1957 283/270hp dual quads drag testing on a 350

[wheezer]

I'm drag testing an original '57 dual quad set up with the WCFB carbs.There seems to be a problem with how to deal with the secondary air valves. These are those seconday air-flappers with a counter weight outside the carb body. How did the class racers in the old days make these things run hard for the complete quarter mile? In testing the car launches normal (3700lb '57chev, 355cid, T400) with1.8 sec 60ft then about 100ft out those pesky air valves can't figure out what they are supposed to be doing. Once in high gear it runs great(high 12.8's at 105mph). Perhaps some old timer will read this and offer some tips. Thanks,Tom, Dallas/FtWorth

[Ed Wright]

I ran the same setup on a 265" in the '60s and early '70s. Some guys made a wire hook to hold them wide open. I tried that, but good ol' Red Anderson made me take mine off. The car ran the same both ways. If you think that is the problem, tie the weights up to see if it actually is. I saw guys drill holes in the thick (heavy) weights to lighten them, they came with two different weights. Mine all had the thin weights. I would be suprised if that is your problem.

Good luck, Ed

[wheezer]

I'll try the wire open suggestion. The real trick is getting the rear booster venturies flowing fuel without leaning out and stumbling more than before with the weights swinging loosly based on the air flow signal. The throttle linkage is set to open both carbs together. What are the other possible explainations for this mid range stumbling? Thanks for the help.

[Bob Pagano]

To much fuel presure

[Ed Carpenter]

Mike Braswell runs this setup on his 57' Chevy Super Stocker. He reads this forum he may chim in.

[55 Chevy]

Do you run wedge plates between the carbs & the spacer/manifold? Because of the small bowls in the WCFB they may run out of gas for a split second then perk up when it gets fuel again. The angle plate helps with this.

[wheezer]

This setup is just like 1957 with 6 inch paper air cleaners.

More background, about 4 years back I called to order a set of Stahl fender exit headers. Fortunately I was able ask Jere Stahl about this dual quad setup as he had championship experience with his 56 Chevy wagon. He indicated the carbs would see a stronger vacuum signal with 350 cid and consequently higher velocity in the venturii meaning if anything it would not run lean with the needle and jet set for the mechanical camshaft specified back in '57. Well...since this setup was really a high performance option for passenger cars in 1957 the power valve booster piston spring is really-really weak in compression, meaning the weak vacuum signal of the 283 with the solid lifter cam would pull the needles down into the jets to allow low power economy and motor idle stability. So...I'm wondering if at the top of 1st gear of the T-400 the vacuum signal is strong enough to pull the needles into the economy position. Then as the motor loads and really starts making more power the vacuum signal degrades and the power piston is pushed up and giving the jets more fuel. I meant to test the k factor of those springs before I rebuilt the carbs last Spring but forgot. The other set of needles and jets I have are for the hydraulic performance camshaft from 1957 and its springs are stiffer. So I've got much more test and tuning. There are several next steps. First is to install a vacuum gauge near the tachometer.

[Dragsinger]

while testing, another option is to completely remove the metering rods and change jets to allow the correct mixture.

This is common with Q-Jet racers. [it is called "no rod jetting"] You remove the rods, remove the power piston spring, then reinstall the power piston in it's original location.

Danny Ashley has a formula that will calculate the need jet size. From memory, I "think" the primary jets will need to be about 7 - 8 numbers smaller. Another source of info is Oliver Race Carbs in Arkansas. Both of those builders have extensive experience with all the Rochester and Carter combinations.

[MIKE BRASWELL]

tom,
call me at 817-371-8874. i'm not sure i can help but i will try.
thanks,
mike braswell ssm/a

[Ed Wright]

Tom may be onto something. I never used those on a larger engine. After we were forced to go to SS (when they killed off Stock) the bigger cam caused low enough vacuum that at high elevations (even at Amarrillo) they would not lean out on the return road, and at idle. I removed the spring and set the lever under the metering rod "tree" to raies them all the way at WOT, no spring needed. It drove so well that way I never put the springs back in. Might try that to verify you not sucking the pistons and rods back down. We also had to wedge the carbs foward after going to SS (no more 7" tires) to keep the rear jets covered up on the launch. 4GCs like wedges too.