I'm doing some fine tunning and am having trouble keeping the idle down. also slow on DownRev. Running a Holley 750 Street avenger on a built 302. fuel mixture screw is at 1 1/2 turns. I'm afraid if I screw it in much more I might starve my eng. Runs nice and smooth otherwise. Idle screw Is backed off all the way. Do I need to adjust the secondary Idle screw? Thanks for the advice in advance
I'm afraid if I screw it in much more I might starve my eng. Runs nice and smooth otherwise. Idle screw Is backed off all the way. Do I need to adjust the secondary Idle screw? >Moore ++++++++++++++ Uhmmm ... two issues here. In order, you must not use the mixture screws to adust idle speed, those should be adjusted for best mixture only, which generally means to peak the idle speed [or manifold vacuum] at whatever idle stop you choose to use. You need to remove the carb, turn it upside down, and figure out why the butterflies are not going completely closed when you have the idle stop all the way out. Apart from whether or not the carb is 'too big', you should still be able to just about completely stop air flow past the butterflies, regardless. I'd pull the various vacuum connections first -- pcv hose and power brake hose -- verify you aren't fighting a giant vac leak of some sort. Regarding size -- at 6k rpm your 302 will suck 524 cfm, assuming 100% volumetric efficiency. A 600 cfm carb would likely be big enough. The formula: 302 x 6000 divided by 3456 = 524cfm.
In addition to what BB suggested,,,Check to see how far that secondary screw is turned,It should only be used as an additional aidd to get and idle HIGHER,,, On the primary side if you are all the way out the throttle plates should still have a very slight opening and approxamately no more than .040 up the transfer slot..My first check would be the secondary adjust,It possible the Idle Feed Restrictions are much larger in that 750 compared to say 600CFM.. THat would be giving additional fuel at idle.You'd have to check the carbs specs..
Just a thought after you check out that you have no vacuum leaks, disconnect your throttle cable/rod just to make sure it's not holding your throttle plates open. That could be your culprit.
NO vacuum leaks . I adjusted the a/f mixture with a vacuum gauge and after that I could not get the Idle down. stiffened the springs. Going to pull the carb now. I have not touched the secondary Idle but I will check when I remove the carb. Thanks All
The secondary idle was open a little bit. Its better but not perfect. It does like to stay on the fast idle cam. I turned the choke down a few notches. I have to drive it a block or two to redo the a/f with a vacuum gauge.
I bought it for my 351C about a year ago. Ive spent to much on eng., trans, and rear end rebuild. I'll have to wait for another carb. It runs good but I have to wait another 450 miles to find out how good. Thanks for all the input everyone. Idle is 600 in D. 16 lbs of vacuum.
As it has vacuum secondaries you can make it work well with your set up if you spend a bit of time with it. You are not so over carbed that you really need to replace it.
