Help with Backfire

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Alex.Sciortino

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I just built a 360 for my Dart. Its basically a stock rebuild with a Hughes Whiplash, a Weiand action plus, and a Brawler 670 carb. I broke in the cam, then started the engine a few other times and everything was good. Today, I went for a short drive, and when I was pulling into my driveway, it started backfiring through the carb. Its about once a second, like a clockwork. I've tried advancing the timing till it was hard to start, and retarding it till it would barely run. I've richened up the idle mixture screws. I'm out of ideas at this point.
 
You have a wiped exhaust camshaft lobe.
 
In the event it is a flat lobe, am I okay to just stab another cam in there, and flush some clean oil through it a few times?
 
In the event it is a flat lobe, am I okay to just stab another cam in there, and flush some clean oil through it a few times?
Well that is a gamble. There will be metal throughout the engine that you won't be able to remove, not all of it. That said, I have gotten away with do that in the past a few times.
 
Well that is a gamble. There will be metal throughout the engine that you won't be able to remove, not all of it. That said, I have gotten away with do that in the past a few times.
If I were to stab a new cam in it, is there anything I can try to do make it safer? I was thinking a strong magnet on the drain plug, and then priming the pump for a good long time with clean oil, then repeating.
 
Any way you clean it will increase your odds of pulling this off. I might use Kerosene and over fill the oil pan and new filter while priming it. A magnetic oil pan drain plug is a good idea.
 
Any way you clean it will increase your odds of pulling this off. I might use Kerosene and over fill the oil pan and new filter while priming it. A magnetic oil pan drain plug is a good idea.
Okay, Ill give that a go! Ill pull the intake this week and get a better idea of what the carnage is.
 
Okay, Ill give that a go! Ill pull the intake this week and get a better idea of what the carnage is.

Change the oil also and see if there is any metal in there....

If metal particles come out in the oil, then I would strip the block down and give it a good cleaning and blast the oil passages with high pressure water and air... Check the bearings and crank for damage, also be sure to check the cam bearings....
 
I would pull the plugs to see if any cylinder is odd looking. Since there was a rebuild here, they all ought to look fairly even, with some variation in plug color due to mixture variations. But no cylinder's plugs ought to look that much different. Then a compression test. These 2 checks are good to (maybe) point to an exact location.

Other possibilities:

- Perhaps a slightly sticking intake valve or one hanging open a bit.
- Could be valve train stackup with milling on the heads and a long pushrod hanging an intake valve open slightly.
- Or the clip flew out of one intake lifter and it has pumped way out, hanging the intake open.
- Electrical misfire in the distributor (crossfire) or crossfire on the wires
- A cam timing issue, but the engine would not likely have jumped time that badly assuming a new chain was used.

OP, can you tell us:
- New timing chain & sprockets?
- Was any head milling done? Any valve work done?
- Original pushrods used?
- What rockers do you have? Stock stamped ones?
- Was cam lube used on the cam and bottom of lifter when installed?
 
Wouldn't a compression check tell if a cam lobe was gone? Is the dist new? I had a situation where the dist shaft was moving up and down too much and I got horrible backfire, changed the dist and problem fixed.
 
I lost two cams in two engines before I learned the proper break-in procedure and the use of the proper oils. Yes there is fine metal particles throughout the motor and the only way to completely remove it would be to completely tear the motor apart. Short of that I replaced the cam and lifters and installed small magnets throughout the motor and large ones around the oil filter. Fired both motors with the proper break-in oil, used new proper break-in procedure. That was 5k miles ago. Both motors running strong. There is life after loosing a cam.
 
Think about your intake valve hangin open theory. If it is poppin, that's the cylinder firing. If the intake valve is hangin open, it would not fire, because there would be no compression. Something is causing the exhaust valve to stay closed, IMO. The cylinder still fires, but the exhaust is forced back up through the intake.

I would pull the plugs to see if any cylinder is odd looking. Since there was a rebuild here, they all ought to look fairly even, with some variation in plug color due to mixture variations. But no cylinder's plugs ought to look that much different. Then a compression test. These 2 checks are good to (maybe) point to an exact location.

Other possibilities:

- Perhaps a slightly sticking intake valve or one hanging open a bit.
- Could be valve train stackup with milling on the heads and a long pushrod hanging an intake valve open slightly.
- Or the clip flew out of one intake lifter and it has pumped way out, hanging the intake open.
- Electrical misfire in the distributor (crossfire) or crossfire on the wires
- A cam timing issue, but the engine would not likely have jumped time that badly assuming a new chain was used.

OP, can you tell us:
- New timing chain & sprockets?
- Was any head milling done? Any valve work done?
- Original pushrods used?
- What rockers do you have? Stock stamped ones?
- Was cam lube used on the cam and bottom of lifter when installed?
 
Check your rockers and pushrods. Not uncommon to poke a pushrod thru a stamped rocker, or bend a pushrod.
Especially if you did any spirited driving, and/or missed a shift.
 
I would pull the plugs to see if any cylinder is odd looking. Since there was a rebuild here, they all ought to look fairly even, with some variation in plug color due to mixture variations. But no cylinder's plugs ought to look that much different. Then a compression test. These 2 checks are good to (maybe) point to an exact location.

Other possibilities:

- Perhaps a slightly sticking intake valve or one hanging open a bit.
- Could be valve train stackup with milling on the heads and a long pushrod hanging an intake valve open slightly.
- Or the clip flew out of one intake lifter and it has pumped way out, hanging the intake open.
- Electrical misfire in the distributor (crossfire) or crossfire on the wires
- A cam timing issue, but the engine would not likely have jumped time that badly assuming a new chain was used.

OP, can you tell us:
- New timing chain & sprockets?
- Was any head milling done? Any valve work done?
- Original pushrods used?
- What rockers do you have? Stock stamped ones?
- Was cam lube used on the cam and bottom of lifter when installed?
I used a new Comp double roller timing set, the heads were milled, then the valves were ground, and new guides were installed. I'm using the stock stamped rockers. I completely slathered the cam with the lube it came with, and also put a blob on the bottom of the lifter.
 
Wouldn't a compression check tell if a cam lobe was gone? Is the dist new? I had a situation where the dist shaft was moving up and down too much and I got horrible backfire, changed the dist and problem fixed.
It is a new dizzy, but it is one of those cheap eBay ones.
 
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