Not Firing on cylinder 2-3

[Doug12382]

Working on my friends 1981 dohc cb750. starting off the entire bike has been rewired with a moto gadget m-unit blue. Bike wouldn't start at all when i received it. I started with the carbs, ultrasonic cleaned them. they weren't that dirty but Definity had water in the bowls, I rejeted for pods & exhaust and bench synced bike started and immediately didn't want to run and was shooting flames out the exhaust with very loud backfires at low idle with a little throttle backfiring stops but its running on only cylinders 1-4. Sprayed fuel around carbs boots and carbs couldn't find any vacuum leaks. noted spraying fuel in to carbs 2-3 no changes in idle. spraying fuel in 1 & 4 causes the idle to drop and bike will shut off. (pulling sparkplugs wire 2-3 while running no change in idle or tone). Pulled all plugs they have bright blue spark. (appears coils, wires and spark units were previously replaced) ohm tested coils, wires and pickups no issues all in spec). swapped coils no change. swapped spark units no change. checked static ignition timing and cam markings they both are correct. compression tests shows all cylinders are close together but are all low under 100 cold ( gauge may be reading low). putting oil in cylinders and rechecking compression they only go up about 5psi per cylinder. checked valve clearances and only cylinders 1-4 exhaust valves are in spec. all others are to tight some being to tight i cant measure them with my feeler gauge set. Not sure what to do from this point. Should i do the valve adjustment? Note I was told the bike didn't have this loud gunshot misfire before it got rewired with the m-unit. Witch would make anyone believe its an electrical problem but i checked out the m-unit wiring and new harness and i couldn't find anything out of the ordinary. but who know what the last guy did with bike while he was rewiring it (he had the bike for over a year i think). I know valves being tight is not good but i don't think it would completely kill 2 cylinders would it? Any suggestions are welcome this bike has really got me confused.

 

[drgnrdr750]

Do the valve adjustments. A open valve will cause low compression, backfires and could burn a hole in the valve. A closed valve can also cause problems such as not allowing the piston to draw in the fuel from the carbs. While doing the adjustments you can also verify that the timing is correct by checking piston and timing marks line up, top dead center.

 

[Doug12382]

Update. Stuck with my gut feeling and Double checked the wiring harness and found that previous guy had the polarity of the 2-3 pickup flipped throwing timing way off causing the backfires. Bike now runs on all 4 cylinders again. Definitely something to keep in mind if your rewireing your entire harness on a bike.