In Reply to: Backfire on Overrun - Mixture or Timing? posted by Hamish on March 07, 2000 at 13:01:04:
: My 76 (32/36, pertronix, blue coil, no emissions eqpt) is increasingly starting to backfire on the overrun. My question is, is this a timing, or a mixture issue?
: The engine conditions are one of high vacuum and low load. With the throttle plate closed, this would make the idle circuit the primary factor in how much gas reaches the engine. However the higher than normal vacuum will pull extra fuel through and increase the mixture richness, correct? The fact that it is backfiring indicates to me that all the fuel is not being burned and is making its way out the exhaust valves.
: Though I have removed the smog pump and thermal reactor, my 76 still has its vacuum retard lines attached (whether they are actually functioning properly I cannot say). Is it possible that the higher vacuum is actually retarding too much and causing this problem? Would I be better off disconnecting the retard altogether? Or is it a carburetion issue after all?
: Thanks for your help.
In my experience it's mostly a fuel thing-- your assessment of the high vacuum/rich idle circuit condition seems sound. Though vacuum retarded timing could make the problem worse, I think the vacuum retard only changes the timing by a few degrees. If your ignition seems OK otherwise (like it's advanced enough at idle, and there's decent power at speed), I would think that the vacuum retard is not the culprit. When you're off idle, the spring/weight mechanical advance mechanism assures that you are several degrees advanced from idle anyway, so the vacuum only brings you back closer to idle advance. In the pre-emissions days, hi-performance carbs (like sidedrafts) backfired readily--even in "good" tune, you'd hear a burbling of rich mixture exhaust popping when on deceleration from high RPM. This overrich condition is one of the main things they tried to fix with all of that mid-70's emissions stuff. EGR is supposed to recycle the HC-rich gases while decelerating, and there is usually a vacuum-operated dashpot that prevents the throttle from snapping fully, immediately closed at high RPM. My advice would be to install the dashpot if it's not there, or fix it if it's broken. You need to choose the "right" vacuum source for both the dashpot and the distributor, too. Although it's tempting to remove all of the emissions stuff, the dashpot doesn't really impact performance, so it's worth leaving.