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Old 06-21-2009, 11:43 AM
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backfire on stock 350.

Hi guys my friend and i Just got done putting a new tranny, and replacing all the gaskets (in the motor) in his 68 el camino.

it has a stock 350 crate motor and had no symptoms prior to us pulling the motor.

it has a quadrajet carb on it, and we have adjusted the timing several times and it still pops when when stomp on the gas and keep the gas on for a short period of time.

I don't know if this is relevant, but we took the headers off and put stock manifolds on it as well as replaced the mufflers.

any and all feedback is appreciated

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Old 06-21-2009, 11:56 AM
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also, it has points, not an hei distributer
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Old 06-21-2009, 12:13 PM
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Does it pop out of the carb., or the tail pipe?

Have you set the point gap, or dwell? You must set this first, because changing this setting will change your timing.

If it ran fine, before the changes you made, I would also recheck your spark plug routing on the distributor cap. You may have crossed 2 of them.

Anther thing you could try, after setting the points, and timing, is the accelerator pump lever adjustment. If it is in the top hole, drop it down to the second hole, if it is in the second hole, drop it to third last hole.

Do you have manifold vacuum going to the vacuum advance? If not, change it to manifold vacuum. When you jump on it, the vacuum drops, retarding your timing to help keeping it from pinging.

What are you setting the timing to, and how much vacuum does it have at idle?
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Old 06-21-2009, 12:17 PM
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Try backing the timing off more and see if it stops? then see if it still has power
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Old 06-21-2009, 01:09 PM
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Timing Tips

IIRC the point gap is .017? The dwell is 28-32 degrees, with the vacuum advance unplugged.

Bring #1 piston to TDC on the compression stroke, and verify your rotor is pointing at the #1 post in the cap.

With the spark plug still out, hook it back up to the plug wire. Now lay the spark plug ground strap against metal, or hook a test lead to it, and ground it where you can see it.

Now turn your balancer to 8 degrees BTDC. Loosen your distributor, and turn the key on engine off. Then turn the distributor, and lock it down, when the spark plug fires. This will set your base timing to 8 degrees BTDC. This is called Static Timing. This is a good place to start.

Now hook everything back up, and test drive it for performance.

Note: You may have to readjust your curb idle speed, and or, idle mixture screws.

If it seems sluggish, you can increase your timing by 4 degrees(with the vacuum advance disconnected, and plugged) to 12 degrees. Do this in increments of 4 degrees, until it pings under load. Then just back it off 2 or 3 degrees.

Post back what you have found.
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Old 06-21-2009, 01:36 PM
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Quote:
Originally Posted by carsavvycook
Does it pop out of the carb., or the tail pipe?

Have you set the point gap, or dwell? You must set this first, because changing this setting will change your timing.

If it ran fine, before the changes you made, I would also recheck your spark plug routing on the distributor cap. You may have crossed 2 of them.

Anther thing you could try, after setting the points, and timing, is the accelerator pump lever adjustment. If it is in the top hole, drop it down to the second hole, if it is in the second hole, drop it to third last hole.

Do you have manifold vacuum going to the vacuum advance? If not, change it to manifold vacuum. When you jump on it, the vacuum drops, retarding your timing to help keeping it from pinging.

What are you setting the timing to, and how much vacuum does it have at idle?

its coming out of the carb.

it seems we are not getting vacum, but the way i am testing it is simply putting my finger over it.

the plugs are new and the gap is good. the dwell is fine.
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Old 06-21-2009, 01:46 PM
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You may have an intake gasket leak- you did just change the intake gaskets, right? Try re tightening the intake bolts. Start from the inside bolts and work your way towards the ends.

Check all other vacuum accessories and lines for leaks. The carb pop is often vacuum leak related, or due to a lean condition because of the carb or carb tune.

A vacuum gage can help pin down whether or not you have a vacuum leak.
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Old 06-21-2009, 01:56 PM
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Quote:
Originally Posted by pittbull7934
its coming out of the carb.

it seems we are not getting vacum, but the way i am testing it is simply putting my finger over it.

the plugs are new and the gap is good. the dwell is fine.
Try the static timing procedure I posted. This should get you close for starting. I do this on all distributor engines, where I have done any major repair's before starting them up. Like this one.




You will need to find a manifold vacuum port on that quadrajet. There may be 3 of them in the front. Although, if it still has a ported vacuum switch, it may have to get to operating temperature before it gets manifold vacuum.

There is one looking from the front, on the lower left side, the one for the choke pull off is another, some QJets have one just forward of the choke housing.
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Old 06-21-2009, 06:46 PM
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thank all you guys for the input, but it turned out the 6 and 8 wires were switched (My friend put the wires on)

so it was a simple fix.

kind of an interesting day!
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Old 06-21-2009, 06:48 PM
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At least it wasent anything major. Cole
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Old 06-21-2009, 06:54 PM
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I'm glad you found it, and it turned out to be something easy.

All in all, there is some good information on this thread, for non-members to read. Hopefully it helps someone else through their dilemma.
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Old 06-21-2009, 08:56 PM
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Quote:
Originally Posted by carsavvycook
I'm glad you found it, and it turned out to be something easy.

All in all, there is some good information on this thread, for non-members to read. Hopefully it helps someone else through their dilemma.
Do you mind if i still give ya a call, I could use some tips for heads i want to pick up, and it would be nice if you could point me in the right direction of used parts, (i have been looking for a 12 bolt)
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