Sometimes I just ignore the light, and reach in there and crank the D to whatever makes it run normal. Then figure who's lying to me.
Once it is idling normally:I hook up the light, and just rev it up slowly.
If it starts running stoopid and the timing light goes crazy,then; there is an excellent chance that the pick-up polarity is backwards.
But if it starts running stoopid without spark scatter, then the rotor or reluctor phasing is probably off. Or possibly the coil polarity is backwards.
But if it takes the revs OK, then probably the the timing index marks are off.
The above assumes the cam-timing is at least close and that ALL the valves are actually fully closing, and that the carb is not fubarred.
An easy check as to closing exhaust valves is to put your palm over the tailpipe during cranking; if it sucks your hand onto the pipe, you have a problem.
You can't really do that same test on a 4bbl, but if you are getting multiple backfires thru the carb with attending flames, there is a good chance the intakes are not closing. Lean missfires may be accompanied by flames, but sound like somebody fired a wet diaper on the concrete floor, or like somebody got punched in the diaphragm. Whereas when the already on fire fuel charge, pops back thru the carb, this is a more serious POW! Peak cylinder pressure is supposed to occur at about 15* ATDC, but the fire was begun at perhaps 10* before. So there is a 25* window there, that if one or more intakes is not closing 100% there will be fire in the plenum, and the pressure in the cylinder is gonna try and blast it backwards thru the venturi, totally destroying the vacuum signal, and with that the engine momentarily quits running. If just one valve is doing this, and the rpm is high enough, it might recover.If it stalls, there will be smoke in the intake and wafting up thru the carb. You can catch the pressure spike on a vacuum gauge.
No matter what, do NOT put your face over the venturis if the crank is turning; If/when you see the fireball coming, you will jerk back and inhale sharply. Your head will stop when it hits the underside of the hood, and you will suck that flame in baaaaaaad. And hopefully the stink of burning nose hairs,will be the only lasting ill effect. Well that and your wife is gonna laugh at you for two weeks until your eyebrows grow out. Not having nose hairs to deal with regularly tho,is not a bad thing.
