Backfiring issue thru carb

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92Raiderburban

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I want to ask after searchin and getting various answers, is this a LEAN or RICH condition when this occurs?

Backstory:
Carb converted L29. HEI has new internal guts. New platinum iridium plugs. For the last 3 weeks I had very bad backfiring out the top of the 4 barrel and truck didnt run right, dead stop acceleration being very sluggish. The choke being completely open would result in worse backfiring than when it would be barely open upon cold starts. 3 of the plugs when pulled were oil fouled, looked nearly the same as valve carbon build up on any high mileage engine, caked with black sludge and grime. One smelled like gasoline a bit. The rest seemed ok but showed their aging. Changed them out about a week ago, and all was good until last night. Started back firing again. Performance is still way better than it was a month ago but I'm noticing a steady decline as I have to open throttle more to get it move. This engine is burning oil, (bluish smoke out the exhaust) and need to top off with 2 quarts every 8 days. Could this have something to do with it? And if so what are my next steps to eradicate this problem?
 

GoToGuy

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Compression check, from what you described, and an unknown history I would do a complete health check. Total compression, Differential compression. Timing, is fuel pressure correct for Qjet (?), regulated or still full flow electric pump? First guess. Anything still pcm controlled?
 

Hipster

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If it ran good with a new set of plugs until they oil fouled again a week later, it needs another set. You need to be making plans to address this massive oil consumption issue. Lean pops out the carb and so can a burnt valve, but you can't really make that determination if the plugs are fouled. That spark enegy grounds somewhere and if it doesn't fire across the plug it grounds somewhere else and can take ignition components out. Like said , you really need to determine the mechanical condition of the engine. Compression/ leakdown tests etc.
 

Schurkey

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is this a LEAN or RICH condition when this occurs?
Maybe. Of the two, far more likely lean than rich.

Carb converted L29. HEI has new internal guts. New platinum iridium plugs.
4-pin module HEI? Or something else? "New" decent parts, or "New" Communist Crap?
I've never heard of "platinum iridium" plugs, and it would be crazy to use either Platinum or Iridium plugs in a carbureted vehicle.

I had very bad backfiring out the top of the 4 barrel and truck didnt run right, dead stop acceleration being very sluggish. The choke being completely open would result in worse backfiring than when it would be barely open upon cold starts.
Worse with the choke open than when closed should be a clue that "rich" isn't the problem.

Do not fail to consider a leaking intake valve, or an exhaust restriction including an exhaust valve that doesn't open. Same with ignition timing problems including crossed plug wires.

3 of the plugs when pulled were oil fouled... ...This engine is burning oil, (bluish smoke out the exhaust) and need to top off with 2 quarts every 8 days.
Intake manifold leak? Oil coming through the PCV system?

As has been said...sounds like this engine needs extensive repair.
 

BeXtreme

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Take all the plugs out and run a compression test dry. Then put some clean oil(couple tablespoons worth) into each cylinder and do it again. If you have low compression in one cylinder and it comes up significantly with oil in the cylinder, you have bad rings. If you have good compression, you almost certainly have a massive intake manifold leak into the cylinders that are showing severe oil fouling.

I would also hook up a vacuum gauge and see what it looks like. If you are pulling low vacuum, it is likely a big intake manifold leak. If you see the needle jumping around a bunch, you should check your valvetrain for broken/bad components. If they all look good, you probably have broken/worn rings.
 

351FUN

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Backfire is usually too lean, but like everyone has said there's a lot more going on here.
 

L31MaxExpress

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If it is burning 2 quarts every 8 days the engine is done. replace it or rebuild it
I have seen a 5.3L burn 1 qt every 500 miles through the valve seals alone. Replaced the valve seals in it and it burned 1 qt every ~2,000 miles.

Need to verify compression and where the oil is being burned before condeming the engine to a replacement or rebuild.

I have also seen engines burning a lot of oil because the oil control rings were carboned up with sludge. A quart of marvel mystery oil in fresh oil and a few short oil change intervals had it burning half the oil in no time because the MMO cleaned up the carbon in the rings and allowed them to free up, expand and work properly.

