I have timed it with a timing light at 10degrees+. Vacum lines all seem to be in excellent shape no cracks or any thing suspect. It does not pop if I am in park revving the engine, it does not pop if I am going down hill and gas it. It only pops when under a load, the steeper the hill the sooner the pop (usually between 2750r's and 3500r's) If I dont let it warm up as long as I usually do (10 minutes or so,) the problem seems worse. It pops through the exhaust. I am running a fairly open exhaust, one shorty glasspack on each pipe, it is a 75 Jeep CJ 5 and I have also run the exhaust in frame, without a crossover. When I adjusted the valves I did not notice any cracked or broken springs. Just today I noticed a little bit (very little) of fuel seepage from the rear of the carb. I already bought a new gasket and will install it tonight after work. I dont think this is the problem, because I never noticed it seeping before and this problem has been an ongoing thing .