Engine issue with my '68 :(
 

Just took the Vette out for a spin.. On my way home, I heard a backfire from the drivers side exhaust and the car would die.. It would start back up but refuses to idle. Was able to drive the car home but it feels like it's running on 6 or 7 cylinders and will not idle.. There is no smoke from the exhaust and oil pressure is good and stable.. I was thinking that it may be a valve spring issue?? I hope that it's nothing on the bottom end..
Any suggestions where to start? I guess pulling off the valve covers and checking the springs may be a good start. Engine only has about 10k miles on it since rebuild by the previous owner.. 10 years ago but only 10k miles on the rebuild..

Check the the easy stuff first: look at the points or electronic ignition module if so equipped under the distributor cap.

Valvespring or headgasket or maybe you have worn down the camshaft?
Would start and removing the valve covers and feel for excessive play in the valvetrain

does it crank at the same speed, or faster. if timing jumped a couple f teeth, it'll run but be weak. and with a lot less compression, you will hear it cranking faster on the starter. pull valve covers and watch valves while cranking for 1 or 2 not moving. then bump starter till #1 exh just closing and #1 intake just opening and timing mark should be right around TDC. quickie valve timing check.

Sounds like the fire went out on one plug. I'm with Chapter 2--- check your ignition system and plug wires first. Maybe you shorted a plug wire on a manifold.

Cranks at same speed. Timing didn't appear to have changed.. Above 2000 rpm, car runs almost normal. I personally would guess that it's a valvetrain issue.. I doubt that it's a head gasket as there would likely be smoke. Ignition is of course a possibility.. Car has a Mallory electronic tach drive distributor..

will check the distributor after I check the valvetrain first.. I had a similar issue when I had my race car.. Turned out that I had a broken valve spring..

Camshaft was replaced 10000 miles ago when they rebuilt the engine.. It has a slightly bigger than factory camshaft in it. Valvetrain is indeed the first thing I will check but of course, it could be a lot of things..
Not sure when I will get to start looking at it.. Kinda busy right now..

Will also pull the plugs if the inspection of the valvetrain will not show anything obvious.. I did check the plug wires already and could not see anything obvious... I was also thinking that it may be a collapsed lifter or broken pushrod..

I just hope that it's nothing on the bottom end.. I don't think so but nothing is impossible.. Good thing is that the oil pressure is good.. indicating no issues with bearings..

What engine? If it is a bbc with Qjet I'd check the base gasket. Everybody leaves out the metal plate and eventually the paper burns through - making a huge lean condition below 1500. Above that it runs fine. I know - this happened to me

It's a 327/300 with a modified Holley 750 with vacuum secondaries.. I highly doubt that it's related to that.. The symptoms don't indicate a lean condition.. Lean usually means higher idle.. My car dies below 1500 rpm...

Mine started to sound like a boat, I had blown the head gasket between the cylinders, no smoke from the engine just a popping out of the right exhaust

Your running a Holley and the engine backfired. Did you blow the carb power valve?

Diagnosing over the internet is impossible. It could be a dozen things. At best you could post a video but until you start pulling parts off to inspect its just a guess.

Thanks.. I will check for that. Unless it's the valve springs, I will have to take off the heads anyways which means that I have to replace the head gasket anyways..

It's certainly possible but I doubt that this is the problem.. Happened to me before on another car and the symptoms were different..

For sure.. Anyways, I will post updates whenever I get a chance to look at it.. Right now, I'm swamped with other things and probably won't get to it until next week :(

What is showing on a vacuum gauge?

What are the results of a compression test?

What are the results of a leak down test?

What happens when you spray carb cleaner around the carb and intake manifold gaskets?

A blown power valve will usually exhibit black smoke out the exhaust from the mega-rich mixture. I'm still betting on an ignition issue.

Haven't even started to look at the issue... Got little time.. Will work my way through all the potential causes..