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Discussion Starter · #1 ·
I own a 1968 Mustang Fastback with a 302 and C4. A couple of years ago I bought a new 600 cfm Holley 4160 to fix the backfire problem with the power valve. Since then on 2 occasions the motor has had a soft backfire that was barely a hiss. Both times the power valve was blown. Has anyone else had this problem?
I called Holley Tech regarding the pressure threshold for blowing the power valve and was told the research had never been done.
Thanks,
Glenn Risley
Medford Oregon
 

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Discussion Starter · #4 ·
What is your manifold vacuum pressure at idle? what size power valve?
alanmacdougall Thanks for you reply. I have never checked my manifold vacuum pressure. I have taken the old power valve to the parts house and gotten the same size so it would be the 'stock size' valve.
Just a little more info on the motor: its 1993 SVT Mustang Cobra 5.0 with a Ford Racing E303 cam, GT40 heads, 1:17 lifters, roller cam and rockers and shortie headers. The block had less than 100K miles which needed a quick honing and standard pistons and rings when I had it rebuilt. I have put about 1,000 miles on it in 3 years.
 

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The cam will affect the vacuum. It would be good to know what manifold pressure you have to make sure you have the correct power valve. But as far as blow out goes, I would try to figure out what causes the backfire. Have you checked your timing? Also I would check the distributer cap, rotor and and spark plug wires. How old was the power valve? Do you use10% ethanol gas? Not sure if they degrade with ethanol since I use pure gas.
 

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Discussion Starter · #6 ·
What is your manifold vacuum pressure at idle? what size power valve?
What is your manifold vacuum pressure at idle? what size power valve?
The cam will affect the vacuum. It would be good to know what manifold pressure you have to make sure you have the correct power valve. But as far as blow out goes, I would try to figure out what causes the backfire. Have you checked your timing? Also I would check the distributer cap, rotor and and spark plug wires. How old was the power valve? Do you use10% ethanol gas? Not sure if they degrade with ethanol since I use pure gas.
I use pure gas. I will look at all the things you mentioned when we go to the house the car is kept next month. I used the E303 cam as the B303 is used in manual equiped cars as it creates less vacuum. My cam was installed at 0 degrees and the inital timing is around 12 to 14 degrees BTDC.
The car runs great, lots of Scoot, 99% of the time. Once in a while when starting it after cranking for 5 to 10 seconds and releasing the key as the engine is slowing down there is a soft noise of a back fire. There is never a hard banging back fire.
Thanks again, Glenn
 
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