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1983 W150 318 auto with ECU ignition and Holley 2V carb.

Recently bought the truck (last March) from original owner. Barely ran. Did some basic repairs, new plugs, replaced the vacuum lines, reconnected the disconnected systems (smog pump, EGR, PCV, evap). Replaced the A/C and everything associated except evaporator. Changed the valve stem seals and that seemed to stop any smoking that it had at idle.

Reset the idle speed and mix with a tach and vac gage (750rpm/18-19 in Hg) and it was running pretty decent.

Until it wasn't. I was driving to work and went to pass a car on the highway and got loud pop with sudden loss of power. I pulled over and there was a regular small backfire through the carb. Made it to work, and about halfway there, power restored somewhat and the backfiring subsided. Still idled poorly and had a really strong stumble off idle.

Started trying to figure out the issue. Noticed the ignition module was leaking down the firewall, so that was replaced with a GM HEI setup and e-coil. Grounded all the components back to battery (alt, voltage regulator, coil and module heatsink). Still no improvement or worsening of the vehicles condition. Went to fill up and noticed a massive amount of pressure in the tank. Checked the evap canister and it was puking carbon out the tank and bowl vent lines and apparently completely clogged up. Disconnected and bypassed all of the evap system, plugged the vacuum lines to the canister and rebuilt carb.

Exact same conditions. Lopey idle and stumble off idle but good power at part throttle to WOT. Noticed chuncks of cat being ejected from tailpipe into my driveway and an occasional backfire when dropping throttle suddenly.

I have no ideas where or what to do at this point.
 

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What's manifold vacuum now,have you rechecked it?

Any mechanical noise associated  with these problems,how many miles on engine?

Cat substrate chunks out of tailpipe could be related to backfire or a symptom of running poorly overheating cat and breaking up substrate. Is cat original,is there a small one on left crossover  pipe to Y.

Put a timing light on it and see if marks are where they should be.
Pull # 1 plug and bump engine around to tdc on #1 cylinder,take cap off and check rotor position in reference to #1 terminal in cap.
Put a ratchet/socket on crank and turn engine slightly back and forth while watching for rotor to turn and see how much slop there may be in timing chainIt may have jumped a tooth or more.

Pull all plugs and look at them for one that doesn't look like the rest. Do a compression test.
Last,pull valve covers and look at all pushrods/rocker arms,any noticeable play in any of them?
Somewhere in doing these checks the cause may be uncovered.
 
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