Vacuum source question.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Let's think about this. The car idles fine but drops low when in drive. Well when in drive the motor becomes loaded with the throttle blades still closed. What you really need is more air & fuel mixture. And a richer mixture requires less advance because it needs more time to burn completely. Adding more advance with higher load but same amount of air/fuel mixture is gonna make less power and drop idle even more.
What you want is what the qjets had were diaphragm pull ons to crack the throttle open more. Some were solenoid operated and some were vacuum I believe but I recall some Caddy qjets had at least 2 off those diaphragm canisters. In your case here the usual fix for carbs other than qjets would be to drill small holes in the throttle plates to allow more idle air. Also some newer Holleys and Demons have 4 corner adjustable idle circuits.
Now Cliff Ruggels of qjet fame claims the ported vacuum on a carburetor prevents the vacuum advance at idle until the throttles become partly open. This can help engines with big cams by delaying vacuum advance during idle with poor mixture due to the larger amount of overlap until the engine begins to breathe better (begins to come up on the cam). Don't know if this will help you but it's simple to try. Just locate the port that enters the carb throut above the throttle blades at idle/closed.
Hope this can help ya.
What you want is what the qjets had were diaphragm pull ons to crack the throttle open more. Some were solenoid operated and some were vacuum I believe but I recall some Caddy qjets had at least 2 off those diaphragm canisters. In your case here the usual fix for carbs other than qjets would be to drill small holes in the throttle plates to allow more idle air. Also some newer Holleys and Demons have 4 corner adjustable idle circuits.
Now Cliff Ruggels of qjet fame claims the ported vacuum on a carburetor prevents the vacuum advance at idle until the throttles become partly open. This can help engines with big cams by delaying vacuum advance during idle with poor mixture due to the larger amount of overlap until the engine begins to breathe better (begins to come up on the cam). Don't know if this will help you but it's simple to try. Just locate the port that enters the carb throut above the throttle blades at idle/closed.
Hope this can help ya.
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
Thanks Cardo I know you put a lot of thought into my problem.
I was able to get back in the garage over the last couple of days and do some trial and error. I tried ported vacuum with a AR12 canister. Car did not respond well to the low initial timing at idle that you get with ported vacuum.
I switch back to manifold and changed canister back to the AR31, added couple degrees of timing. Fine tuned my AFR with vacuum gauge and was able to raise my vacuum to over 10 in gear. Now my vacuum advance is steady in park as well as drive. I'm not getting a surging idle and have very little rpm drop going from drive to park. Thanks to everyone that offered suggestions.
I was able to get back in the garage over the last couple of days and do some trial and error. I tried ported vacuum with a AR12 canister. Car did not respond well to the low initial timing at idle that you get with ported vacuum.
I switch back to manifold and changed canister back to the AR31, added couple degrees of timing. Fine tuned my AFR with vacuum gauge and was able to raise my vacuum to over 10 in gear. Now my vacuum advance is steady in park as well as drive. I'm not getting a surging idle and have very little rpm drop going from drive to park. Thanks to everyone that offered suggestions.
