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Super Stock / Pro Street and Modified Discussion 55-56-57 Super Stock / Pro Street and Modified Discussion 55-56-57 Talk engines, race tracks , setup's etc

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Old 04-10-2012, 05:00 PM   #1
Rogue56
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Default Troubleshooting a backfire

As all things come to pass, I was just to the point of being able to drive my 56. The paint was just finished, I put all the newly polished stainess on the body, the 700R4 tranny was attached to my (fairly new) 454 crate motor. And then as you probably can guess, something went wrong. The motor - 454 510hp used to run flawlessly but when I tried to start it a couple of days ago, it started backfiring throught the carb. I figure timing is off. I also heard some tappet noise on the passenger side. Anyway, I checked the timing. At idle it sounded fine with no backfiring. When I upped the rpms, it started backfiring. I checked the timing and it was at 16 btdc, just where I had set it before. I loosened the distrib and turned it a bit to see if I could get the backfiring to go away, no luck there. I put in new plugs and checked the plug wires, dist cap, and rotor and they all looked good. As a matter of fact, I've probably only got about 50 hours on the motor total. It's just taken me years to get it to the point I was able to drive it.
I took off the valve covers and found that the #8 intake & exhause roller rockers were loose, so I did a static valve adjustment on the whole thing. I did notice that the #6 exhaust push rod only opened the valve about 1/4 inch where the others were a good 3/4 inch. I took a compression reading on all cylinders and they were at 145psi.
Sooooo, my question is is there anything else I might be able to check to find the backfire problem. Would the #6 exhaust valve not opening all the way cause the backfire problem. I guess it's possible the new cam has a flat lobe and that's why the #6 exhaust valve is not opening all the way. I haven't checked the oil for metal yet, but will do that within the next day or two.
Any suggestions/advice anyone might have is GREATLY appreciated.
Thanks. Steve
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Old 04-10-2012, 05:23 PM   #2
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What type of lifters do you have? If it's Hydraulic you might have a broken spring in the lifter.
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Old 04-10-2012, 07:57 PM   #3
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Originally Posted by Rogue56 View Post
As all things come to pass, I was just to the point of being able to drive my 56. The paint was just finished, I put all the newly polished stainess on the body, the 700R4 tranny was attached to my (fairly new) 454 crate motor. And then as you probably can guess, something went wrong. The motor - 454 510hp used to run flawlessly but when I tried to start it a couple of days ago, it started backfiring throught the carb. I figure timing is off. I also heard some tappet noise on the passenger side. Anyway, I checked the timing. At idle it sounded fine with no backfiring. When I upped the rpms, it started backfiring. I checked the timing and it was at 16 btdc, just where I had set it before. I loosened the distrib and turned it a bit to see if I could get the backfiring to go away, no luck there. I put in new plugs and checked the plug wires, dist cap, and rotor and they all looked good. As a matter of fact, I've probably only got about 50 hours on the motor total. It's just taken me years to get it to the point I was able to drive it.
I took off the valve covers and found that the #8 intake & exhause roller rockers were loose, so I did a static valve adjustment on the whole thing. I did notice that the #6 exhaust push rod only opened the valve about 1/4 inch where the others were a good 3/4 inch. I took a compression reading on all cylinders and they were at 145psi.
Sooooo, my question is is there anything else I might be able to check to find the backfire problem. Would the #6 exhaust valve not opening all the way cause the backfire problem. I guess it's possible the new cam has a flat lobe and that's why the #6 exhaust valve is not opening all the way. I haven't checked the oil for metal yet, but will do that within the next day or two.
Any suggestions/advice anyone might have is GREATLY appreciated.
Thanks. Steve
I took my'56 out for a drive this past Saturday. It started backfiring and running really bad. it was the first time it had been driven in over two (2) months. The gasoline in the tank had also been in there for two (2) months. I filled it up with fresh gasoline and drove it really hard. The backfiring did stop after the fresh gasoline! I also discovered that the negative cable on the battery was loose! Maybe my backfiring was bad gasoline? The loose negative cable or the combination?? Dennis
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Old 04-10-2012, 08:05 PM   #4
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If not a broke spring I'd worry about lobbed cam.

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Old 04-10-2012, 08:16 PM   #5
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A few things to check for this backfire problem would be a lean
carb mixture,choke too lean and eng. cold, two mixed up spark plug wires,or
leakage across the dist.cap may cause backfire by enabling spark
to occur in a cyl. which is on its intake stroke.Hope this helps.
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Old 04-12-2012, 08:48 PM   #6
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I have seen bad gas cause this, have no intelligent explanation for it, other than I've seen it before. Only other thing I can think of is an exhaust valve hanging open, will let it back fire through the carb. Check the plugs, they can cause this too. Keep us up to date, kinda curious to see what ya find, good luck! Forgot to answer your other question, exhaust valve won't cause carb backfire, intake valves will.

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Old 04-12-2012, 10:09 PM   #7
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Hello there Rogue56, I hate to be the bearer of bad news, but I think you have a worn out camshaft!!---You have just described classic symptoms, of a wiped out exhaust lobe on the cam!!---The reason it starts popping or backfiring as you try to accelerate the engine, is the exhaust valve is not opening enough to expell all the spent exhaust gasses for that firing cycle, so when the intake valve opens on the intake stroke, it trys to send the trapped exhaust gasses back out the intake,because that's the only place they have to go, this is the popping you're hearing!!--The more RPM the engine has the more rapid and louder the popping gets!!---Good luck milton
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Old 04-13-2012, 04:01 AM   #8
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Hello there Rogue56, I hate to be the bearer of bad news, but I think you have a worn out camshaft!!---You have just described classic symptoms, of a wiped out exhaust lobe on the cam!!---The reason it starts popping or backfiring as you try to accelerate the engine, is the exhaust valve is not opening enough to expell all the spent exhaust gasses for that firing cycle, so when the intake valve opens on the intake stroke, it trys to send the trapped exhaust gasses back out the intake,because that's the only place they have to go, this is the popping you're hearing!!--The more RPM the engine has the more rapid and louder the popping gets!!---Good luck milton
I agree about the cam or a broken exhaust rocker could do the same.
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Old 04-13-2012, 08:33 AM   #9
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The #6 exhaust valve not working properly is likely the cause of your backfire. Now you need to find out why it is not opening all the way and you will find the real problem.
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Old 04-13-2012, 12:52 PM   #10
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The #6 exhaust valve not working properly is likely the cause of your backfire. Now you need to find out why it is not opening all the way and you will find the real problem.
I had the same problem years ago and I had some lobes wore off the cam. Since your push rod is loose, something is wearing to give you extra clearance.

One other time I had a motor (stock 350-350hp) that run great. After I run it hard for a race, I could hear a weird noise out the exhaust. To make a long story short, I lost 2-3 c-clips of the top of my hydrolic lifters. After running it hard the lifters would pump up a little more and hold the those 3 valves open a hair. By the time I got it slowed down the lifters went back to normal and valves went closed. That was a hard one to find what was going on. Didn't pop out the exhaust or out the carb.

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