Before you start chasing all those other things, start with a basic compression test. It will show if you have a weak hole and if it warrants pulling off the valve covers. If that is good, then you can start looking for tuneup and carb issues.

I’d be suspicious of a stuck valve.

Good on you for reaching out the new guy to the trade. Thanks for the video

When those engines were new they were known to have a problem with valve stem to valve guide clearance. They often would hang a valve open when they got hot, After sitting a long time, wouldn't take much rust to stick a valve open.

This is some great advice for tracking down a backfire through the carburetor. If he follows through he should be able to find the fix!

Thank you all for your tips and sharing your knowledge. I am in college and work on the car one weekends when I've got time. I'll be sure to report back and fill you all in what the cause was!

Hey AG, Tim here, i just finished a 409 build for a real '62 409/409 #'s matching 4 sp car..... motor ran great!!!....i mean AWESOME!!!......comp chk, then leak down chk.......if ign is proper, we got a wiped lobe, bad lifter, or a valve/head issue.....but u touched base on all the things he shud chk, other than my said comp chk/leak down chk.....i wish the young man the BEST!!..our 409 runs spectacular!!!...LOTS of detail was paid attention to.....and getting machine work done, requires an older shop that has the fixture to bore/hone, as the bore is NOT square to the deck, as i'm sure u know.......we went with a hyd roller.....i measured for the pushrods...patterns were good....motor INSTANTLY fired up, and ran awesome, PLENTY of oil pressure......hope the young man listens to you/us, and finds the issue!!....LOVED this vid, as i just got done with our 409 2 weeks ago!!

Compression test please!

Our old tbird, sits fo long periods of time, last time it started backfiring through carb, took off valve covers, found 2 bent pushrods, hammered them on a flat metal table, ran great after that

Good video, Allan. One of the things I like about the car community is the willingness of people, like yourself, to share their wealth of knowledge and experience with others. It's a very rare thing when one car guy won't offer sincere help to another who's having an issue with their car. Thanks for being a great mentor and teacher, Allan. I know I've learned a lot from your videos and I'm sure Blake and many, many others have too. Well done!

Can’t wait for that 409 build!!

A few years ago we struck a bad condenser. Brand new. Caused a scattered miss. Swapped the old one back in and all was good.

Great tips for checking an old engine with that problem.

Very Lucky kid get to work on grandpa's sweet '62 409 !! ...Back in day (Early '60's) you could find factory Dual quad 409 4-speed Chevys roaming streets even in Small towns.....Saw a few Tri-power 406's but never any Max wedge street cars....409" 409/425 for avg dude street win !!

It sat for a long time ... I would start with a compression test ... hopefully it was sitting inside but especially if outside it could actually have corrosion on a cylinder ..or valve is sticking .... But this will let you know how it's doing and where to look

It sounds like it isn't doing it all the time, or consistently. So that usually leads to cross firing spark or a valve hanging up.

First I would ask is the backfire constant like a repeating pop through the carb? Indicating an open valve when firing. Check manifold vacuum to see if the gauge is steady or fluctuating. Second check the distributor cap and rotor (if not new) for a crack or spark tracking. Meaning a dark line from one post to another or to the distributor body.

Electrical or engine timing related whether it be electrical or valve train. Flat cam, push rod, stuck valve from varnished gas distributer cap, Outdated points( throw them in the trash and get electronic ) timing chain slipped, crossed plug wire, rocker stud pulled.

You covered most everything Likely .....Really Like the "Ray Gun" exhaust manifold individual Temp test !! ....If a suspicious problem cylinder(s) is found, a Leak-Down comparison test is Very helpful but Don't use the terribly Miscallibrated Harbor Freight brand !! ....Most Faa--Aircraft maintenance related brands are good....A diy 0.040" diameter leakage Orfice "Hole" should test 'bout 50% indicated Leak-Down on good Testers regardless of air Pressure source but 80--100 Psi recommended.....

Sitting that long the valves and seats are probably corroded. Time for valve job. Or lapping . My 2 cents.lol

'62 409, is it using a points distributor? Points setup incorrectly can foil shit up in a hurry! Valve stuck open?

Ever done a video on the benefits of a spread bore carb compared to a square bore carb?

You hit most everything. One possibility could be an electronic ignition was installed at some point and the car has a resistor or resistor wire for points thats dropping the voltage to the coil.

Let him figure it out !

I'd check the points, dwell, timing , etc. Opps ... I typed that seconds before you said it. lol

check fireing order of the plug wires or a crack in the distributor cap will cause all kinds of problems

First things first it's usually something simple so don't go doing ridiculous tests to the engine when it's probably tune-up related. Always stay with fundamentals. An engine needs fuel, air, spark, ignition and mechanical timing, and compression to run. A gasoline engine needs a minimum of 90 PSI but in reality it should have a lot more than that. Very first thing is make sure the plug wires are in the proper firing order Second thing is make sure your ignition coil, points and condenser and cap and rotor button are in great condition He is also correct about the lean fuel condition so carburetor still could possibly be an issue Thirdly because it sat so long it is possible you have knocked the lobes off of the camshaft. And that has a very distinct sound that the engine makes when running and I'm not able to hear it but it's just something you want to look for. Pull the valve cover off and spin the engine over and watch the operation of the rocker arms make sure they are all opening and closing exactly the same because if they're not you got cam lobe problems or valve spring problems, pushrods, etc

General Motors til I die ☠️

Hopefully they drained out the old oil and primer it will new zinc oil.

Sticking valves or maybe needs new valve springs ?

stuck valve if rhythmic Or maybe a bad coil if not.

I had a 71 Torino 500 with a 351 and I bought brand new spark plugs for it for out of the eight were bad and it was backfiring through my carburetor

Backfiring through the carb is either an intake valve not closing, or if it is a Holley carb then it could be a blown power valve.

I had a very rare experience once where a lobe had worn off the distributor and wasnt firing that cylinder. Needless to say, it had me scratching my head for awhile. Its the only instance ive ever seen where that happened. Pretty strange

I've heard people with the 348 or the 409 are having problems with the after market distributor I have a 348 that's on the engine stand now I made sure I kept the original one just in case don't give up

I was told a little motor oil can be slowly poured down the intake while at high idle and that will lube the valve seats and guides if its been sitting up a long time. Be careful not to hydro lock it you have to go slow pouring it

Flat camshaft on the exhaust valve lobe AG... Look at the rocker arms going up and down. Might be the intake lobe or intake valve not closing.

How are you doing Sir? I am doing good here. Still working on my Ls3 swap with my grandkids. Slow going, but going. Great video again Sir. I should prob wait till the end of the video before I comment lol. I am just worried I'll forget lol.

30 years, no oiling... Parts sit that long and weird things do happen. Could be a valve most likely sticking, and that happens being off the seat for years on end and corrosion on stem prevents proper seating. Slightly off topic, lifter maintenance [I assume hydraulic] is worth doing at this time: get one of those plastic boxes that keeps them ordered just as they came out of the engine and... take them apart [one at a time], and clean them all. Mineral spirits are great at removing sludge or varnish. A little 30 wt needs to bless the parts before being reassembled. Good plunger clearance will prove itself when trying to remove it: there's some vacuum that will try to pull the plunger back in. I find a gasket pick helps on disassembly: find a weephole or groove to grab with the point - there's always something. Good luck Blake.

Stuck valve or timing?

It would be very helpful to send a video of what it’s doing! Because who knows what’s been done? If it was done correctly? Are the new parts any good? Lots of new stuff from parts stores for old cars, is garbage!