3.0 litre GM 4- cylinder LP-GAS--Impco-Spectrum fuel system- no codes-basics all checked-ran program on lap top-fuel controls OK--backfires through intake-underload off idle-no vac leaks-- fuel inlet hose from LP convertor OK-timing control-OK feedback from some folks, they say crank sensor? engine jerks like a wet dist cap-Cap,wires,rotor and plugs all new--initial timing set--OK --system simular to Yale but has crank sensor at front of engine, HELP! IF ANY ONE CAN HELP,IT WOULD BE GREATLY APPRECIATED THANKS