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Discussion Starter #1
Hi i run a 307 with vortec heads ,xe262 cam , headers , rpm inlet and a carter 625 AFB competition series carb . Now i have always had a problem with the motor stuttering with light throttle , it runs ok normally cruising on the road but when i drive in town at low speed using light throttle it stutters badly . The idle mixture screws only need a 1/4 turn out for good idle and i can turn them almost shut and still get it to idle . Is the carb to big and the idle restriction to large , i think that maybe it needs more throttle than the 307 uses before the main circuit starts to work , and the stutter is because iam still in the idle circuit with the screws only 1/4 turn out . Does anyone know about the AFB and be able to offer some advice on it, or should i go down to 500 CFM carb.
Pete.
 

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Re: anybody know about carter afb,s

The present carb is alittle big for a 307 V8 as GM only installed a 2-bbl carb on them.
I believe you can make it work by advancing initial timing to say 10°-12°BTDC because a 307 has 8:1 static compression ratio and you can run a lot of initial advance, but, you can go too far as it will crank very slow when starting a hot engine, then retard timing by two degree increments until it starts normally when hot.

If timing is advanced, then you can bring down the curb idle to have better usage of the idle circuits and adjust mixture screws for best vacuum reading on the vacuum gauge.

You might even get by using full time vacuum on the distributor vacuum advance allowing better revving from idle.

Good luck. Trial & error.
 

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Re: anybody know about carter afb,s

I run at 16deg initial with 32 deg max by 3000 rpm because vortec heads dont seem to like too much more than 32 deg . The comp is about 9.1 with 15thou shim gaskets and 1 get 16wg manifold vacuum at idle. I have changed everything on the ignition side apart from the Pertronix ignitor module . I have just checked over the carb and the only thing i found was that the gaskets under the primary boosters were wet with fuel , so maybe i have a leak there . I will put it back together with a little bit of sealant and run it tomorrow.
Pete
 

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If you get 16 wg at idle, you're doing good because of the short stroke, design of the engine, you can't change it and of the overlap of the aftermarket camshaft.
You might try adding more fuel during the accelerator pump stroke - go to the inside hole to give more volume during the pump stroke when throttle is opened more.
 

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I would also check the pump accelerator shot. There are several positions the little S clip or hook can be placed.

I would also look to see if the power piston (that the meter rods hang off of) is not sticking or the accelerator pump material is not rolled back or has a cut in it.

These were old AFB issues and I am sure the newer eddy models inheriated some of this.
 

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Discussion Starter #6
I did the squirters a while back because it would spit back when i hit the gas . I have fitted an O2 sensor and it showed a lean spot just off idle but was ok cruising , so i thought i would put in a richer rod and the lean spot got a bit smaller ,so I tried another rod and the lean spot has almost gone . I have gone from a 68/42 rod with a 100 jet to a 63/47 rod .The AFB came with98 jets and 71/47 rods but would spit back bad so i put in 100 jets which was better but i could never get rid of the light throttle stutter ,so going to 63/47 rod seems like a big jump but it works . I think the vortec heads can cruise on quite a weak setting just my set up had a lean spot at light throttle , so it was more fuel for the fire . Thanks for the replys and 02 sensors are handy to have.
Pete.
 

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I did the squirters a while back because it would spit back when i hit the gas . I have fitted an O2 sensor and it showed a lean spot just off idle but was ok cruising , so i thought i would put in a richer rod and the lean spot got a bit smaller ,so I tried another rod and the lean spot has almost gone . I have gone from a 68/42 rod with a 100 jet to a 63/47 rod .The AFB came with98 jets and 71/47 rods but would spit back bad so i put in 100 jets which was better but i could never get rid of the light throttle stutter ,so going to 63/47 rod seems like a big jump but it works . I think the vortec heads can cruise on quite a weak setting just my set up had a lean spot at light throttle , so it was more fuel for the fire . Thanks for the replys and 02 sensors are handy to have.
Pete.

I've had similar issues before with my Edelbrocks, and could usually tune out the slight stumbles with a different step-up spring on the metering rod circuit. I assume the Carters have that option?? I guess I'm not absolutely sure on them though. Just a thought....
 

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I've had similar issues before with my Edelbrocks, and could usually tune out the slight stumbles with a different step-up spring on the metering rod circuit. I assume the Carters have that option?? I guess I'm not absolutely sure on them though. Just a thought....
Edelbrock = Carter (they bought the rights from Carter)
 

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I did the squirters a while back because it would spit back when i hit the gas . I have fitted an O2 sensor and it showed a lean spot just off idle but was ok cruising , so i thought i would put in a richer rod and the lean spot got a bit smaller ,so I tried another rod and the lean spot has almost gone . I have gone from a 68/42 rod with a 100 jet to a 63/47 rod .The AFB came with98 jets and 71/47 rods but would spit back bad so i put in 100 jets which was better but i could never get rid of the light throttle stutter ,so going to 63/47 rod seems like a big jump but it works . I think the vortec heads can cruise on quite a weak setting just my set up had a lean spot at light throttle , so it was more fuel for the fire . Thanks for the replys and 02 sensors are handy to have.
Pete.
Still , you need to look into the accel pump shot.
 
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