Backfiring Problems

[71_resurrection]

So I have been having problems with the Mustang lately. It backfires terribly on acceleration. I recently changed the spark plugs, checked the timing, it is dialed in at 10 degrees BTDC and the carb has been adjusted where I would like it with idle set nicely. My car is still backfiring coming up the hill to my house on pretty much every other ignition and when I accelerate in any situation. What the hell is wrong with my car. I noticed a lot of water and a bit of oil in my exhaust "spray", indicating it may be time for a rebuild (93,000 original miles on the original bottom end). I just dont think that this is causing my backfiring. I have an Edelbrock 1406 that is not even a year old, do you think my accelerator pump in it is done? As always any input helps! I want to paint the car this summer, not spend it rebuilding the motor! Thanks guys!

- Nik

 

[caspianwendell]

Is the accelerator pump squirting a good steam of fuel? I did not see any reference to the vacuum advance...are you sure it's working properly? Are you running points? what ignition system do you have?

 

[marks73]

Cross a plug wire maybe?

 

[Bill73Ragtop]

Distributor. Check for play in the shaft.

 

[will e]

Old plug wires will do it too. Start the car in the dark, look for any crossfiring of the wire. It has to be pretty dark to see it.

 

[71_resurrection]

The plug wires are a couple of years old. As for the distributor it is stock with a pertronix ignitor II upgrade and a flame thrower coil. As for the vaccum advance, I dont know enough about it to go messing with it. If someone can explain it to me, I am a quick learner. I will check my wires for crossfiring as well.

 

[CZ-75]

Can you tell if the backfire is happening in the exhaust or intake?

 

[72HCODE]

ok,

Backfire through the carb means too lean, not enough fuel

backfire from tailpipe means too rich, too much fuel.

assuming the car was ok one day, you didn't change anything else and started to have issues then we can be looking at ignition problems or carburetor problems.

you said the car is fine unless it is accelerating or climbing a hill. that is a high load situation, first it could be a ignition wire. but i'm leaning toward a blown power valve inside the carb.

a bad accelerator pump would cause a carb backfire but you should check the accelerator pump. push the throttle and watch the shooters if fuel is spraying out of the shooters then the accelerator pump is fine.

make sure no ignition wires are loose if you find them all tight, you can run a timing light and move the pick up through all 8 wires and just make sure all the plugs are firing the strobe correctly and nothing is miss firing. a miss fire will look like the strobe light is not consistent.

firing inconsistent then you know you have a problem in the wires or rotor or cap or the electronic ignition has gone funny, a problem with points will usually effect all 8 cylinders firing, a problem with electronic ignition can effect 1 or 2 cylinders by themselves.

after checking them and the problem still persisting. open up the carb and change the power valve and see what happens.

a blown power valve can cause tailpipe backfires, the valve is suppose to open under 6.5 HG" then close as vacuum rises. so when you punch it if the valve is blown it will go, then hesitate then backfire then kind of go and may keep backfiring.

are you getting black smoke out the tailpipes when you floor it as well as the backfire?

a blown accelerator pump will, Backfire through the carb, either stall the engine or massive hesitation and then go.

a blown power valve, Car starts to go, then hesitation, then tailpipe backfire.

some new power valves have anti blow out protection. basically one backfire or carb backfire can damage the power valve and rip the diagram, ethanol gas can eat the rubber inside of carbs. the 10% and now 15% is nasty stuff it traps more water also and eats the fuel system from the inside out over a few years. its possible that your area switched to ethanol in the last 5 years and the fuel finally ate out the older edelbrock power valve and the accelerator pump can be effected, new rebuild kits for carbs are suppose to be ethanol friendly, they use a new rubber that is green colored the old was either black or red colored.

To give you an idea about 8 years ago my area switched to ethanol year round. we used to have normal gas in the spring/summer and ethanol in the winter/fall only, so my ford never got ethanol since i only drove it when it was warm out. they switched to ethanol year round.

with in 1 week my rubber fuel lines turned into GUM and i had leaking fuel on my garage floor, i had to change out my fuel lines from the Fuel tank to fuel line from the fuel line to my fuel filter, the rubber isolator on the engine bay apron by the booster and the fuel line going into the carb.

next it ate my accelerator pump diagram, i had a pool of gas sitting on my intake. i had to swap the original black rubber for the new green rubber version. i did change the power valve also. and my idle air bleeds started to leak and i had to change the o-rings on them also.

then 2 months later i get a notice in the mail from the state that the ethanol change has caused massive frustration for everyone with a car older then 2006 and you could not sue the state for car repairs. Wonderful government a-holes.

