Quadrajet
Race Director
Joined: Jun 2000
Posts: 10,045
Likes: 675
From: FL
Ok, today I went out and worked with the timing. The vacuum line to vacuum advance was removed and plugged. My initial setting of
8 degrees was already set (factory specs). I double checked the idle setting with and without a rubber band on the weights, no change. At about 2800 RPM (any higher RPM's did not change the reading) the timing was 14 degrees more or at about 22. I turned the distributor until the reading was at 32 degrees (8 initial + 24). The can added on about 14 degrees. The engine seems to run ok, but now the initial setting is at 16-18 degrees at the slowest idle I can get. Ok now if the difference between idle and high RPM should be about 24 degrees difference, what do I need to do to correct the issue. It seems to me I may need lighter springs? I have a stock HEI distributor that has never been modified as far as I know. The manifold vacuum at idle at the higher initial setting of 18 was better. Maybe I am confused so I am looking for thoughts.
8 degrees was already set (factory specs). I double checked the idle setting with and without a rubber band on the weights, no change. At about 2800 RPM (any higher RPM's did not change the reading) the timing was 14 degrees more or at about 22. I turned the distributor until the reading was at 32 degrees (8 initial + 24). The can added on about 14 degrees. The engine seems to run ok, but now the initial setting is at 16-18 degrees at the slowest idle I can get. Ok now if the difference between idle and high RPM should be about 24 degrees difference, what do I need to do to correct the issue. It seems to me I may need lighter springs? I have a stock HEI distributor that has never been modified as far as I know. The manifold vacuum at idle at the higher initial setting of 18 was better. Maybe I am confused so I am looking for thoughts.
The reason that GM retarded the timing to 8 BTDC at idle and used ported vacuum for vacuum advance was in order to pass emissions. How often is your engine at less than 2000 rpm when driving?
You are now tuning for performance.
Last edited by SteveG75; Mar 17, 2010 at 02:46 PM.
Thread Starter
Intermediate
Joined: Oct 2009
Posts: 37
Likes: 0
From: West Michigan
Idle fuel is metered and restricted by the oriface in the Idle Fuel Restrictor Tubes. Power piston position has very little effect on idle quality if the carb is idling on the idle circuit. If the carb is set up so lean that it won't idle on the idle circuit, the raised power piston will cause an extreme rich condition through the main metering circuit if the carb is idling on the main circuit.
The 77 carbs tend to run very lean, both at idle and at WOT. You may need to enlarge the idle tube restrictions along with a significant jetting change. As a last resort, you can block the idle air bypass holes to force engine to pull more fuel through the idle and transition circuits.
Lars
The 77 carbs tend to run very lean, both at idle and at WOT. You may need to enlarge the idle tube restrictions along with a significant jetting change. As a last resort, you can block the idle air bypass holes to force engine to pull more fuel through the idle and transition circuits.
Lars
