Larry, A lean condition can cause backfiring through the carburetor. There is a burning charge in the cylinder(s) as an intake valve opens. Lean burns late. A restricted exhaust can cause this also. There could be a timing issue. This could be something related to valve timing and this can be a worn camshaft or even weak valve springs or a sloppy timing chain. A leak down test will uncover valve seating problems. Before you ask, no, a compression test will not spot a worn out cam or a restricted exhaust except in cases where the exhaust is plugged. Probably not ignition related unless plug wires are not installed correctly and firing order is off or there is cross firing due to plug wire(s) routing error. But then again, I have known big block Chryslers to be very sensitive to the condition of ignition parts. Warren Anderson Sedona,AZ The situation: 1963 300 sport...413 w ith 105K miles, 152-160 pounds compression in all 8 holes, brand new plugs, wires, Edelbrock 650 carb, Mopar electronic ignition system. At WOT at 30 miles per hour in third, it coughs (backfires) through carb repeatedly. Is this a result of too much or too little advance of the timing or is it a carb problem concerning too rich or too lean? (New thought) today with 600 pounds of weight in a utility trailer behind, it did not do the coughing. Shouldn't be worrying about this with Lake George eminent, but hoping some tune up genius tells me what path to take to correct. [Non-text portions of this message have been removed] [Non-text portions of this message have been removed] ------------------------------------ To send a message to this group, send an email to: Chrysler300@xxxxxxxxxxxxxxx To unsubscribe from this group, send an email to bob@xxxxxxxxxxxxx or go to http://autos.groups.yahoo.com/group/Chrysler300/join and select the "Leave Group" button For list server instructions, go to http://www.chrysler300club.com/yahoolist/inst.htm For archives go to http://www.forwardlook.net/300-archive/search.htm#querylangYahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Chrysler300/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/Chrysler300/join (Yahoo! ID required) <*> To change settings via email: Chrysler300-digest@xxxxxxxxxxxxxxx Chrysler300-fullfeatured@xxxxxxxxxxxxxxx <*> To unsubscribe from this group, send an email to: Chrysler300-unsubscribe@xxxxxxxxxxxxxxx <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/