Blowback is caused by the 4-way activating too quickly relative to the sear releasing the hammer. To cure blowback, you want to create a longer time period between the time the sear releases the hammer and the time the 4-way activates. You do this by 1)turning the hammer lug counterclockwise (thereby shortening it and making the sear release it sooner); 2)lengthening the 4-way rod (thereby causing the 4 way to engage later in the tirgger stroke); 3)doing both 1 and 2 (you'll probably have to do both).
Lengthen the 4-way rod far enough that you can pull the trigger to get the hammer to drop, but NOT have the 4-way activate. From there, its just a matter of shortening the 4-way rod to speed up your firing cycle, but not short enough so you get blowback.
Make sure not to adjust your hammer lug out too far, or else the lug won't catch the sear.