[Ktaves1971]

My current project is a 400 sbc with World Products torquer heads, 268 Extreme Energy Comp cam, 480 lift, 9.5 compression, rpm intake and a 750 vaccuum holley carb that is rebuilt. It has a new baseplate that I think has the powervalve protection built into it. But this baby backfire a few times in a big way when I was setting it up and starting it. Sounded like a BOMB went off! Well now it doesnt run quite right, it stumbles off idle and seems to be lacking the power that it should have.

Could I have blown the power valve? Also what no of power valve should I be running? Most rebuild kits come with the 6.5 but someone is recommending a 4.5 to me. I dont understand the concept of the power valves yet.

Sorry for being so longwinded.

[Tmod]

Well the power valve adds extra fuel when the vacuum falls below the rating of the power valve.

This is done so you can run a leaner main jet yet not suffer any detonation or any other problems from a lean mixture when under power.

I am not confident that the power valve blowout protection will stop all damage to the power valve and especially when several good backfires take place. They are cheap and easily replaceable so I would suggest just changing it out.

Usually the power valve rating is determined by having the vehicle at idle and in gear (With the parking brake on) and select a valve that is 2" lower than the current vacuum reading. If you have a short duration cam that builds alot of low RPM power the vacuum will be somewhat high and you won't be able to find a valve with a rating that high.

For instance on my truck it idles in gear at 16" vacuum and I will be putting a 9.5 power valve in as that is where my carb goes lean when accelerating from a cruise mode.

There I outwinded you!