1980 C3 with L-48 350 and 4 speed manual trans. Stock engine with 101k miles. Mods include, Weiand Street warrior intake, Holley 600 cfm carb with mechanical secondaries. DUI street/Strip HEI distributor with Taylor plug wires and Accel shorty plugs. Exhaust is Stainless headers with side pipes.

Issue presented itself this past weekend. This isnt a daily driver so it sits for a quite a while between use. 

At idle (cold or hot) i get a consistent miss on the drivers side of the sidepipes. I pulled all spark plugs to check the gap which all checked out fine with exception of some had fuel and oil (which i know i have an issue with valve stem seals leaking on the drivers side). I then checked all plug sires for cuts and chafing which checked out fine. Next i pulled the cap and rotor to inspect for wear, that also checked out fine.

This miss does get worse when you accellerate to the point of seeing a backfire through the carb. I watched a youtube video where a guy shows putting a piece of paper against the exhaust to show a proper working engine versus a burnt/stuck valve. On the bad engine, it sucks the paper into the pipe versus a good one only pushing out. I did this last night and it definately sucks the paper in and has a very rich smell with black soot in the pipe.

at this point i'm feeling i have a problem with the drivers side head and plan to pull the valve cover this weekend for visual inspection and try a cylinder leak down test on the drivers side. I think this should validate my thinking.

my two questions for the group.

First, do you think on on the right track ?

Second, if this is the case, next will be a set of heads will run around $1k and i would assume i should replace the cam and lifters while it's all apart which is another $300. So at this point i'm at $1300 plus miscellaneous gaskets etc, so i'll round it to $1500. Based on a compression test i did a while back where all cylinders are around 125psi i would assume i still have a few miles left in the motor, or would it be worth my while to wait a bit longer, add another $1000 or so for a crate motor ?

This could go either direction... if i do the heads and cam now and get another year or more out of the motor, i could re-use the heads, intake and carb on a new motor so they wouldn't go to waste, or just bite the bullet now and do the whole thing.

Thanks in advance

DarinM said: ...Based on a compression test i did a while back where all cylinders are around 125psi..

Adams' Apple said: Before you start ripping into this thing, let's do some basic checks.
Did you Ohm out the plug wires? Just looking at them(or the plugs) will not tell you if they are capable of carrying the spark. Use an ohm meter, and check the resistance of all of the wires. You're looking for consistency here, not specifics. If you have one(or two) wires that ohm far differently than the others, those wires are suspect.
Use a spark tester if you have one....test each wire to see if they will all fire across the same gap.
If you have a dead miss on one cylinder, check the wires first, then move the spark plug to another cylinder and see if the miss moves with it, or just install a new(or different) plug in that hole to see if the miss goes away. If it doesn't, check compression again on all cylinders, ignition unplugged, throttle held wide open. If that one cylinder is down on compression more than 10-15% of the others, then it's time to dig into it deeper.

Those two wires are routed beneath the L48 motor and it's easy to get them cris-crossed.

Happened to me that's how I know..Jim

DarinM said: On the compression test, they all came back at 125psi except cylinder 1 and 3 at 120psi, but i can certainly perform that again as well. 

Adams' Apple said:
Those readings are fine, but...was it missing when you took those readings? I think you're really looking at an ignition related issue here, not a mechanical one. I've had new plug wires fail within months of installing them, so new(or new-ish...or even high dollar) don't mean diddly.

See this is what i love about these forums. You can stare at an issue for hours and not see something in front of your face. It takes a new set of eyes to view from another angle !

jim in oregon said: Yep..ALWAYS check-doublecheck the most likely offenders first....:)Jim