Reply to "Ignition, vacuum advance question"

[QUOTE]Originally posted by No Quarter: Just added a Summit electronic distributor,SUM-850306 to my 351C/Longchamp. I set it w/o vacuum advance to 16 degrees. I noticed the engine had to fight preignition when starting, so I changed that to 12 degrees. Cars seems to run fine, steady idle and takes the gas fine. Vacuum however is only 14. Then I added the vacuum advance, taken from the port vacuum on the Holley 600. Idle unchanged (of course, it's ported vacuum), but when accelerating it pings and explodes in the exhaust, clearly too much advance.

Mikael, are you sure the harmonic balencer's timimg mark is correct? Its rare that a factory harmonic balencer hasn't shifted- always to the retard side. I have quite a collection of junk balencers, all with the mark moved. One is 160 degrees out! If the engine's 'fighting' the starter, you have way too much initial advance! A second thing that will cause popping under acceleration is a cold engine. Before hammering the accelerator pedal, be sure the engine is smoking-hot. A third thing is a too-stiff secondary diaphragm spring if the carb is a common vacuum-secondary Holley. Finally, don't get too locked into a particular number for advance. Just adjust the intial advance, drive the car and see if you can hear any engine rattling. If not, add a few degrees until you can hear pre-ignition or detonation. Then back off a degree of two and call the job done. Not all combinations of Clevelands behave the same. FIWIW, Euro octane ratings are measured differently than the U.S: 95 euro-octane is about 91 U.S.- what we call 'premium'.