To lostinmyhead: If you have recently worked on the ignition system I would check the following (and basically start from the beginning): 1. Check to make sure the spark plug wires are on the correct plugs, the No. 1 wire is in the correct location in the distributor cap and the wires are in the correct sequence. This info can be found in a Motors Manual or online. 2. Make sure the rotor tip hasn't been broken or bent (sometimes this happens when putting the distributor cap back on). 3. You are using the No. 1 plug wire when hooking up the timing light. 4. If your timing marks are off only by a 1/2 inch I wouldn't think it would be enough to cause the backfiring you are describing. Aligning the timing marks to the recommended point shouldn't cause any problems (but, please be sure to read suggestion No. 5 next). 5. My last suggestion may cause this type of backfiring - Check the crankshaft dampner to see if it has slipped. I don't know for sure about Buicks but other GM dampners (Chev and Olds) are actually made of two pieces with a piece of rubber in between. After several decades of age the two pieces can slip in relation to each other and cause all kinds of havoc when you go to time the engine. I had this happen on a 1960 Olds 371 and I had to time the engine by "ear" - another words, by turning the distributor back and forth until the engine ran/started the best (The marks were a good three inches off). A dampner replacement cured ALL problems. Determining if the dampner has slipped will be difficult - you will have to compare yours to a known good one and see if the relationship between the timing marks on the outer piece of the dampner is the same to the crankshaft keyway on the inside dampner piece. I hope this helps you. These recommendations are the result of my years of messing around with GM cars and I hope you get yours figured out. Thanks, BluesBrothersBuicks
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