backfire through exhaust

[coloradostang 10]

New 351W, edellbrock performer rpm heads, cam and manafold, quick fuel carb. I adjusted the valves when engine was warm. Idles well but backfires out the exhaust when acellerating and coasting. not a harsh backfire but it is backfiring. but not all the time. Have not messed with the carb. Have tried different settings with the timing. Same result. I am going to look for any loose bolts. Does it sound like I need to reset the valves again? I am running premium fuel. Header bolts were a little loose, but nothing else.

Edited June 11, 2013 by coloradostang
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[FrozenMonkey 12]

Is it a backfire or an afterfire? There is a difference. Backfire is an ignition in the intake. Sounds like afterfire to me, which would likely mean you are running rich and somehow igniting the unburnt fuel in your exhaust. Your timing could also be advanced too much or you could have a really free flowing exhaust. I'd check your A/F ratio's and timing and go from there.

 

As an example, my 99 Trans Am sounded fine until I added longtubes, deleted the cats and did a 3" exhaust. It would pop a bit when decelerating, but A/F and timing were dead on.

[coloradostang 10]

Sounds like it's afterfire. I have headers and 2 1/2 exhaust. I was running sidepipes but it was too loud. So I put a short lenth of pipe after the glasspacks and added a couple supertrap mufflers as a guick fix. Could that cause the problem?

[DocWok 30]

For what it's worth I'd recheck how I adjusted the tappets, sounds like you are right on the borderline of too much preload and as the motor warms up the exhaust valves are expanding and just starting to stay off the seat, causing the popping you can hear in the exhaust system.

[DocWok 30]

For what it's worth I'd recheck how I adjusted the tappets, sounds like you are right on the borderline of too much preload and as the motor warms up the exhaust valves are expanding and just starting to stay off the seat, causing the popping you can hear in the exhaust system.

[Max Power 74]

Describe your procedure for adjusting the valves.

[Max Power 74]

Describe your procedure for adjusting the valves.

[DocWok 30]

There are lots of different ways to go about valve adjustment, but here's how I'd do it.

Firstly take out all your spark plugs, it makes it easier to rotate your engine.

Next use a socket and ratchet attached to the harmonic balancer retaining bolt and rotate your engine in the normal direction of rotation (clockwise).

Look for any rocker that is starting to compress a valve spring, when the rocker is no longer travelling down and stop for a moment, from this point you rotate the engine one full rotation, this will make sure that you are then on the cam base circle for that particular rocker.

Next back off the rocker adjusting nut until there is a bit of clearance between the rocker and the valve when you rock it back and forwards with your fingers.

Then grip the push rod with your fingers and rotate it back and forth while at the same time you slowly start to tighten the rocker adjusting nut, there will come a point where you start to feel a drag on the push rod as you are rotating it, stop there. This indicates that you have taken out the clearance from the valve system. Now you continue to tighten that rocker adjusting nut a further 1/4 turn, this will give you enough lifter preload. That's it, your done for that particular valve.

Now repeat this same procedure for each of the remaining valves, there is no sequence you have to follow just look out for a valve that is on its way down, and remember which valves you have previously adjusted! I know this is a slow way of doing it, but it's a no brainer method for getting it done correctly. Good luck.

[DocWok 30]

There are lots of different ways to go about valve adjustment, but here's how I'd do it.

Firstly take out all your spark plugs, it makes it easier to rotate your engine.

Next use a socket and ratchet attached to the harmonic balancer retaining bolt and rotate your engine in the normal direction of rotation (clockwise).

Look for any rocker that is starting to compress a valve spring, when the rocker is no longer travelling down and stop for a moment, from this point you rotate the engine one full rotation, this will make sure that you are then on the cam base circle for that particular rocker.

Next back off the rocker adjusting nut until there is a bit of clearance between the rocker and the valve when you rock it back and forwards with your fingers.

Then grip the push rod with your fingers and rotate it back and forth while at the same time you slowly start to tighten the rocker adjusting nut, there will come a point where you start to feel a drag on the push rod as you are rotating it, stop there. This indicates that you have taken out the clearance from the valve system. Now you continue to tighten that rocker adjusting nut a further 1/4 turn, this will give you enough lifter preload. That's it, your done for that particular valve.

Now repeat this same procedure for each of the remaining valves, there is no sequence you have to follow just look out for a valve that is on its way down, and remember which valves you have previously adjusted! I know this is a slow way of doing it, but it's a no brainer method for getting it done correctly. Good luck.

[j69302 11]

Since it does it while accelerating, the first thing I would do is verify the ignition timing and then fuel mixture. And make sure you're not hearing miss-fires, they can sound a little similar.

 

If your ignition timing is late, or A/F ratio too rich, this is what will cause it. Not necessarily valves. However, if it feels like it is lacking power, then it can likely be the valve adjustment.

 

Anyways, If you use DocWoks procedure, but follow this order, it will save time and there is less chance of missing a valve or making a mistake.

 

First start with the #1 cylinder at top dead center on the compression stroke. You can now adjust BOTH valves at the same time as described by DocWok.

 

You can put your finger in the #1 plug hole, rotate engine over slowly and wait for air to come out. At this point, use a flashlight, or even a small screwdriver for feeling the piston move, or looking into the hole. You should be able to see/feel the piston come up, then stop and go back down. Spin it back and forth small amounts until you find the piston's "dead spot" at TDC. Just make sure you dont jam the screwdriver in there Use care if using a screwdriver.

 

Once done, rotate the crank shaft 90 degrees in the correct direction while following the firing order of the engine.

 

So, for a 351w (I pulled this from a google search, you should verify it is correct), adjust

 

adjust #1, rotate crank 90*

adjust #3, rotate crank 90*

adjust #7, rotate crank 90*

adjust #2, rotate crank 90*

adjust #6, rotate crank 90*

adjust #5, rotate crank 90*

adjust #4, rotate crank 90*

adjust #8, rotate crank 90*

 

And you are done.

Edited June 25, 2013 by j69302

[j69302 11]

Since it does it while accelerating, the first thing I would do is verify the ignition timing and then fuel mixture. And make sure you're not hearing miss-fires, they can sound a little similar.

 

If your ignition timing is late, or A/F ratio too rich, this is what will cause it. Not necessarily valves. However, if it feels like it is lacking power, then it can likely be the valve adjustment.

 

Anyways, If you use DocWoks procedure, but follow this order, it will save time and there is less chance of missing a valve or making a mistake.

 

First start with the #1 cylinder at top dead center on the compression stroke. You can now adjust BOTH valves at the same time as described by DocWok.

 

You can put your finger in the #1 plug hole, rotate engine over slowly and wait for air to come out. At this point, use a flashlight, or even a small screwdriver for feeling the piston move, or looking into the hole. You should be able to see/feel the piston come up, then stop and go back down. Spin it back and forth small amounts until you find the piston's "dead spot" at TDC. Just make sure you dont jam the screwdriver in there Use care if using a screwdriver.

 

Once done, rotate the crank shaft 90 degrees in the correct direction while following the firing order of the engine.

 

So, for a 351w (I pulled this from a google search, you should verify it is correct), adjust

 

adjust #1, rotate crank 90*

adjust #3, rotate crank 90*

adjust #7, rotate crank 90*

adjust #2, rotate crank 90*

adjust #6, rotate crank 90*

adjust #5, rotate crank 90*

adjust #4, rotate crank 90*

adjust #8, rotate crank 90*

 

And you are done.

Edited June 25, 2013 by j69302