I have a 1977 toyota celica gt coupe that has a built 20r and has an offinhouser intake with a holley 390 carb on it. When i warm up my car and then go about 5 miles then turn it off it will backfire out the exaust. it doesnt do it all the time just in the mornings after it has been running for about 20 minutes then i turn it off and about 3 secounds later it backfires but other then that it responds great under ecelaration and at an idle.