BIG BLASTING BACKFIRE
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BIG BLASTING BACKFIRE
87 F700, engine is a 429. Rebuilt carb, new distributor cap, plugs, wires. Exhaust manifold resurfaced, new gasket, new tail pipes. Timing set to perfection, truck purrs until rpms increase.
To much air from somewhere??? Its been suggested to change the EGR valve ( not sure this is it)!!
Have spent lots of time, energy, and money & still no resolution.
If you have any suggestions, would love to hear them?
John
To much air from somewhere??? Its been suggested to change the EGR valve ( not sure this is it)!!
Have spent lots of time, energy, and money & still no resolution.
If you have any suggestions, would love to hear them?
John
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Joined: Aug 1999
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BIG BLASTING BACKFIRE
A little old for me, but heres a starting point.
Backfire is f/a mixture igniting in exhaust manifold.
Start with valve timing. Maybe the cam jumped a tooth? Check your valve lash/clearance. Maybe you are holding a valve open too long or opening too early?
Next, see if the ignition system isn't firing well. A loaded up cylinder of the size you are talking about will really pack a wallop if the fuel is just pushed out to the exhaust.
EGR valve? Maybe, but unlikely. The valve should only open at times when there is high combustion chamber temps (lean condition) and it will prevent the charge from detonating which could cause a backfire. But that is relatively easy to check, just block the valve with a plate so it cannot inject any exhaust into the intake charge.
Other than those two things (and igintion timing) not much else.
My money is on incorrect cam timing or stretched timing chain.
Randy
Backfire is f/a mixture igniting in exhaust manifold.
Start with valve timing. Maybe the cam jumped a tooth? Check your valve lash/clearance. Maybe you are holding a valve open too long or opening too early?
Next, see if the ignition system isn't firing well. A loaded up cylinder of the size you are talking about will really pack a wallop if the fuel is just pushed out to the exhaust.
EGR valve? Maybe, but unlikely. The valve should only open at times when there is high combustion chamber temps (lean condition) and it will prevent the charge from detonating which could cause a backfire. But that is relatively easy to check, just block the valve with a plate so it cannot inject any exhaust into the intake charge.
Other than those two things (and igintion timing) not much else.
My money is on incorrect cam timing or stretched timing chain.
Randy
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From: Linwood, N.J. USA!
BIG BLASTING BACKFIRE
John, I have learned that It can be the nature of the beast sometimes. All of my F-700's have had backfire hissy fits at one point or another.
One thing to look out for is that by the time you find the original culprit, the things that you may have replaced with new may be need to be replaced again because of backfire damage. My trucks still backfire on occasion when cold and first starting out. After that, normally no problem. Sometimes it could be plugs. Sometimes carborator. Many times I would replace exhaust gaskets and have them blown back out after several nasty backfires because of a bad coil. (Cracked or oil soaked. Had this happen on two trucks).
Pollution components that are old and worn can sometimes allow cool air into the exhaust and create a nasty backfire. Components like the check valves that go from the pollution pump to the exhaust pipes. I have had them ware out and actually make the hose from the pollution pump break down and fall off creating an exhaust leak. Just some thoughts. Good luck.
Jakegypsum // Ford fleet, soup to nuts.
One thing to look out for is that by the time you find the original culprit, the things that you may have replaced with new may be need to be replaced again because of backfire damage. My trucks still backfire on occasion when cold and first starting out. After that, normally no problem. Sometimes it could be plugs. Sometimes carborator. Many times I would replace exhaust gaskets and have them blown back out after several nasty backfires because of a bad coil. (Cracked or oil soaked. Had this happen on two trucks).
Pollution components that are old and worn can sometimes allow cool air into the exhaust and create a nasty backfire. Components like the check valves that go from the pollution pump to the exhaust pipes. I have had them ware out and actually make the hose from the pollution pump break down and fall off creating an exhaust leak. Just some thoughts. Good luck.
Jakegypsum // Ford fleet, soup to nuts.
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