. for now it seems to run good, but ill find out for sure tomorrow how it works. thanks for the help eric

Ahhhh.. the old idle air bleed. Can't say I ever played with that. Just left it at factory stock position. Do you have a procedure for adjusting that or just playing with it? I could take a look at the GM service manual for adjusting that if it would help.

Getting closer....

well it still stumbles on occassion, so i will now adjust the float, which hasnt been set at anything specific. i really wish i lived near someone who was a thirdgen expert so they could diagnose something right away.