holley power valve

[70gtx]

what does it do. is it that fuel goes though jets than when vac. drops below the # on PV, the PV opens to give a bigger flow of fuel.

[rumblefish360]

YOU GOT IT!

[Rictek]

What happens if it goes bad? Can a backfire break it? How would I know this from a different problem?

[road chicken]

If your carb is an early one without the protection valve, a backfire can blow the valve,(s) some have two, primary and secondary. If it goes bad, you will get a lean spot in your power curve because the flow from the PV(s) won't be supplementing the jets, until the vacuum allows them,( the jets), to deliver the full fuel charge. I would think a bit of pinging would be in order due to the lean mix, but I've never lost one so I'm not exactly sure.

[70gtx]

i'm not an expert but have been really studing this carb so i know how it works.you will know if it is not working because it will run rich off an idle to probably mid rpm range but will be fine at full throttle. it will put black smoke out tail pipes. vacuum holds it closed till vac. drops below the number power valve you have. (if its a 6.5 PV it will open when vac. drops below 6.5"). if float is to high it will drip from venchuri's at an idle .other wise you should be good to go.
the holley .com was some good tech on carbs.i found that i may have to open front and rear carbs a little to compensate for large cam.(6pack)
summit sells the valves to protect your power valve from back fire.