74 Valiant 318 Backfire when hot

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ThatPolarBear11 said:
It's ran like this since we got it, it came with that manifold and carb, and like I said it only backfires when warmed up
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Still doesn't mean that then PO timed it right or the carb isn't bad. Do my simple stuff first and MOPAROFFICIAL's manifold recommendation, then remove the valve covers and go deeper.
 
It reminds me of my dad’s 77 Plymouth station wagon,,,400 engine .
After about 175,000 miles,,,,it started getting hot and overheating a lot from a leaking radiator .
One night my brother got it pretty hot,,,and the next day it acted just like that .
My Daddy decided it needed to be rebuilt,,,,so he had his best mechanic do the job .
After the rebuild,,,it acted exactly the same .

As it turns out the catalytic converter got hot and melted together,,lol .
After he replaced that,,,it ran for another 150,000 miles,,,lol .

I know the Ops car doesn’t have a Cat,,,,,but it might still be a blockage somewhere .
That is something to consider .

Tommy
 
Desoto291hemi said:
It reminds me of my dad’s 77 Plymouth station wagon,,,400 engine .
After about 175,000 miles,,,,it started getting hot and overheating a lot from a leaking radiator .
One night my brother got it pretty hot,,,and the next day it acted just like that .
My Daddy decided it needed to be rebuilt,,,,so he had his best mechanic do the job .
After the rebuild,,,it acted exactly the same .

As it turns out the catalytic converter got hot and melted together,,lol .
After he replaced that,,,it ran for another 150,000 miles,,,lol .

I know the Ops car doesn’t have a Cat,,,,,but it might still be a blockage somewhere .
That is something to consider .

Tommy
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Similar indeed to a cat that's clogged, had that happen before too, more pedal it just slows down
Maybe he has a blown head gasket between two cylinders, but you got to go through the steps to pinpoint whatever it is.
 
I have seen the video…….
Nothing I heard sounded like a “backfire” thru the carb to me.

I’ll be pretty surprised if any type of “tuning adjustment” solves whatever is going on there.

I’d probably start the diag with a compression test.
 
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MOPAROFFICIAL said:
Similar indeed to a cat that's clogged, had that happen before too, more pedal it just slows down
Maybe he has a blown head gasket between two cylinders, but you got to go through the steps to pinpoint whatever it is.
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True,,,,but I’m wondering if his muffler could have collapsed and is blocked off ?
I have heard of that happening .

Might be worth checking,,,,at least to cross it off the list .

Tommy
 
ThatPolarBear11 said:
Just picked up a 74 Valiant w/ the 318 and a 3 speed. Someone swapped an edelbrock carb and a Weiand super street manifold onto it. Runs good when cold but backfires when hot. Haven't shot base timing yet (adjusted it by ear) but wanted to get opinions.
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OP says it runs "good when cold." Jus saying.
And you got him changing cams already?

Will it rpm if you gently and slowly open the throttle; instead of snapping it open?

I was talking to an old friend about a month ago, with a 318/1407/streetmaster/headers/new exhaust, that he said, was doing the exact same thing. He said the vehicle was already sold, As-is; and he was not interested in spending money on it. But I talked him into bringing it over, just for a quick look-see.
In the short time I had to look at it, I was able to ascertain, that it wasn't; fuel,
valve lash, nor ignition timing, nor the accelerator pump, nor the T-slot sync, nor rotor-phasing, and it had a new mandrel-bent dual exhaust on it.
The vacuum was low, but it did have a "bit of a cam"; specs unknown.
The Idle-Timing was 15*, and it demanded it, it got worse with less.
I suspected cam-timing, but ran out of allotted time before I could prove it.

It rpm'd Ok if I slowly opened the throttle,
and it drove fine, once it got rpm.

Happy HotRodding.
 
Super lean? You said runs good cold, after warmed up try holding choke 1/2ways shut and hit throttle. Agree,there’s lots to look at before digging into engine internals.
 
ThatPolarBear11 said:
Idk if the secondaries are supposed to be open all the way but they're staying shut basically
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They are doing that because the carburetor's choke linkage is keeping them shut since the carburetor is not on the car and operating and it is on its end.
 
Ahhh I see, but yeah I had to hold the choke open to get it to rev better. That and the new cometic gasket on there helped, I'll pull the valve covers Thursday now that I've eliminated the easy stuff
 
ThatPolarBear11 said:
Ahhh I see, but yeah I had to hold the choke open to get it to rev better. That and the new cometic gasket on there helped, I'll pull the valve covers Thursday now that I've eliminated the easy stuff
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I hope it's not what I think it is, but it is. lol
 

sounds like you're fighting battle on many fronts...

before investing any undue time or effort, whip off the valve covers and see what's up. if there's chicanery afoot, you'll know immediately. which may prevent a whole round of chasing your *** on timing and carb related "issues".... which isn't to say isn't also a problem (no carb base gasket? c'mon son!) but i don't think that's the main issue.
 
So a massive vacuum leak, possibly no accelerator pump cuz we haven't heard back on that yet... well that sure is a hell of a lot better than going after burnt valves and flatten cams... but i guess it ain't over till the obese lady farts.
 
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