msd distrubitor question

[Joe-JDC]

Any time you change between boxes you should recheck the timing.  My MSD 6AL-2 boxes are 4* different.  Joe-JDC

[GerryP]

As Joe suggests, you should check the timing.  Under what conditions is it backfiring?  Is the sound coming from the intake or the exhaust?  Are you running + battery voltage to the B terminal of the module and that wire also feeds the coil+?

[fryedaddy]

it backfires randomly while idling or at low rpms and it backfires when it idles down from a rev.could low fuel pressure cause this,i was worried the foam in my tank might have stopped up my holley black fuel pump.but i havent checked the timing either.

[Falcon67]

Any time you change between boxes you should recheck the timing.  My MSD 6AL-2 boxes are 4* different.  Joe-JDC

This, very much.  And check your wire polarity on any mag pickup as it can vary between models. Get them backwards and you get all kinds of weird behavior.

[GerryP]

it backfires randomly while idling or at low rpms and it backfires when it idles down from a rev.could low fuel pressure cause this,i was worried the foam in my tank might have stopped up my holley black fuel pump.but i havent checked the timing either.

Okay.  I think I know what's going on.  Make sure you have the distributor wires on the correct module terminals.  If you reverse the wires, the ignition timing ends up very advanced.  Most people don't know how the pickup works and how the module interprets the signal.  It's electromagnetism, so it just works mysteriously.  All modules work on the same "zero voltage" principle.  As the reluctor approaches the pickup's magnetic field, a negative signal is generated, then zero voltage when the reluctor is centered on the pickup's magnetic field, then voltage rises as it continues to pass center and out of the field.  It is the negative to positive swing over zero voltage that produces the ignition signal.   If the wires are reversed so that the signal goes positive first, the unit will trigger early (as the rotor approaches the pickup). This will cause early timing. The trigger point will change depending on RPM, as higher RPM will create higher voltages.

[fryedaddy]

its wired up like the diagram says,i cleaned the foam out of my tank,new filter and cleaned holley fuel pump,put new plugs in and it quit backfiring.dont know if it was fuel related or if i had a bad plug or plug wire connection.i remember i went for a ride a couple years ago and it started backfiring.i went home and found a plug wire off.

[fryedaddy]

everything is running good now,i checked my timing,it was over 3 degree more advanced than it was with the box.it was 33 before and almost 37 after switching to a module.i backed it back down to 33 and now its so responsive you would think its a small block.