Servere backfiring solved - ignition distributor at fault

[jjs3rd914]

Thanks to this forum's search function which allowed us to diagnose the following.

Recently purchased 74-911 (stock with CIS) was backfiring thru exhaust pretty bad upon decel. Checked the timing with vacuum connected at 900 RPM and was at 0 degrees. It is suppose to be 5 ADTC. Reset timing and car was even worse. Then checked at 6000 RPM with vacuum disconnected (all per manual) and found there was no mechanical advance upon reving motor.

Pulled dizzy, (stock Bosch 0 231 169 008) disassembled and found the mechanical advance was frozen solid. See photo. By applying penetrating oil and WD-40 finally got the mechanism to work properly. But we cannot figure how to get the rotor (top) portion of the mechanism off so we could clean up the shaft. The felt plug in center of the top of shaft was removed but there is not set screw or clip, so we are at a loss of how to remove.

By continually flushing the parts with penetrating oil, the mechanism is now free. Dizzy reinstalled, timing set per factory and car runs great-no more backfire. But would like to thoroughly clean the parts so how does the top portion get removed?

Thanks again

Jeff
-74 911

PS, during reviewing prior posts, there was some discussion where the vacuum advance was totally disconnect to allow car to run even better. If this is true what are the gains by doing so? So what would the timing be set at at 900 RPM with vacuum disconnected or does one and just set the timing at the 30-35 degrees BTDC at 6000 RPM and be done with it?

[T77911S]

if you want to remove the vacuum retard, leave the timing where it is, remove the vacuum and reset the idle. full advance is what you are concerned about. the factory gives you a spec at idle to get the full advance you need. then note the idle timing for future reference.

since your full advance does not change, there are no power gains, perhaps some drivability changes in the lower RPM's. also, the retard is out by 11 or 1200 RPM's anyway. you can check that by removing the vacuum, note the timing advance, then connect it back up. then rev the engine and note the RPM when the timing is at the previous noted timing.

mine is still connected, i prefer the sound of the idle with it connected and i could not tell any improvements with it removed. i can drive my car down to 1000 RPM's in 5th, and back up to speed and it is very smooth.

also, if you remove the retard, the mixture at idle will be richer and there is no way to lean it back out without changing it across the entire RPM range.

removing it can make the engine temp, AT IDLE only, go down, but who spends a lot of time at idle.

most on here remove it, i am one of the few that does not.

[Two Rivers]

To remove the advance top part I use dental picks to remove the small steel ring.
It is held in with a steel ring under the felt, the picks are angeled a bit so you can spread the ring with the tips of the dental picks.

Putting it back together the steel ring just snaps back over the shaft.
Do not use grease on the shaft just oil when putting it back together. The grease will not let the advance work proper when cold.