 

[71_resurrection]

ok,

Backfire through the carb means too lean, not enough fuel

backfire from tailpipe means too rich, too much fuel.

assuming the car was ok one day, you didn't change anything else and started to have issues then we can be looking at ignition problems or carburetor problems.

you said the car is fine unless it is accelerating or climbing a hill. that is a high load situation, first it could be a ignition wire. but i'm leaning toward a blown power valve inside the carb.

a bad accelerator pump would cause a carb backfire but you should check the accelerator pump. push the throttle and watch the shooters if fuel is spraying out of the shooters then the accelerator pump is fine.

make sure no ignition wires are loose if you find them all tight, you can run a timing light and move the pick up through all 8 wires and just make sure all the plugs are firing the strobe correctly and nothing is miss firing. a miss fire will look like the strobe light is not consistent.

firing inconsistent then you know you have a problem in the wires or rotor or cap or the electronic ignition has gone funny, a problem with points will usually effect all 8 cylinders firing, a problem with electronic ignition can effect 1 or 2 cylinders by themselves.

after checking them and the problem still persisting. open up the carb and change the power valve and see what happens.

a blown power valve can cause tailpipe backfires, the valve is suppose to open under 6.5 HG" then close as vacuum rises. so when you punch it if the valve is blown it will go, then hesitate then backfire then kind of go and may keep backfiring.

are you getting black smoke out the tailpipes when you floor it as well as the backfire?

a blown accelerator pump will, Backfire through the carb, either stall the engine or massive hesitation and then go.

a blown power valve, Car starts to go, then hesitation, then tailpipe backfire.

It doesnt smoke at all, just a little oil and water blows out when revving. I will look at the elctronic plate inside the distributor to see if there is any visible damage. However, when I punch it, it does backfire, go, backfire, hesitate, go, backfire and so on. This morning wasnt so bad, but i was very easy on acceleration due to this fact. Looks like I will be changing the power valve if all of my other troubleshooting turns up negative. Thanks for the very knowledgable input!

 

[72HCODE]

damage to the electronic ignition will not be physically visible, that is why you want to check the plug firing with a timing light to just make sure everything is fireing right, a misfire will show up REALLY quick that way. if the strobe isn't strobing constantly you know you have a problem on one cylinder.

check the accelerator pump you can do that with the car turned off in the driveway. take off the air cleaner shine a light down the barrels and push the throttle fuel should come out of the shooters and spray down the ventries the second the throttle lever moves.

that means the pump is working if you push the throttle and then nothing happens then the pump isn't working, if you move the throttle and nothing happens then 3/4 of the way to WOT the shooters spray then you know the carb linkage is messed up and needs to be looked at and adjusted so the spray comes on the second the throttle moves.

that will tell you condition of the accelerator and electronic igntion. but i suspect the power valve has blown. to change it you may need a carb rebuild kit for the replacement gaskets, in addition to a new power valve.

on a holly to change the power valve you have to take the carb off the intake, then you will need 2 replacement gaskets one for the meter block and one for the power valve itself.

some Double pumps have 2 power valves one on secondaries, that requires 4 gaskets to change both. you have to take the fuel bowls off to get to the center meter blocks. on a edelbrock things are different so consult your manual or you can get a PDF online to show you how to change a power valve.

 

[71_resurrection]

damage to the electronic ignition will not be physically visible, that is why you want to check the plug firing with a timing light to just make sure everything is fireing right, a misfire will show up REALLY quick that way. if the strobe isn't strobing constantly you know you have a problem on one cylinder.

check the accelerator pump you can do that with the car turned off in the driveway. take off the air cleaner shine a light down the barrels and push the throttle fuel should come out of the shooters and spray down the ventries the second the throttle lever moves.

that means the pump is working if you push the throttle and then nothing happens then the pump isn't working, if you move the throttle and nothing happens then 3/4 of the way to WOT the shooters spray then you know the carb linkage is messed up and needs to be looked at and adjusted so the spray comes on the second the throttle moves.

that will tell you condition of the accelerator and electronic igntion. but i suspect the power valve has blown. to change it you may need a carb rebuild kit for the replacement gaskets, in addition to a new power valve.

on a holly to change the power valve you have to take the carb off the intake, then you will need 2 replacement gaskets one for the meter block and one for the power valve itself.

some Double pumps have 2 power valves one on secondaries, that requires 4 gaskets to change both. you have to take the fuel bowls off to get to the center meter blocks. on a edelbrock things are different so consult your manual or you can get a PDF online to show you how to change a power valve.

So I spoke with an Edelbrock tech this morning and there are no power valves in their carbs. He suspects the accelerator pump or maybe even as far as a ground down lobe on the cam, not allowing proper intake. I still need to check the wires and the firing with a light, just need the time. Picked up an accelerator pump in the mean time and will keep troubleshooting if the swap out doesn't solve the problem. What electronic ignition do you suggest if I decide to replace the whole thing? Thanks again!

 

[Kermit 460]

Would"nt a compression test rule out a bad cam? :huh:

 

[caspianwendell]

The plug wires are a couple of years old. As for the distributor it is stock with a pertronix ignitor II upgrade and a flame thrower coil. As for the vaccum advance, I dont know enough about it to go messing with it. If someone can explain it to me, I am a quick learner. I will check my wires for crossfiring as well.

