if i hold the starter and the trucks fires off i can get it to run at 2000 to 4000 rpms but let it drop to 1500 or less it backfires and dies. also the electronic tach is jumpy and does not hold steady. i have a 235 with a pertronix ign. any suggestion, i have been trying for 2 weeks to figure this out. HELP!!!!!!!!!!!! RON