Technical Strangest backfire issue

Hey guys, I got a project years in the making. 5 years ago, I dropped in a new Chevy 350 long block into my 52 GMC truck, I was able to perform the initial break in, but I had life get in the way of the build for YEARS.

SBC 350
Stock cam
Holley 600 double pumper
Eldelbrock performer air gap intake (all I had on hand)
Stock heads
10w30 Brad Penn grade oil

Anyways, I finally got back into it, fixed the brakes, and did tons of new wiring, and was testing my cooling system letting it idle for an hour or two... just about ready to do my very first test drive on the road...

Engine started to idle about 100rpm lower, started to hear a low toned pop out of the exhaust... And then clap, clap, clap! Full blown backfire through all rpm range, and I turned her off.

Never heard of a backfire develop like that, researched everywhere, and nobody seems to describe this.

Engine fan would kick on at 200f, cool to 185f then turn off. Sounded PERFECT before the backfire developed.

By the way, this is my fourth engine through all these years, each one failed from rare and strange reasons, so I'm crazy paranoid. Getting worn out, and scared I lost yet another engine lol.

 

Check distributor cap for carbon tracing. Plug Wires for cross firing in the dark. Pull plugs and check for weird coloration. Then consider bad cam lobe juju. Good luck. Maybe look at push rods and rocker arm movement with it running and valve covers off...

 

Letting it idle that long did nothing to splash oil on a virtually new cam shaft.

 

HEI distributor has a new cap, rotor, wires, and spark plugs...

I'll pull the plugs tomorrow, and maybe do a compression test if I can find my tool

 

If its running very rich and you have an exhaust leak at the manifold, that might be it. Try retightening the exhaust manifolds or headers if you have them

 

I’m sorry to not offer anything meaningful, but for me a 5 year delay on a project is like a “power nap”.
Good luck.

 

Sounds like a collapsed lifter, (clack, clack, clack) with the rocker arm making noise. This would produce
the symptoms you described. You can easily feel the exhaust headers/manifold to isolate the cylinder, or
squirt some water on each pipe to find the dead cylinder, then go from there.

 

Bob, you’re too old school. We all now have stepped up the the HF laser temperature gun

 

I'd guess a stuck valve. Pull the valve covers and take a quick look to see if all the pushrods are still in place.

 

You may have wiped a cam lobe. Sorry.

 

If this has a flat tappet cam and not roller, after breaking the engine in for (I****ume the typical 20-30 minutes) you then (five years later) let it idle for a solid 1-2 hours ... my guess is the cam is wiped. It could be a collapsed lifter but I would****ume that would have clacked instantly.

I hope you changed the oil and filter 5 years ago, after the initial run ... who knows what kinda mess the break-in lube made if it was left in there for that long. You did use break-in lube on the cam right ... not just Brad Penn?

 

Check rocker nuts and studs too. Seen them break and pull out on brand new engines.

 

I am betting on a flat cam lobe or two, to much idle time on a new cam. Not even the best oil will help with 2 hours of idle.

 

Dang it...I KNEW I overslept too often....gotta get into at least the 90's, haha....But,I can't get all those coupons
on my flip phone...thanks, Budget36!

 

Oh ya, it's had about 3 oil changes since then. Over the years, I would just fire it up about every 6 months, act like I'm doing a cam break-in, then shut off.

I just finally got all the cooling issues worked out, and let it idle while I was tying up wires, and moving junk out of the way for the first drive... I was excited, and didn't think about the cam. Looks like that just took too long.

About 6 years ago, I tried to have a mechanic do the initial break-in, and road test if possible. He was able to do the initial break-in using large shop fans, but was unable to road test, as the cooling system just couldn't keep up.

So, over the years, I just spent 10min here, 10min there installing an aluminum radiator, fan shroud, the biggest electric fan I could fit, and a temperature switch connected to a relay.

To make things take even longer, I had issues where it turned out my relays didn't have a blocking diode, causing a reverse power surge. An engine temp gauge was bad (reading really high), two doa fan switches, fan switch location was wrong.

I do have one more cam and a set of lifters... But I'm wondering... Maybe I should just go roller cam? I don't even know if my brakes are going to work correctly, and may have to cut the break-in short. I don't want to fail another cam and do further damage to the main bearings...

I'm that guy where you make every mistake in the book, and have every strange issue.

 

Maybe just pop a valve cover as suggested before, use a 25 dollar HF magnetic base/dial caliper gauge and check lift. Before just putting in a cam, etc.

 

Is that new but 5 years old or actually new.
Maybe you got a wire crossed or go bad/burnt.
I’d pull the plugs, pop the valve covers off and run a compression test and look for something obvious,

 

It's probably something simple and not a big deal.
Start with checking the obvious, easy checks. Did a plug wire get too near the exhaust manifold? Did you lose a plug tip for some odd reason? Water in the fuel tank? And so on with the suggested checks/inspections from the easiest to the most involved last.
It's a process of elimination. Find it. Fix it. Drive it. I wish you good fortune and happy highways soon.

 

I'd check the obvious like plug wires just because it's easy, then pull the plugs and look at them, just because. Then I'd pop a valve cover and try to rattle every rocker arm.

 

My car developed a bad miss suddenly. Decided to pull the plugs and take a look, first wire I pulled off I saw the problem. Wire had gotten against the manifold, burned a small hole, was arcing direct to the manifold. I had moved the wires when I recently changed the alternator and didn’t realize it had gotten against the manifold. Couple of turns of black tape, no more miss.

Got some burn proof plug sleeves on the way. Won’t have that problem again.
Maybe yours is something similar.

 

its a bad condenser

 

Well there’s that

 

A modern electronic fuel injected engine can idle for days with no harm done
the
Idle circuit in a carburetor is usually very rich
letting the engine idle for more than an hour with fowl the plugs and valves
run some Seafoam down the carb and clean out all that crud.

 

Finally got under the hood, and I am hoping you may have hit the nail on the head! (At least it seems like it)

I siphoned out about 3 gallons of gas, which apparently was all that was left in the tank, despite my gauge reading full. Replaced with fresh 91 octane.

I checked each plug, and they were all fouled with a thick oily film smelling like gas. Like a complete******, I apparently put on way too much antiseize and had the gap way off at .030. I wire brushed them clean, wiped off all antiseize, and set the gap to .040.

I checked the oil, and it smelled like gas... So I think I have an A/F mix issue, mega rich. I understand a misfire can make the oil smell like gas also, so we shall see.

I started her up real quick, started easy peasy, no backfiring, revved to 2.5k even let her back down to idle and everything seemed fine (crossing fingers).

I shut her off after a few minutes, as I need to change the gasoline contaminated oil, and check the carburetor.

 

The sea foam will clean all that outa there
I would imagine your valves look the same
Get the sea foam in the aerosol can
you will need to rev the engine when you spray it

 

May I ask why you are using anti seize on the plugs?

 

That one plug has enough anti-seize for 20 plugs.
You want a little dab, like a sesame seed sized.
And about 2 threads up from tip end. It will coat the entire plug as it threads in. Personally I’d start with new fresh plugs. And I’m pretty sure there’s enough anti seize in the head to skip it as well

 

So he can end up looking like this!

 

Get a little on you and it’s on every thing you own before you know what happened

 

It makes it easier on the next guy changing the plugs, saves on gauling, rust build up, carbon, needed especially on aluminum heads- it’s a very good habit but it takes just a tiny bit.