Crazy issue with timing marks

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21jtb12

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Trying to figuer out why my timing mark jumps around when using timing light. Iv replaced the whole distributor,tps,iac,map,new egr but have it capped off on egr and vacuum from throttle body. Im having difficulty setting timing due to its like its never the same from start up to start up. The timing light when using it goes from 10-12* advance down to 0ish* while idling. My tach i installed jumps rpm itll show steady 6-700 rpm at idle and jump to 4-5000 rpm but no rev from motor. If timing set it will either be harder to start but run good when warm or itll fire up perfectly but run like crap when it qarms up. Please help anyone im lost on it at this point but its the only truck I got so need to figuer it out 88 gmc c1500 5.7 tbi sbc 350
 

21jtb12

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I tried it and a couple times wouldn't start. So hooked back up and have been adjusting it by ear and drivability a little at a time. Its hard to set when its not the same every time. Im thinking my ecm or I think its called electronic spark control might be bad. Here's a pic of the spark control(i think thats what it's called)
 

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Nad_Yvalhosert

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The EST is a 1 wire: brown with a white tracer (or is it black) right by the distributor.
Start the engine, unplug the 1 wire, THEN set timing.
Shut off the engine, reconnect the 1 wire, unplug the battery to clear the codes, step on the brake pedal to deplete any capacitors in the ECM, then reconnect the battery.
Start, drive, and enjoy.
 

Nad_Yvalhosert

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I tried it and a couple times wouldn't start. So hooked back up and have been adjusting it by ear and drivability a little at a time. Its hard to set when its not the same every time. Im thinking my ecm or I think its called electronic spark control might be bad. Here's a pic of the spark control(i think thats what it's called)
Wrong module, too many wires.
 
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