Backfire and I can't found the cause!!!

[Nitrometano]

Hi,
My Porsche have a backfire and I can not found the cause. Is a '72 911T with Weber carburetores, headers, MSD 6A, Crane Cams XR-700 electronic ignition, Accel 140001 coil and MSD Timing control. I installed new spark plugs, new fuel filter, clean the distributor cap and rotor, fix and air leak between the right Weber and the intake manifold, clean all the idle jets and mixture screws and passages and air filters. I check the ignition timing and is correct, both in the distributor and in the timing controler.

Since 3 weeks ago, I notice that everytime that I shut off the engine, after 1 or 2 seconds, the engine make a loud backfire. Some times it blown fire thru the exhaust pipe at the time the backfire occures. Also I check the carburetors for calibration and mixture adjustment and notice that the right side Weber, the idle screw that moves the linkage are more tight that the other side, in other words, the right side have more opened the throttle butterflies to get an accurate calibration like the left side. The engine is not "dieseling". Also the engine is not running very strong before that happened.

Please help me with the problem to found the cure for that poor guy.

Thanks,
Nitro

[Joe Bob]

Does it it do it as well on decel?

[Nitrometano]

No, is only whent I shut off the engine. I shut the engine, 1 to 2 second pass and then...the backfire comes. Only one. Is like the last breath of the engine.

[Rick V]

I had this problem on an old truck of mine the cause for me was the carb was leaking down when it got hot. It was allowing fuel to run into the intake. When shutting the engine off the ign would shut down (eletronic) and the engine would finish about 1/2 of a revolution. This created just enough vacume to pull fuel into 1 cylinder BANG.
I would start looking at your carbs. Make sure the butterflies are fully closed and that your floats are properly adjusted. Hope this helps. Good luck

[Nitrometano]

Hey Rick, that is interesting. I would check the buttlerfies of the right hand weber because they are more opened than the left side. Also were I synchronized the carbs, the right bank need to be more opened that the left side bank to get an equal reading.

What can cause this? Clogged air neddles?
Also what are the basic setting for the air adjusting screw? One, two turns?

Anyone knows were I can buy a float gauge for the Webers? or if there is a home method to check it? Also what is the best way to clean all the passages of the carburetor. Thanks.