DBez93

To start, my car is a 1978 L82, but the engine is a "hecho an mexico" lm1 (working on the fix for that now)
About a month and a half ago the car began running extremely rough anywhere past idle, & backfiring (confirmed through exhaust only & not carb)
I've replaced:
Plugs
Plug wires
Distributor cap & rotor
Coil
Fuel filter
I'm still running the original quadrajet on the engine, & it's done fine until now if it were the problem, no definable flat spots through the curve.. I'd also like to point out the timing has been reset to 10 past tdc w/o vacuum & I've resorted to filling with real gas instead of gasahol because there's still a few stations selling the good stuff out here, I'm at a loss, & I was looking for any input or ideas as to what could be causing the problem, any HELPFUL input would be greatly appreciated.. I'm not trying to drop $300+ on a carburetor for this engine when I've got the bottom end for a 383 on a stand in the living room (much to the frustration of my girlfriend)

'75

Sounds like the cam has a bad lobe, not opening one of the valves or something wrong in the valve train. I would remove plug wires to see what cylinder is causing the problem, then remove that valve cover and investigate to see whether all valves and rockers are moving correctly. Also 10 deg after tdc is way too retarded, should be timed well before tdc.

leadfoot4

Just for the sake of information, what kind of motor oil have you been using in the car? Is it oil suited for flat tappet cams (ie, oil with a high ZDPP content)?

DBez93

I meant to say 10 degrees adv, at idle without vacuum, I've used royal purple for the last 2 years, & still added zinc addatives for peace of mind trying to be nice to the cam & lifters, between 2000-2500 rpm it rears it's head, but if I back off to barely maintaining speed at those rpms it mellows out to being much less defined, under load it sounds exactly like a full on miss, 10° was what they rolled off the assembly line stock with, & considering the stock cam would be an upgrade I don't see any need for further advance

DBez93

So today I tried to start the car & it was unusually difficult to start, but after finally getting it running it was running normal, perfectly through the rev range, right up until I reached 3k rpm, then almost like someone flicked a switch, it began backfiring every combustion stroke, I let out of it, & it was giving me the same misfire I'd had the entire time before today since the problem reared it's head, when it did start it was clouds of blue smoke..

MajD

Blue smoke at start-up can be caused by oil dripping on the piston after shut down. I wonder if you may have a bad exhaust valve. That could explain the backfire through the exhaust because the valve no longer seals the chamber. It probably didn't backfire right after start because the exhaust needed time to get hot enough to ignite the fuel mixture. As you increased the engine rpm, the exhaust heated up quicker and bang.

I'm not sure pulling the plug wire will help isolate the problem if the fuel is igniting in the exhaust system. If the valve is not sealing, the cylinder should fail a compression test. I wouldn't run the car anymore as you could rupture the exhaust system. Just my thoughts...