Jims66

How do you know if the power valve is blown ?? I was adjusting the idle mixture screws the other day and had it a little too lean as it backfired (with a flame) when I hit the gas. I've since re-adjusted (richen) and it seems to be running ok. I've heard if the motor backfires there's a good chance the power valve got blown. The final adjustment is about three quarters of a turn out on each mixture screw. The carb is a 750cfm w/vacume secondaries on an L79 4 speed car. The power valve is a 6.5.

Donald #31176

I believe if you were to put a finger over the front vent of the carb if the engine was to stop the power valve is defective. If the engine runs but eventually dies the valve is okay.

Don Keefhardt

Well...this is going to sound weird...but you can suck on them to test them. Really.

Remove power valve. Place in mouth, with one side exposed to atmosphere and the other side exposed to tonsils. Suck, and if it's good, you can hear/feel it go "click".

Really.

Ron Miller

That's a fact, as well. 6.5 inches of vacuum will open the valve if it's on the right side of the valve.

But, I believe you're OK without having to remove the valve. Any time mine has blown in the past (when I used a Holley), it would idle overly rich, impossible to set the idle screws properly to get a correct idle. I think you dodged the bullet this time.

Jims66

That's how I've tested them too. I guess I should have said that I was trying to avoid pulling the bowl off the carb due to a tad of laziness and was wondering if there were any tricks to testing them while the car is running. I'll try Donald # 31176's method and see what happens.
Thanks folks.

65 vette dude

Turn idle screws all the way in,if motor still runs,pv is blown. I think I read somewhere that since the mid nineties,all Holley's are made with blow-out proof power valves.

Ron Miller

Blow out resistant!!
I've even blown the blow out proof power valves!!

Dave64

If your car is still running well, the PV is probably OK. If it ruptured, the car would not want to run, as gas would be flowing down the carb and fouling your plugs to the point they may no longer be any good.

toddalin

I backfire mine at least once or twice each time I start the car (no choke horn so you gotta get it real rich) and I've never had one blow out on either my 650 or 600 dp.

Car always seems to run fine afterwards and when ever I do take the carb apart they've been fine.

Maybe I've been lucky. :o

Jims66

Thanks. I did the idle mixture screw test and the motor died as you said. I bought my Holley new (out of the box) except that was back in 1989-90 timeframe......... I guess I don't have the blow-out proof power valves kind.

olescarb

The power valves in the later holleys are not blow out proof, they have the same basic power as the earlier holleys. The change that makes the later holleys have power valves that live longer is the addition of power valve check valve in the vacuum passage. Holley sells a kit to retrofit earlier carbs, the Holley part # is 125-500.

I hope this helps Henry @ olescarb