Originally Posted by martyminnesma
ok so background info. its a 1972 chevy c10. rebuilt 350 with a mild cam, hydraulic lifters, roller rockers, edelbrock aluminum e-street heads, performer intake and a 1406 carb.
it used to run just fine, but slowly over the course of a couple months it ran richer and richer. to the point where it was dieseling every time i turned it off. I even changed the jets and metering rods twice to lean it out, but it still ran really rich. I'm currently at the 13 setting on edelbrocks little jetting chart.
most recently i drove it on a 300mile trip and i couldnt put the gas pedal past a gentle cruise without the motor shaking around and stumbling. If i dropped a gear and revved it up it would accelerate slowly but pop and bang out the exhaust like crazy. gas mileage went down the tube on this trip. i originally did the 300 miles on one tank, the way home with it running like this took almost two full tanks.
upon getting home ive done a few things that have helped lesson the symptoms of my problem, but i think the main issue is still there.
- I changed the weights and springs in the HEI distributer. total timing was coming in at 44, with idle at 8. vacuum advance was on ported.
- after the new weights total timing was a proper 36
- I tightened the T-fitting for the brake booster manifold and sealed it more properly
- i redid the intake manifold gaskets, thinking they were leaking, but they were not, same with carb gaskets and anywhere else that uses vacuum. Im 99% sure vacuum leaks are not my problem, I've checked with carb cleaner and ether. nothing found.
- ive disassembled the whole carb, cleaning it with carb cleaner and the compressor. also double checked the floats and needle and seats. cleaning it single handedly made the biggest improvement, but still didn't entirely fix the problem
- i changed the vacuum advance to manifold. definitely runs better, more low end power, a little less rich running.
- gone through and metered out the spark plug wires, all are fine. all are in the correct order.
i still have to get a reading on my fuel pressure and I also want to get a vacuum gauge to make sure i dont have any leaks.
also to describe the sound more clearly. when i'm accelerating hard in second or third, the truck switches back and forth between running great with strong acceleration and a smooth sounding exhaust to running rough with poorer acceleration and a louder crackling exhaust. it goes back and forth every second or two.
my best guesses are a bad coil,
a bad ground (is a solid number 12 awg good enough to ground the block? worked before this issue came up),
possibly a bad spark plug that comes on and then quits.
perhaps a sticky lifter on an exhaust valve (but would that cause the terrible gas mileage?)
sorry for the huge essay, just want to get you guys as informed as possible so that you can have the easiest time helping me figure this crap out.
The after-fire into the exhaust is an indicator of 3 things.
1) Rich mixture
2) Failing ignition
3) Cam falling out of time
Unfortunately all 3 can make the exhaust appear rich in the first case because it is, in the second and third cases because of failed or incomplete burns.
Happening over time shows a progressive change going on which doesn't eliminate any of the three suspects.
1) Rich mixture:
- Excess pump pressure at the carb to where the float controlled shutoff valve can't
- One or more of the floats is becoming fuel logged then sinks some or completely.
- The vacuum controlled piston that operates the primary metering rods is sticking, or the vacuum isn't applying do to a clog somewhere in the carb or a gasket failure or missing or incorrect type for the carb model, metering rod bent or not in the jet.
- Exhaust plugged
2) Failing ignition:
- Can be the trigger, if points it can be the points themselves or the shaft on the distributor is worn so it wobbles giving a constantly changing dwell, or the condenser can be bad or the voltage from the key switch unstable, points use two circuits from the key, battery voltage while cranking and resistor (usually resistive wire on GM) that supplies 6-9 volts to the coil when running. If an HEI this can be the module or wiring, these use battery voltage all the time , one needs to be careful when introducing HEI to a points vehicle to rewire the "run" circuit with no resistor or resistive wire.
- Certainly the coil could be going out, they fail in a host of ways including almost but not quite. When you demand more power the temperature and pressure in the cylinders goes up, this increases the dielectric strength of the mixture (to say it resists the making of the spark) so this requires a greater effort* on the part of the coil, if it is not receiving sufficient voltage or itself is failing that margin will not be there so when the power demand on the engine goes up the spark misfires or late fires, either of these will make the engine appear to be running rich and in the case of a late fire will appear to be out of time.
*actually the coil has no way to increase its effort based on cylinder pressure, the coil is pretty constant in output, but if voltage or switching time is/are insufficient the output voltage will not be high enough or shaped correctly to sustain jumping the gap in the face of higher pressures.
3) Cam getting out of time.
- One thing that struck me was your redo of the springs in the distributor and stating before advance was 44 degrees and after is 36. First the spring should not be depended upon to regulate the maximum advance; that is the job of stops in the plate that limit the movement of the counterweights with a positive stop.
- A characteristic of the cam getting out of synch with the crank is the need to frequently have to readjust the base advance setting. This is also a characteristic of the distributor gear wearing---to that end you indicated that this is running a hydraulic roller cam which would not be native to a 1972 pickup, roller cams being made from billet steel or CGI iron are not compatible with 1972 distributor gears they wear the old gear fast. The racing community uses bronze or carbon/epoxy composite gears but these wear pretty soon if used on the street. The factory makes a Melonized treated steel gear on roller cams which provides the necessary long term wear for the street. So the question here is what gear is on the bottom of the distributor?
For the cam to be getting out of synch with the crank would be a wear or failure of the timing set, gears and chain. Normal operation can show timing with the light drifting 2 maybe 4 degrees, if you rev the engine and suddenly close the throttle that amount of wander should remain, if it gets more than 4-5 degrees the timing set needs replacement. The cam falling out (retarded because how it rotates demands that outcome, the engine will lose power first and most up in the top RPMs and/or under load, fuel mileage will drop as now the cam events are late regardless of ignition timing settings and the engine will appear to be running rich.
I'm adding this to say that once the plugs are fouled it only gets worse for them. Even if you fix the problems, they have developed conductive paths on their insulators that allow coil voltage to bleed to ground rather than jumping the gap, so you may need to go through several spark plug replacements before this is settled or that you can even tell the problems are settled.