Tunning Dual Carbs

[eric lipper]

Ok, for the idiots like myself.  I sent my carbs off to be restored on the 67 GT500.  I have done lots of single Holleys but never a set up with two.  I need to set them back up from scratch.  Any tips?

[gt350hr]

    My reply #1 . You can set the idle mixture screws at "one turn out" from bottoming. ( all four screws) Then "crank in' some throttle opening on both carbs. Start it up and adjust the idle speed back and forth until adjusting one or the other changes idle speed , effectively making them even. While all of this adjustment has been happening the engine should come up to temperature so you can set the idle mixture screws ( some guys hook up a vacuum gauge and go for the highest vacuum reading ) and tune them to the smoothest condition. This may take a few "rounds" . Your idle speed may have increased so you may have to lower the speed screws. at that point  , "I" would go for a drive , come back and re check all my adjustments . Yes it is a tedious process but that's the price you pay for the power you get from dual quads.
    Randy

[1109RWHP]

Got some time today to mass with the carbs again. I opened the secondary plates with the stop screws 1/2 turn. I removed all of the choke parts from the one carb also. Checked the fuel pressure and it s 6 psi at the regulator. Got it warmed up and it still idles at 1,000 rpm but the primary idle speed screw is all the way out now. I can however adjust the mixture crews though. I used a vacuum gauge to set them to the highest rpm. It has about 8-9' of vacuum at idle. Set the float levels lower so it is just below the sight plug opening. I will have to adjust them again once the motor is in the car at it's normal angle. I have this trick adjustable billtet pcv valve that I want to mess with but I need to do it once everything is in the car and plumbed the way it will be for good. I have a oil separator can to go on it also. Idle is good and it sounds pretty crisp.

[shelbydoug]

Got some time today to mass with the carbs again. I opened the secondary plates with the stop screws 1/2 turn. I removed all of the choke parts from the one carb also. Checked the fuel pressure and it s 6 psi at the regulator. Got it warmed up and it still idles at 1,000 rpm but the primary idle speed screw is all the way out now. I can however adjust the mixture crews though. I used a vacuum gauge to set them to the highest rpm. It has about 8-9' of vacuum at idle. Set the float levels lower so it is just below the sight plug opening. I will have to adjust them again once the motor is in the car at it's normal angle. I have this trick adjustable billtet pcv valve that I want to mess with but I need to do it once everything is in the car and plumbed the way it will be for good. I have a oil separator can to go on it also. Idle is good and it sounds pretty crisp.

8-9" isn't much vacuum. It doesn't matter terribly IF you don't need vacuum for accessories. I get uncomfortable under 12".

I would think it is MOSTLY the cam but I have no experience with the idle characteristics of that intake manifold/head combination. Randy does. I'd guess 85% is cam, the balance is the runner configuration of the intake?

All considered, 1,000 is livable. More of a Pro Street type set up. I've seen those not being able to idle under about 1,400 rpm though and the things just spit through the exhaust because of the amount of overlap.



The Cleveland in my Pantera idles at 12 to 13". It was a lot of finagling to get it that high. Normally 10-11 would be about what to expect with the cam, head, carb combination.

After MUCH time and effort working on vacuum, I gave up and just installed a CompCams vacuum pump. It gives me a stockish 17" for the brakes. The trade off is you can hear the compressor running which annoys me. Such is life. That's much about compromises of what you can live with and can't?

Normally the Webers alone will drop you to about 11 to 12" at idle.



By the same token my 68 GT350 idles at about 800 hot, with about 14" BUT it has a milder cam and uses the "street" 2x4 intake and not the T/A that Randy is running.