So I took advantage of the long weekend and swapped my factory 305 quadrajet for an Carter AFB 4760S I had laying around that was in good condition. It was my first time swapping a carb and it went better than I would have expected. The AFB however is a 750 CFM carb, which I realize is immensely over sized for a factory 305. I've got it dialed in pretty well right now but my fuel line needs to be all properly done up as it's mostly rubber from the pump forward right now. It runs pretty rich and doesn't like to idle below ~1000 RPM, is the high idle just a symptom of running such an over sized carb? I've got a 5 speed manual so taking off isn't an issue for me because i can blip it up to an RPM it's happy at. The exhaust is cut off just before where the cat would have been and then there's a 2.5" glasspack and that's the setup for the time being because I needed to get rid of that restrictive cat. There are no emissions requirements where I live so it isn't an issue. In regards to running rich would that be a matter of changing jet sizes? I've got it leaned practically all the way out with the screws on the carb and it still crackles like crazy out the exhaust. I'm hoping to get a video up on the new carb soon and I'll link it here, like I said I've never worked on a carb before so any help is appreciated

 

Jets don't typically affect the idle. Something else is out of adjustment...

 

So the unstable idle isn't the result of the jets or the carb itself being too large? I've heard a big carb can ruin bottom end, Mine seems to be an acceptable power from 1500-3000 RPM, very comparable to the way it performed before, maybe slightly better or worse. The top end seems to have really picked up, it was wheezing if I even got close to 5000, which wasn't necessarily a bad thing because I'm not to comfortable taking a nearly 200,000 mile motor that far. But after the new carb it really seems happy to get up past 3000 until redline. I'm going to put a vacuum gauge back on it today and try to get it running better if I can, I was pulling about 14-15 inches at idle which seemed pretty low for a stock 305, but it held pretty steady there and I checked for vacuum leaks but maybe one snuck by. I'll take another look once the neighbors are out and about because I don't think they'll appreciate me tuning a carb on a straight piped car at 6 AM

 

if this thing is running pig rich at idle the float is probably set a bit high, if you have to balance it out by opening the throttle blades to keep it running at 1000rpm as TL said, something is out.

go back to the beginning and start again.

warm the motor
unplug, the vac advance on the dist, set timing to zero
screw the air bleed screws all the way in gently, the back out a couple of turns
wind down back on the throttle blade adjustment screw to bring the throttle blades to a more closed position
engine will quit on you
start the car again and wind the air bleeds out at the same amount of turns until the idle doesnt increase anymore
check the tach for the current idle.
turn the throttle blade adjustment screw to set the kurb idle as per the sticker on the car or the manual

take it for a drive and see how it goes.

 

Did you retain the factory distributor? Without an electronic carb, the timing will not adjust. Use an aftermarket distributor ( I used an ACCEL) and set timing around 8*. The size of the carb is not the issue. The Q-jet is around 750 cfm, too.

 

Yeah I kept the factory distributor, Is it electronically advanced? It looks just like a regular vacuum advance distributor. I haven't had a chance to get out and mess with the car as it's been raining quite a bit here and my daily killed a coil pack on me the other day. If I've got to change my distributor out it isn't a huge issue but I'm leaving town for work next week so it might take a while. I didn't know the factory carb was also around 750 CFM

 

If you haven't done it already; it's a REAL good idea to install a Fuel Pressure Regulator,......Edelbrock and Carter carbs don't handle ANY excess fuel pressure very well.

 

Yeah you will have to replace it with a non computer controlled vac advance distributor.

 

+1 on the non-CC distributor. I replaced the Quadrajet with a Holley years ago, and didn’t realize the distributor was computer controlled. I could never get it to idle right until I put a non-CC distributor.

 

Remember the stock 305 Q-jet is a 750 cfm unit!
Why swap for another 750 brand X ?

 

I changed my CC distributor with an ebay cheapie because anything 30+ years old and American is hard to come by in Australia. The unit is absolutely amazing and never stepped a foot wrong.