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Old 09-19-2013, 01:02 AM
martyminnesma martyminnesma is offline
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Running rough under load mystery.

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.
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