74 Valiant 318 Backfire when hot

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ThatPolarBear11

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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.
 
When does it backfire? When you give it gas and let off? Or just sitting there idling? Or going down the highway at cruising speed?
 
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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By the way, welcome to the site !!
 
ThatPolarBear11 said:
It would backfire when you give it any sort of aggressive throttle when in neutral or when load is put onto it i.e. going up a hill in 2nd. Appreciate the warm welcome!!
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Is the backfire through the exhaust or through the carb?
 
The fact that it doesn't do it cold but does it hot eliminate a few things (kind of strange to). Could be as simple as when it warms up, a leak occurs, leaning it out and it's getting the "lean snap". Maybe when its' fully warm and idling you could spray carb cleaner around the carb base/vacuum hoses/and intake. If it revs up a little, your on to something.
 
If it is repetitive like a machine gun, look for something keeping an exhaust valve closed. A broken exhaust rocker arm, bent exhaust pushrod, wipe exhaust cam lobe and or lifter. If it only does it on acceleration, then check the initial timing. It likely needs more. Can you get us a video? This IS 2025.
 
RustyRatRod said:
If it is repetitive like a machine gun, look for something keeping an exhaust valve closed. A broken exhaust rocker arm, bent exhaust pushrod, wipe exhaust cam lobe and or lifter. If it only does it on acceleration, then check the initial timing. It likely needs more. Can you get us a video? This IS 2025.
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Yeah I can get a video tonight for you. That will probably make this easier
 
ThatPolarBear11 said:
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That sounds like a diesel !! you might not want to run it anymore and start by pulling the valve covers. And that really didn't sound much like a backfire, but like it's gasp'n/missing as a results of whatever is going on in there
 
Well crap, really strong start thankfully it's a bit easier than the euro stuff I'm used too which is nice. If it is a cam what cam and timing kit would yall reccomend?
 
Disclaimer, I can't see the video on this computer, so members don't crucify me. Set the timing to 10-12 degrees BTDC (Good starting point) and make sure the accelerator pump is working by actuating the throttle with engine off and verifying a good shot of fuel. Make sure and do these things first, there easy and free. If you're getting a good shot of fuel when you actuate the throttle, you've verified timing and still seeing the problem, then perform a compression check (engine cold, throttle fully open) and report back with numbers for each cylinder. If you don't have a compression gauge, you can rent one (deposit returned when you bring it back) from one of the major auto part chains. You need a rollback timing light for this hobby if you don't have one. Do all this before you start tearing into the engine under the doom and gloom advice about a worn-out cam or bad valves. Incorrect timing, vacuum leaks, etc. can make a car run awful. If it turns out to be one of those things, then you get to do some upgrades!
 

I agree with the above, if this thing ran fine until the manifold and carb swap then why don't we start with the simple **** first. It very well could be a mechanical failure, but let's go through the Diagnostics first.
 
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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Here's a simple one then, fire it up let it idle until it's fully warm, then take a screwdriver handle, the plastic kind... and drag it against the exhaust manifold right up next to each port on the head. If the handle slides melting like butter, it's firing just fine and there's probably no issue.. if it sticks or drags roughly across the manifold then that's the cylinder with the problem.

This is easier with headers, but works with manifolds.

If you find the problem cylinder, pull the valve cover and rockers... and simply see if the valve stem height matches the others...if so, then check the plug , then onto a leak down test... or you can put a piece of paper up to the tail pipe and if it goes from blowing the paper away to.. sucking the paper to the tail pipe.. that is a burnt valve. It could have a section melted and broken away.

The boys can surely be on to something, just figured id offer my approach.
 
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