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Discussion Starter #1 (Edited)
To start off, I just did the infamous 351w swap. The motor has roughly 100k miles on it and has a backfire.
My exhaust currently dumps straight back and stops behind my seat.
I also have a half-dollar sized hole below my seat on the drivers side floor pan. It seems that the backfire in my car has caused the carpet to catch fire and 1/8th of my carpet is now gone...

My exhaust header on the drivers side has two bent in pipes (In order to have clearance for the steering shaft), those two pipes are 50-55% restricted. Could this be the cause of my backfire?

Please help! This is Urgent!!

Thanks Everyone,

Andrew S.
 

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Rockin the "letter" cam!
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backfire is usualy timing. thru exhaust means to retarted. what is the timing set at? check your wires and firing order as well.
 

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Discussion Starter #4 (Edited)
backfire is usualy timing. thru exhaust means to retarted. what is the timing set at? check your wires and firing order as well.
Well I got the motor and had no idea what was in it, so I tried running the 351w firing order. It ran like crap, only 4 cylinders were firing.
I then tried the 302 firing order and it seems to run great... Until I let off the gas after I get the engine up in the RPMs, we then get a bunch of backfire!

The timing is set at 10 degrees, I believe!
I just replaced the ignition control module, coil, plug wires, spark plugs, distributer, rotor & cap... :(
 

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Need more specs on the set-up. Is this an original HO EFI car? Are you still using EFI or is this carb'd? What cam? What's the rest of the set-up as far as performance mods? What type of headers and what size primaries?
 

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Discussion Starter #6
Need more specs on the set-up. Is this an original HO EFI car? Are you still using EFI or is this carb'd? What cam? What's the rest of the set-up as far as performance mods? What type of headers and what size primaries?
The car is a 1983 original 5.0 gt from factory, it was factory carb'd.
I dropped in a 351w, this motor has very limited information as I pulled it out of a wrecked 1985 F150 for $400. It has a RPM air gap intake manifold and headmen shorty headers, we believe it may have a 302 cam in it, though we are not sure because no one knows anything about the motor.
 

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If it runs like crap on the 351 order and runs better on the old 302 order, then you probably do have a 302 cam in there. If you want to be sure, pull the valve covers and spin the engine with a breaker bar and watch the rockers.

Pull your plugs and see what they look like. And you need to make sure that your timing is where you say it is. What about the distributor, is it a mechanical or vacuum advance? Is it working properly?

If you're stock headed, I wouldn't worry about the primaries being dinged in. Lots of guys clearance their headers. I personally hate it when people do it, but it shouldn't cause any backfiring issues like what you've got. You're not making enough power with that engine.
 

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Discussion Starter #8
If it runs like crap on the 351 order and runs better on the old 302 order, then you probably do have a 302 cam in there. If you want to be sure, pull the valve covers and spin the engine with a breaker bar and watch the rockers.

Pull your plugs and see what they look like. And you need to make sure that your timing is where you say it is. What about the distributor, is it a mechanical or vacuum advance? Is it working properly?

If you're stock headed, I wouldn't worry about the primaries being dinged in. Lots of guys clearance their headers. I personally hate it when people do it, but it shouldn't cause any backfiring issues like what you've got. You're not making enough power with that engine.
I'll take the vavle covers off to verify the firing order of the cam. When I take out the spark plugs what should I be looking for? I feel like I have a sticky vavle, I put in a quart of tranny fluid in hopes to clear that up, no improvements though!

The distributer is a vacuum advanced and seems to be working great!
The vacuum line is going to the port on the carb that pulls no vacuum at idle!
 

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That could be part of your problem. You should be pulling some kind of vacuum at idle. Depends on the cam, but if it's anything like an EFI car, you should be seeing over 15in at idle. I'm no good with carb'd cars so I could be off on that figure, but regardless you should have some vac at idle. Maybe someone could chime in and give you a better example of roughly what you should be looking for on a carb vehicle.

Do you have a vac gauge? A stuck valve could be causing this and no vacuum at idle points to that too.
 

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Discussion Starter #10
That could be part of your problem. You should be pulling some kind of vacuum at idle. Depends on the cam, but if it's anything like an EFI car, you should be seeing over 15in at idle. I'm no good with carb'd cars so I could be off on that figure, but regardless you should have some vac at idle. Maybe someone could chime in and give you a better example of roughly what you should be looking for on a carb vehicle.

Do you have a vac gauge? A stuck valve could be causing this and no vacuum at idle points to that too.
There are two ports on the carb for the vaccum advancement, one pulls vaccum at idle and one does not! I'm not sure which to run it on! My friend told me to run it on the port that has no vaccum at idle, I'm not sure how true that statement was though!

Thank you so much for helping me with this!
 

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If you dont know what cam is in it, you cant know what firing order to follow.
 
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