Mike, The electronic conversion kits are probably the best $100 upgrade you could make on that motor. The time you will spend trying to diagnose the issue is probably worth more. The 4.3 is basically a 5.7 without 2 cylinders, I recently replaced the 4.3 in my 2001 Silverado (had a rod knock). I would expect the timing to be about 10-15 degrees btdc without vacuum. My first guess would be ignition, determine which cylinder has the miss by pulling wires while it is running (with insulation one at a time). Its possible you have a faulty new wire, swap some wires and see if the miss follows the wire, next try the plugs, try the old cap. If the miss is still there do a compression test. A poorly seated valve can cause a miss and low power. If the compression test reveals a low cylinder do a leakdown on that cylinder. Listen at the carb and exhaust to determine if it is an intake or exhaust valve. Hopefully that's not as you will have to pull the head and lap the valve seat. Too bad I just tossed my old 4.3 with good heads...
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