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Old 02-18-2008, 07:04 AM
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Carb choice help

Here's my car stats:

63 Corvette convertible
Weight 2880lbs dry
4 speed M-20
3:08 gears
70' LT-1 block
461x iron heads (worked)
AirGap intake (recently replaced a Torker II)
Holley 3310 (4160/750cfm, vacuum secondaries)
Flat top 10.5:1 pistons
290/490 cam (?)
MSD vacuum advance ignition (recently replaced a Mallory mechanical)

I have chased a dead spot that I all but eliminated. It's only evident with a stab at the accelerator (at any speed), and is a fraction of a second. I have adjusted this carb every imaginable way there is and adding fuel is what has gotten me from a bog to a slight hesitation, but I am out of adjustment. I am running a 30cc accelerator pump on this carb, and could go up to 50cc(?). But, am debating if I would gain anything by putting more money in this carb, or should I start over. I have researched the carb selector sights and everything points to 650cfm being more ideal for my combination. Even read some testing of three carbs, 600, 650, 750cfm on a similar engine and the 750 didn't make any more HP or torque than the 750, only used more fuel.

Given the light weight of the Corvette, and the 4 speed transmission, I am thinking about going to a 650cfm carb, possibly a Holley 650 HP Double Pumper, mechanical secondaries. I have read and re-read all the threads about not running a DP on the street, and for every negative post there is a positive post based on the weight and transmission combination. I do not need a choke, never drive in the cold.

Anyone got any experience with a similar set up?
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Old 02-18-2008, 07:38 AM
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3310 is a good carb.

what accelerator squirter size are you using?

what spring in the secondary are you using?

what color accelerator pump cam are you using and what position is it on?

What is the initial ignition timing and how much total timing are you using?

I would use a regular RPM intake over an Air-gap RPM and especially over a torker 2. A street car needs that exhaust cross over to warm the carb, otherwise it will take a long drive to get the air/fuel mixture up to temp. An RPM intake will make more power than a torker 2 everywhere in the power band.
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Old 02-18-2008, 11:20 AM
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42 Squirter
White spring
Factory, set at second hole (quickest movement)
18* initial, all in by 2500 at 36*
I like the AirGap, not concerned about the warm up
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Old 02-18-2008, 12:08 PM
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I bet it is the white spring that is causing your trouble. Try a purple or a tall yellow.

You could also try a little more squirter size.

I think 38 or even 40 degree total would run better with those old heads. 36 isn't enough for 461's due to slow flame travel.
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Old 02-18-2008, 01:08 PM
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I have a 37's I could put in it. I tried all the springs, and settled on the White. I did extensive curving on the Mallory before replacing it, and was at 40* and it just didn't make a lot of difference. Coming in earlier made the most difference. Never heard that iron heads required more timing before.
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Old 02-18-2008, 01:18 PM
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No, 37's are smaller than 42's. you need more fuel. 45's maybe would help the bog.

The white spring is the lightest one. It opens right away. That is most likely the cause of the bog. Try a heavier spring like a purple or yellow.

it isn't that iron heads need more timing. It is old iron heads that need more timing due to the shape of their combustion chambers. They have a slower burn rate so they need more advance to get the job done. GM Vortec heads are iron but have a fast burn combustion chambers, so they do need as much timing. 35 or 36 is enough for them (vortecs).
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Old 02-18-2008, 01:41 PM
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More rear gear for that cam would make a great difference. A 42 squirter is very big for your combo.
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Old 02-18-2008, 01:56 PM
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Carb Choice

I recently helped solve a similar problem. Same carb on a Z28 Camaro. There was too much clearance between the accelerator pump cam and the arm bar leading to the pump. I bent the arm a little and it solved the problem. I set it so there was just a slight clearance, maybe ten thousandths. Cant tell you this will work but might be something to check.

Bob
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Old 02-18-2008, 04:00 PM
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454C10, your right about the squirter size, sorry for the brain fart. I tried other springs last fall, it's not the vacuum secondaries causing the hesitation. With vacuum advance I should have plenty cruising, but will have try advancing a couple degrees, won't hurt to have more initial.

baddbob, have considered a gear change, but I drive the car everywhere during the summer months, and am already at 2700 rpm's at 65mph. With side pipes, it gets a little loud. When I save enough to put an overdrive tranny in it, gears would be a natural change.

Quarter Flash, already adjusted the accelerator cam arm so there is no wiggle, getting a shot with slight movement.
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Old 02-19-2008, 11:37 AM
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Carb Choice

Are the bushings that hold the butterfly shafts in place in the foot of the carb warn out. Sometimes it's hard to find because they are under spring load. Check power valve size and condition.

Bob
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