Cam going flat??
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Elder User
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Cam going flat??
I've never heard this before but just recently my 79 302 short bed has been backfiring throught the carb under a light load. It started a week ago one time going down the highway then 2 times the other day and today I lost count. It was under a light load (heavy rainstorm and a head wind of 60 mph) The carb is a Edelbrock and love it no problems whats so ever. Basically everything has been replaced except internals. I called Edelbrock and spoke with the tech guy and he told me it sounds like the cam is starting to go flat. He said it's because of the new oils no longer have Zinc and vehicles with flat tappet cams will start having these problems. Something to do with the exhaust lobe is staying shut longer and popping through the intake. The only way to tell is pull the cam and check he says. Does this sound normal?????? I was planning to change the cam anyway but not this soon so I'd like to be for sure. Oh I did change the timing chain and oil pump back last winter so those are the only interal parts that's been changed.
Post Fiend
Joined: Jul 2006
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Before I would even consider any changes to the carb, I might be checking the most obvious things.
Vacuum leaks can and will cause a backfire like this. You backfire though the carb is not necessarilly the cam going flat since the intake valve has to still be open in order to send a backfire through the carb. This does not support a flat cam since the flat cam would cause the valve to not open as far or not at all.
Ignition timing is the key here, the plug is firing when the intake valve(s) is (are) open, and this creates the backfire.
Could be as simple as a a failing ignition module or just a poor cap / rotor combo.
I am probably not that convinced that the cam lobe is causing this problem.
Vacuum leaks can and will cause a backfire like this. You backfire though the carb is not necessarilly the cam going flat since the intake valve has to still be open in order to send a backfire through the carb. This does not support a flat cam since the flat cam would cause the valve to not open as far or not at all.
Ignition timing is the key here, the plug is firing when the intake valve(s) is (are) open, and this creates the backfire.
Could be as simple as a a failing ignition module or just a poor cap / rotor combo.
I am probably not that convinced that the cam lobe is causing this problem.
Hotshot
Joined: Apr 2009
Posts: 14,333
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From: Stanley, VA
It could be any of the above but the symptoms you describe could also be what the tech said, a flat cam.
I've run across this a couple of times in the past, once on a Cadillac Seville and once on an older Mercedes, they both had the same symptoms.
If you have a dial indicator you don't have to pull the camshaft to check it. Use the dial indicator to check the lift at each valve.
I've run across this a couple of times in the past, once on a Cadillac Seville and once on an older Mercedes, they both had the same symptoms.
If you have a dial indicator you don't have to pull the camshaft to check it. Use the dial indicator to check the lift at each valve.
Postmaster
Joined: Dec 2008
Posts: 3,940
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From: Southern Oregon Coast
I've got this problem with the 318 in my Fury - 2 of the cam lobes are worn down significantly and valve lift is noticeably affected as a result... Check the stuff the others have mentioned first though before going with the cam - you described the problem as happening under a light load - mine happened under a somewhat heavy load, like when I romped on it to try to get up to freeway speeds an whatnot - got to the point where I couldn't get it up past 40mph without it popping through the carb constantly...
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Elder User
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Excellent ideas guys. I do have a vacuum gauge in the truck and she's reading a good 17 down the road. At idle it's around 20 or more and steady. I will check the timing and pull the cap back off. I have not replaced the ignition module though so that could be a possibility. Is there a way to check the module? I do have a meter.
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Elder User
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I was thinking about the cap and rotor deal. I don't even have 4000 miles them and were bought at Napa, Ekler I think was the name on the box.
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Posting Guru
Joined: Jan 2005
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From: The Vieux Ford, Kansas
I suspect simple ignition cross fire. The heavy rain you mentioned got me thinking this. You get some water in the distributor, or soak the plug wires good and it will cross fire and fire cylinders at the wrong time or kill it entirely. You get them wet enough and even good components can fail in this manner. Open up the distributor and see if there is any condensation in it and try to route your plug wires so they don't touch each other or a good ground.
-Johnboy
-Johnboy
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Fleet Mechanic
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From: Middle TN
Running lean, check for vac leak, check egr if it has one, is it sealing is it working properly.
Is motor running a little hotter than normal, Does it want to keep running after you turn it off. Has the idle speed changed from normal. These are all signs of lean fuel air mixture.
Usually when the cam lobes wear out of spec you just lose ome power and gas mileage.
Is motor running a little hotter than normal, Does it want to keep running after you turn it off. Has the idle speed changed from normal. These are all signs of lean fuel air mixture.
Usually when the cam lobes wear out of spec you just lose ome power and gas mileage.
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Elder User
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Running lean, check for vac leak, check egr if it has one, is it sealing is it working properly.
Is motor running a little hotter than normal, Does it want to keep running after you turn it off. Has the idle speed changed from normal. These are all signs of lean fuel air mixture.
Usually when the cam lobes wear out of spec you just lose ome power and gas mileage.
Is motor running a little hotter than normal, Does it want to keep running after you turn it off. Has the idle speed changed from normal. These are all signs of lean fuel air mixture.
Usually when the cam lobes wear out of spec you just lose ome power and gas mileage.
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Elder User
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Well I believe so, I can even feel it through the accelerator and when she pops the truck looses power that very instant then comes back.
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