Backfire

[MustangMatt69]

I have a 1969 Cougar XR-7 with a 351 Cleveland that is far from stock. It has compression in the 11's with 4 barrel quench heads and a big hydraulic roller camshaft. It also has all of the MSD ignition upgrades in it such as: The 6-al ignition pro billet distibutor 15 degrees timing control, a 750cfm carb with edelbrock intake. When ever I am cruising around it back fires. It usually always backfires when I first hit the gas or get off the gas. I know that it is an exhaust backfire and not intake but what can I do to fix it. Right now I am running premium fuel with no octane booster. Should I add some octane booster and lean out the carb mixture a little bit. The 750 cfm I put on about a month ago, but I havent played around with the mixture screws, do you think that the mixture screws would be a good place to start? The old carb did not backfire nearly as much, maybe once in a blue moon.
Thanks alot
MustangMatt69

[Phillyfanfrombirth]

Hmmmm....

Causes of backfire...

Choke.

Vacuum leak / hose connected to wrong port.... the vacuum line that connects to the metering block is the one that goes to the vacuum advance, if you have one. The others are the full time vacuum sources.

Ignition timing (too retarded)

Crossed plug wires... or defective plug wires.

Poorly adjusted valves (people like to torque the hydraulic lifters down too far, then when they pump up with oil, they end up leaving the valves hanging open.)

Bent exhaust valve.

Timing chain set in a retarded position.

As far as the carb... after checking to make sure that the choke is in good working condition, which shouldn't be too difficult this time of year... the fuel bowl floats should be set at a level that allows the gas to touch the threads of the sight plug. The fuel mixture screws (2 if you have a double pumper) should be slowly turned in until the engine starts to sputter, then back off by 1/4 turn.

[MustangMatt69]

Thanks for the advice. I will try it out and let you know what happens. The ignition system doesnt have a vacum advance, becasue I got the pro billet with out it. The MSD timing control, alows me to adjust the timing 7 degree retard or 7 degrees advance, and it seems to me that it back fires even when it is in advance. Yesterday I found the 3 of the hearer bolts had loosened up and one had actually fallen off into the engine bay. So that explained where the exhaust leak was coming from. The car has always had a little leak but I could never locate it. I have heard the to rich of a mixture mixed with an exhaust leak can create an explosion in the exhaust pipe, which is know as a backfire. Let me know what you think, and once again thanks for the great help.
Mustangmatt89

[Phillyfanfrombirth]

Exhaust leaks can be the cause of backfires... your scenario does seem plausible.