One of my 8.1Ls had oil fouling plugs when I bought it, it had oil in all 8 intake runners and all 8 intake ports had broken plastic around the rubber gaskets. The carb intake/gasket choice may not be sealing the intake ports and allowing oil to be pulled into the cylinders. Would also explain the lean, crappy running, air that bypasses the carb is not getting fuel added to it.
 
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92Raiderburban

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Compression check, from what you described, and an unknown history I would do a complete health check. Total compression, Differential compression. Timing, is fuel pressure correct for Qjet (?), regulated or still full flow electric pump? First guess. Anything still pcm controlled?
Removed the in tank pump. Have an external low pressure pump that is maxed at 6 psi. Nuthin related to ignition or trans is pcm controlled. Bypassed and disabled.



Maybe. Of the two, far more likely lean than rich.


4-pin module HEI? Or something else? "New" decent parts, or "New" Communist Crap?
I've never heard of "platinum iridium" plugs, and it would be crazy to use either Platinum or Iridium plugs in a carbureted vehicle.
New decent parts, replaced since this HEI was in my last truck. And what? I thought an OG like you would've heard of them. I've been running them on all my GMTs and 70s Cadillacs for over 10 years. Platinum iridium last longer than copper and there is noticable difference in performance too. Any carbed engine below 10:1cr needs HEI and hotter spark that platinum gives. That's just the standard these days with piss gas, if you want the most out of it .


Worse with the choke open than when closed should be a clue that "rich" isn't the problem.
Yea what got me thinking tho since it runs better when the ambient temp is 75 and lower vs when it hits 90. Seems sluggish when this warmer. Which is a rich indication


Do not fail to consider a leaking intake valve, or an exhaust restriction including an exhaust valve that doesn't open. Same with ignition timing problems including crossed plug wires.


Intake manifold leak? Oil coming through the PCV system?

I'll check for more leaks, haven't seen the back of the block, might be one there but as of now no drips from oil pan or bottom end

As has been said...sounds like this engine needs extensive repair.
That's funny, cuz I've seen these BBCs do this before. Wasn't my trucks so I didn't diagnose then, so now this time figure that out since Chevy never corrected that problem in 30 years of BBC design. Even seen the 8.1 do the same in U-Haul trucks

I have seen a 5.3L burn 1 qt every 500 miles through the valve seals alone. Replaced the valve seals in it and it burned 1 qt every ~2,000 miles.

As I mentioned above this too even LS had the oil burbing problem.

Need to verify compression and where the oil is being burned before condeming the engine to a replacement or rebuild.
Of course, Im gonna start tracing as some of these answers are vague in this thread. Chevys being notorious for oil burning altogether and me being a Pontiac guy where that just doesn't happen. Might have a leak at the back possibly as I noticed early today theres a bit wet near exhaust port where the exhaust manifold juts down to the pipe.


I have also seen engines burning a lot of oil because the oil control rings were carboned up with sludge. A quart of marvel mystery oil in fresh oil and a few short oil change intervals had it burning half the oil in no time because the MMO cleaned up the carbon in the rings and allowed them to free up, expand and work properly.
Nice to know. Been lookin around for answers on Marvel, heard nuthin but good stuff about it.

One of my 8.1Ls had oil fouling plugs when I bought it, it had oil in all 8 intake runners and all 8 intake ports had broken plastic around the rubber gaskets.
Yep I did a move in 2016 and the U-Haul truck with the 8.1 had consumed 3 quarts in under 700 miles. A widespread Chevy problem lol

The carb intake/gasket choice may not be sealing the intake ports and allowing oil to be pulled into the cylinders. Would also explain the lean, crappy running, air that bypasses the carb is not getting fuel added to it.
Yea I have an aluminum rectangle port intake (with rectangle port gaskets) to the oval port Vortec heads. Done this before on a Gen/Mark IV and had no issue whatsoever. I search for any other possible cracks that might be pulling in air around it. I did so a few weeks ago but that could have changed again. After all is done I will then begin with the valve seals
 
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