Blown Holley power valve symptoms?
 

Just got it all back together and it tends to backfire through the carb under quick acceleration or blipping the throttle more than a little bit. Accelerator pumps appear to be working fine, so I'm assuming this may be a power valve problem? Timing is at 6 to 8 degrees initial, haven't confirmed the total or timing of it yet, but distributor has a fresh rebuild.

The carb is a 650CFM Holley DP. Are there different power valves as well, I seem to have a collection of them. And is there any way to test or inspect for being 'blown'?

Thanks

If it backfired you need a power valve! Set your timing before you blow another one.

Timing is set, but may not have been on first couple of attempts at starting. Is there a specific model or version and what are the symptoms when one is blown?

when a pv suddenly blows, it stalls. it wont idle unless u turn the idle screw way in. plugs will be black. it will get 4-7 mpg.
Have u by chance, messed with the valve lash?

well now
 

:iagree: If the power valve blows it leaves a big leak of fuel into the motor making idling impossible, you need to constantly blip the throttle to keep the motor from loading up.
Power valves are not easy to blow. Most holleys have a check valve preventing this.

Pv
 

I had this problem early on with my 66 327/300. Never could get the carb right, and always backfireing.

A friend suggested a PV protector sold by Jeg's # 720-1511G for $9.99. We got it and installed it, and the problem was solved.

Hope this will be of help

Frac :) GB

To check the power valve, put your finger over the front bowl vent with the engine idling - if it takes several seconds for the engine to die, the PV is OK; if it dies almost immediately, the PV is leaking. Plugging the bowl vent forces fuel into the engine by way of the PV vacuum passage; if the PV is good, the fuel will be forced out of the boosters to stall the engine and this will take several seconds to occur. :thumbs:

In addition to the correct procedure for checking the PV as JohnZ stated above, you might want to note that a backfire up through the carb is an indication of a lean condition - not a rich condition. When the PV is blown, the carb goes rich, so backfire through the carb will normally not occur unless you also have a severe timing problem or other issues going on.

Thanks to both of you. It idles fine once warmed up and does seem to be pretty cold-natured, so the tip about being lean is probably spot on.

For Matt, messed with the valve train...you mean took it all apart and left it that way for years? Just kidding, but yes, it has been apart and back together. Valve adjustment was performed cold per specifications. I am expecting to do it again once when hot after I've had a chance to drive it a bit, so far it's only been around the block. But it does start and run quite well with the exception of this backfiring issue.

I'm going to run John's test, but I think the PV is fine from all indications, and then I'll check my carb adjustments as well. There's a part of me hoping I can find a reason to trash this carb and get one of the newer ones from Lars.

Be aware how the power valve works. The intake manifold vacuun holds the valve closed. When the intake vacuum drops below the rating of the valve, like when under load, it opens richening the mixture.
Each power valve has a vacuum rating stamped in the flange of the valve. This is the closed vacuum rating. The best way to test the valve for proper operation is with a hand vacuum pump and gauge. Most hand pump kits have a variety of accessories, one is a large, 1" dia shallow cup. Remove the valve from the metering block. Grease up the surface of the cup and hold the valve in the cup while pumping up the vacuum. The valve should close at the value of the rating on the flange. If it closes too early, the engine will be lean at load, if it takes extra vacuum, it will be rich.
If the cam has ever been changed affecting intake vacuum, the power valve rating should be changed. Measure the intake vacuum at idle and part load. The valve should never open below idle intake vacuum and should open just at part load. Ideally this vacuum should be determined under actual load conditions but can be approximated off the part load.
Holly has made several different series of power valves. Some for racing, others for economy. A modern version for retrofit is staged to enrichen the mixture in steps.

Thanks jsdailey, that's a very helpful link. It confirmed my antique memory on how the power valve functions and gave me further setup tips.

RGS, good information too, very similar to the link above. Thanks.