[68lemansman]

I have a holley 650 i downgraded from a 750 (save gas) I instaled the rebuilt carb and it works fine when driving and starting up. my problem is is when im at a dead stop it dies and hesitates then it takes off. I can't figure out why it is doing this. Any suggestions?

[regamble]

So, it wont idle at all and hesitates or tries to die when accelerating from a stop? What's the list number from the carb and have you done any work like rejetting, PV swap etc?

I presume that the only thing you changed was the carburetor when this started, i.e. it was running fine before this carburetor.

[68gtoMN]

vacuum signal. Do you have a vacuum leak? Check by starting car and spraying carb spray all about carb (not in carb). If idle smooths out or increases, you have a vacuum leak.

[antique69lemans]

Were both carbs double pumpers (assumption from the 650 cfm) ? I know the idle circuitry between a 600 and 750 vac secondary carbs behave different. The smaller carb always transitioned and idled better. You need to start with the settings for that list number as suggested and tune from there. Float levels etc. will all come into play. Can you state your engine combo and more detailed info on the carb before and after ? Without knowing your skill level and experience it is hard to help especially without data to draw from. A seemingly direct swap could have resulted in a different vacuum source being used for the distributor and then being blamed on the carb.. I have been fixated on curing a problem, that never was the culprit before. Outside objectivity helps sometimes.

[68lemansman]

I have a 1970 gp 400 with a msd rotor and the carb i had on it was a 750 but i replaced it with the 650 for gas reasons and also because when i got the car the 750 was butchered basically the whole choke flap assembly was missing. The car ran fine with the old carb on it even without the choke flap. But the car now runs and starts it has a little bit of rough idle when starting but after warming up she runs fine. I just did a tune up on the car (new wires and plugs) When i come to a stop and then go to get on it the carb pops like and hesitates and stalled out but it then takes off. I dont know the number off the carb i can get them. Im not near my car right now but i can post them later. I havent done anything to the distributor or nothing else, but just replaced the carb.

[mike nixon]

Ruptured power valve.

If it's backfired or popped back thru the air horn this is probably your issue.