I wish someone would tell me for sure what the "13" means...

 

The back fire protection is located on the base plate. The picture you show here is a two stage power valve that I would just get rid of completely. those are usually only good for large trucks and motorhomes. Sounds like you have an idea already how to choose the right power valve opening so just get a single stage and you just may have solved your problem.

 

problem solved, thanks!

 

Have a number of them with 1 and 3 stamped on them they are all different sizes. Not sure what the significance is.

Ran into something yesterday where the 4 corner idle screws were non responsive turned out the rear float was stuck down.

Dont know if that helps. Usually something simple.

 

I just pulled out one of my spare 6.5's It has a 6 and below it a 5 on the face of the bronze colored big end

With a radical cam a 6.5 is a good starting place.

In a correctly running carb the engine should die if you turn in the idle screws. If your car can run it has to be getting fuel from another curcuit.

 

If that is the power valve that came out of your carb get rid of it anyway.
get a single stage 6.5 or better yet 4.5 and you will be done with it.

 

I'll take a look at that as well...

 

I ordered the 7.5 power valve yesterday (before you guys posted). Wish I would have gotten the 6.5, but the 7.5 should be ok. I really pull at least 15 in of vacuum at idle so that may be another thing to chase down is why the vacuum is so low.

 

if its out of tune it will drop some vacuum.
Either one of those PV is fine with 15

Only make 7 in on a good day use a 4.5 no issues.


That 1/2 vacuum rule is a wives tales.

 

Re power valve size, according to Lars, you should select a valve that is 2" hg less than your idle idle vacuum. So if you havve 11" at idle an 8.5 would work. BUT if you have an auto, you need to check the vacuum in drive.
On my car with a mild cam I have 14" in neutral and 8" in drive. so I went with a 4.5. A 6.5 would most likely have worked. The wrong poere vale effects off idle and low speed operation
Another issue is the vacuum advance canister. My current one starts to advance at 8" hg and all in by 10". Works great in Neutral, but in drive with 8" I am having an unstable idle... just my 2 cents!!

 

I have a LIST 80555C Model 4175 on my truck. I bought it new, and it already had the power valve protection built in.

 

All holley carbs manufactured since '92 have power valve protection installed from the factory; excepting the 4500 series.

 

Off Holleys web site-

QUESTION How do I tell what size power valve I need?
ANSWER To properly size a power valve, take a vaccum reading at idle and if it is above 12" for a standard transmission a 6.5" will be safe to use. For automatic transmissions take a vaccume reading in gear at idle and if the vaccum is below 12" divide that in half for proper size. Example 9" of vaccume in gear at idle will require a 4.5" power valve.

http://www.holley.com/TechService/FA...ory=Carburetor

The 2" below idle vac rule is also found in many tuning books since dirt was invented. I usually use that rule personally.