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It has been 10 yrs. since the engine was rebuilt and at that time I made a modification to a distributorless ignition system. The HPV.1 by Electromotive, Inc. uses a magnetic sensor to count the teeth on the trigger wheel mounted to the face of the crankshaft pulley wheel and the signal from the sensor to a computer broad that will cause the coils to fire the plugs. Over the years I have replaced 2 magnetic sensors which failed and caused engine to run eratic, & one of the 2 coils failed caused 2 of the cylinders to have no spark.
When the engine was rebuilt a machinest/engine builder that does racing engine rebuilds did the complete reassembly. New valves and springs were installed. He ordered shims from Delta as required to set up the clearances. A new camshaft belt was also installed. It has been 9 years 4 months and 11,645 miles since then and I have not checked the valve clearances, since I have not reached the suggested 15,000 miles as noted in JHPS standard, Dec. 2001 "Valve Adjustment on the 907 Eng - Part 1."
So with all as noted as some reference data, what would most likely cause the backfiring?(1) Camshaft belt jumping notiches on camshaft pulleys? (2) Valve clearance needing to be checked and adjusted. (3) Some type of failutre with the distributerless ignition system?
I would appreciate some thoughts that anyone may have as to what could be the cause of the backfiring.
'74 J.H. #14120
Robert Rawe