If the pickup in the pertronix is not adjusted right it will do that too...check your clearance, don't remember the number but it is really close. I would keep the ignition...I have had good luck with these. but it does seem like when you run one a while, touch it, or move it, you have to reset it everytime

 

[goodnigh]

The plug wires are a couple of years old. As for the distributor it is stock with a pertronix ignitor II upgrade and a flame thrower coil. As for the vaccum advance, I dont know enough about it to go messing with it. If someone can explain it to me, I am a quick learner. I will check my wires for crossfiring as well.

If the pickup in the pertronix is not adjusted right it will do that too...check your clearance, don't remember the number but it is really close. I would keep the ignition...I have had good luck with these. but it does seem like when you run one a while, touch it, or move it, you have to reset it everytime

A piece of plastic came with my Ignitor setup and if I am reading the

micrometer correctly it is 0.030". About the thickness of a matchbook

which we once used to set the points.

mike

 

[Jeff73Mach1]

Would"nt a compression test rule out a bad cam? :huh:

only if it is an intake valve and it is worn completely flat.

I would look at ignition advance.

With a timing light hooked to plug wire 1 and a mark on the damper of white chalk around 30-36 degrees, engine running everything hooked up advance the throttle to 3000 rpm. If you don't see the white mark close to where the timing tab points you aren't getting full advance. I full circumference of the damper is 180 degrees, not 360 so 30 degrees of advance will be 1/6th of the way around the damper.

Simple test that can be done in a minute or two and will tell you if your distributor is advancing properly.

the timing light can also be used to test individual cylinders for spark signal.

a vacuum gauge would be a helpful tool as well to check for worn cam lobes and such

 

[marks73]

Would"nt a compression test rule out a bad cam? :huh:

only if it is an intake valve and it is worn completely flat.

I would look at ignition advance.

With a timing light hooked to plug wire 1 and a mark on the damper of white chalk around 30-36 degrees, engine running everything hooked up advance the throttle to 3000 rpm. If you don't see the white mark close to where the timing tab points you aren't getting full advance. I full circumference of the damper is 180 degrees, not 360 so 30 degrees of advance will be 1/6th of the way around the damper.

Simple test that can be done in a minute or two and will tell you if your distributor is advancing properly.

the timing light can also be used to test individual cylinders for spark signal.

a vacuum gauge would be a helpful tool as well to check for worn cam lobes and such

You and 72Hcode should be writting a book! Dead on advice guys!

 

[Kermit 460]

Years ago ,I had a 71 F250 w/ a 352 that did that on me.Being a kid ,I just threw in a used cam,no bearings,lifters or anything ,would you believe it actually ran for 6 months?

 

[caspianwendell]

The plug wires are a couple of years old. As for the distributor it is stock with a pertronix ignitor II upgrade and a flame thrower coil. As for the vaccum advance, I dont know enough about it to go messing with it. If someone can explain it to me, I am a quick learner. I will check my wires for crossfiring as well.

If the pickup in the pertronix is not adjusted right it will do that too...check your clearance, don't remember the number but it is really close. I would keep the ignition...I have had good luck with these. but it does seem like when you run one a while, touch it, or move it, you have to reset it everytime

A piece of plastic came with my Ignitor setup and if I am reading the

micrometer correctly it is 0.030". About the thickness of a matchbook

which we once used to set the points.

mike

Sounds like too much gap to me...look all I know is from experience with a couple around here. The module had to be almost touching the pickup coil for the engine to run right. it would not even fire at that much gap

 

[72HCODE]

if the cam did wipe you would see something in the oil or get the oil filter out and carefully cut it open and see how full of metal it is.

you could pull the valve covers and have a look at the top end, maybe roll the engine over by hand and see how the valves are opening and closing, i don't think either would be conclusive unless you pulled the cam.

going on that the problem is not that major. we are still looking at a carb issue or ignition issue.

some edelbrocks do have power valves, some don't.

if you want to eliminate a possible issue then ditch the electronic ignition and install a set of points for diagnostics. if the module or pickups have malfunctioned using points will tell you instantly.

if the points have no effect, you could keep going swaping wires, rotors and caps, or pulling plugs and seeing if any of them look very different since that would pinpoint a cylinder that has problems.

otherwise the problem is back in the carb. it might be simple like a piece of trash from the tank is clogging a fuel circuit inside the carb. you could rip the carb apart and try to find it, or swap a known good carb and see if the problem stops , just like switching to points it would pinpoint where the problem is either spark or fuel.

it is hard to diagnose through the net. a compression test can be inconclusive also.

All the electronic ignitions drive you insane when they go bad. there are so many ways they can go bad. either the pickup goes wrong or the magnetic base that triggers the pickup can go wrong. I've had both happen.

now the coil could of gone bad too, causing a weak spark. but now you get into the change parts until the problem stops situation.

A vacuum gauge would help in diagnosing a problem with valves, but that is hard to do also unless it stands out like a sore thumb.

"General Ignition Troubles Or Sticking Valves

With the engine idling, continued fluctuation of 1 to 2 inches may indicate an ignition problem. Check the spark plugs, spark plug gap, primary ignition circuit, high tension cables, distributor cap or ignition coil. Fluctuations of 3 to 4 inches may be sticking valves."

thing is it is possible for miss adjusted idle air bleed to look just like a sticking valve