71 LA 340 timings moved!! how?? looking for help

Hope we all had a good Christmas.... so today I thought I'd get back on it and while there replaced the MSD Blaster 2 for a Blaster 3 coil, checked timing once again and still sitting at 32 at warm idle, so I adjusted the timing back and its now sitting at 17 at idle.... so at that point I took it just up the road, as it was idling nice and sounding? I don't know? stronger? lol!! anyway not out the close give it a little bit of throttle and got a bloody loud back fire out the exhaust, then it stalled out on me... I was super low on fuel so put a gallon in got it to start after a few attempts, but now don't like to idle and the back fire is still there. god dammit!!! so now parked up as dark now....

Any ideas? and to add I noticed today a noise from what sounds like driver side valve cover, kinda like a rattle, not a knocking or tappety sound??? could this be related?? its been -5 temp wise here in England and its not been used in well over a week.. I'm guessing valve cover off, but other than anything obvious not sure what I'll be looking for?

I'd really appreciate any advice people, I must say of all the cars I've owned this is my first old old muscle and the steepest learning curve I've experienced to date!! I guess no experience with timing, carbs and coils etc is really screwing me over