idiling problems??

90-5.0-GT said:

I just picked up a 73 fastback and im restoring it. I did the timing over again from scratch because it wouldnt start. It starts up now but the problem is as soon as i stop cranking the starter it dies. its a edlebrock 600cfm 4brl carb and i just got new plugs and wies for it any sugesstions on how i get it to idle would help. thanks

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The chances are the idle circuits in the carb are not functioning, thus when you give it a shot of gas using the accelerator pump, it runs. As soon as you let off the idle circuit in the carb is supposed to kick in. It likely doesn't due to clogged passages in the carb.

1. Rotate the idle mixture screws on the carb fully closed (light touch). Open them about three turns each (count them). Look at the butterflies in the carb primary venturis. The butterflys should just be barely cracked open. About the amount of a very thin piece of wire. If not barely open, adjust the idle speed screw to achieve a barely open throttle. Pump the accelerator several times then attempt to start the engine. It should start and idle. If it does, then set the idle speed down to 600 rpm (use tachometer). If it does not idle down then close the mixture screws 1/2 turn and try again to reset the idle speed to 600 rpm. Repeat closing the mixture screws 1/2 turn until you achieve a satisfactory result. Once the rpm is low and controlled, then set the mixture screws as follows:

Edelbrock and Autolite: turn mixture screws down until an rpm drop of 20 is observed (this isn't very much so look at the tach closely, better to use a tach/dwell meter with large meter face than small dashboard tach). If rpm doesn't roll off then there is likely something wrong with the idle circuits in the carb. This procedure sets the Edelbrock and Autolite carbs to "lean idle roll off, the most economical setting".

Holley: close the idle mixtures until the engine starts to bog then reverse the direction and open the idle mixtures. Continue opening the mixtures and watch the tach for a rise in rpm. As long as idle continues to rise, continue to open the mixtures. When the rpm stops rising, then stop opening the mixtures. Make sure both idle screws on the Holley are exactly the same, hence move one a little bit then the other to obtain "max idle roll off". This is where the Holley will work best.

After adjusting the mixtures do a final calibration of the idle speed with the idle speed screw. This should be about 550 rpm for the stock V8 engine and about 600 rpm for a stock hot rod engine. Aftermarket street cammed engines usually idle better at about 650-700 rpm. Note that if the car is an automatic with stock torque converter, 650 is about as high as you can set the idle and not have the car creep at idle. Final adjustment is done out of gear in a manual trans car and in "drive" in an automatic. When doing a carb idle speed adjustment in an automatic, be sure to have someone in the drivers seat holding the brake (parking brake on) while the final touch is made. Don't want any cars getting away from you.

2. If the procedures above fail, rebuild the carb paying attention to the idle passages. Make sure low pressure air passes through them but be very careful to use filtered air. If you get a particle in the orifaces and jamb them in with high pressure air, they're likely never to come out again. Look at the idle mixture screws. If they're buggered up from a ham fisted mechanic that got to the car before you did then its likely new carburetor time.

Good luck,

Jeff