I'm running a 351W with MSD 6AL, Blaster 2 coil and Mallory 5755401 distributor with magnetic breakerless ignition. If I leave the car for a few days, it sometimes won't start due to total lack of spark. So I then do the spark test (disconnect coil wire to distributor and place 1/2" from exhaust and then ground the white trigger wire. After two goes, I get a spark. Immediately after I do this and put everything back together, the motor fires instantly. It's as though the ignition unit won't trigger without the spark test. Once the motor has been running, it will start all day, everyday until I leave it for a few days. Battery, starter motor, alternator etc all fine.

Any ideas? Is my 6AL on the way out?

Thanks in advance.