QuestionMy 78 Triumph Spitfire 1500 backfires when downshifting or when letting off the accelerator after hard acceleration. The timing was checked and is right on the money. What else could cause this.
AnswerI assume that your 78 still has most of the emission equipment, which is likely the cause.
Also, I assume that your backfire is out of the tail pipe, and not out of the carb.
Given that, I would check the connections on your vacuum lines to your emission components; the likely culprit is the "gulp" valve that is on the air rail, or the EGR valve & hose that activates it. Look for one cracked, fallen off, or rotted away. The EGR valve and/or gulp valve are original, they may need to be removed/replaced.
