Power Valve Sizing
 

The carbs came with the engine from the builder with 6.5 power valves installed. After getting my engine back into running condition, manifold vacuum measured 12 inches. According to what I've read, I should be running a 5.5 power valve.

What size power valves are you guys running in regards to your manifold vacuum?

What are the consequences of running a too high or low rated power valve?

rule of thumb is to divide vacuum by two, and if in between sizes round up.
so 12 in of vac is a 6.0 so a 6.5 sounds right is there some sort of drivability
problem that is leading you to question the power valve?

Fred

The power valve opens when the manifold vacuum goes below the number on the valve. The number has NO effect on the amount of gas used ... it only regulates WHEN the power valve opens. You might want to measure the manifold vacuum at WOT under a load. Helps to have a passenger look at the gauge, especially if you do not have one in the dash. Check to make sure the vacuum is below the number of the power valve.

Standard Holley power valves are rated between 2.5 and 10.5 "Hg vac. There can be two problems with using the wrong valve.

If you put a 2.5" Hg valve in your car and the minimum vac at WOT was say 3.0" HG, it would never open. Tremendous loss of power.

On the other hand, if you used a 10.5"Hg valve and the engine had a very rude cam with a lumpy idle and was at say 8" HG at an idle, the power valve would be open all the time. Your exhaust would be black and it would run like a pig.

The most common power valve is a 6.5" Hg, so you are all right with the valve you got now.

You might want to get a book on Holley Carbs. The best one IMHO is by Dave Emanuel

http://www.buy.com/retail/product.as...51&dcaid=17902

I recall reading a recent thread that mentioned how inaccurate the power valve rating from the package was. They recomended a tester to check and verify the rating before installing. I guess not all 6.5's are a true 6.5 for example. Also a good backfire can damage a power valve.

John

The exhaust is slightly dark, and smells of gas (rich), with black soot in pipes. Just had carbs tweaked, everything adjusted except for power valve. Just wondering if this could be the cause of the rich exhaust fumes.

Test your power valve by taking it out of the carb and placing a quality hand vacuum pump to the diaphragm side.

Squeeze the pump until the diaphragm pulls in. Look at the dial reading at that point. YOU will need to use the cup that comes with the pump.

If you use a good pump ( one that has a bit of accuracy ), you should be able to get an idea of when YOUR valve begins to work.

Hope this helps.

:D

I would first adjust the idle mixture screws. Turn them in (Clockwise) about 1/8 turn and see what the idle is like. This should lean out the rich condition at idle. It would help to have a good vacuum gauge hooked up when making the adjustment. Many people say that the idle screws should be about 1.5 turn off the seats for best idle mixture, but the 750 cfm carb on my Cobra runs best at 3/4 of a turn off the seats. If it improves the idle, adust the screws cw a little more and see what happens.

Of course, I'm assuming that the power valve is NOT bad. And adjusting the idle screws are quick and easy adjustment as compared to disassembling carbs.

I'm also assuming that the floats are adjusted properly, and not set too high so that gas spills out of the bowls at idle.

You said that you just had the carbs tweaked. Was this done while the engine was running? Hopefully the person doing the tweaking is a good tuner.

Here is the vac pump I have.
http://www.tooltopia.com/index.asp?P...OD&ProdID=8121

I also have a 427 SO with dual inline Holley 600 CFM carbs. My side pipe exhaust are all sooted up also. I just had new primary metering blocks prepared with replaceable idle restricters. The new primary metering blocks came with .026" idle fuel restricters and #5 powervalves ( I had #6 powervalves before). I changed the idle restricters in the Quick Fuel Technology secondary metering plates from .038" to .032". The problem I had was mostly the Idle was way to rich. Startup in the garage is a mess . With all the soot in the pipes and the engine creating water (initialy in liquid form), I have black spatter blubering out of the ends and joints. I have single car garage so correct placement of cardboard is a great help. It appears to be running better and I hope the side pipes will clean out over time. In reality, the secondaries are influencing the idle but probabably have never opened. If you read the thread on this forum "inline dual fours" you will see some detail. I'm still running #65 primary and #67 secondary jetting. The move downward on the powervalves helped the surging under a light load at near idle (second gear in a hospital parking garage). Recent state emission testing, cruise HC was over limit but close. Idle HC was over 10 times the 220 PPM HC limit. I'm hopping that most of the sooting was from the idleing. It is a work in progress.

I would suggest getting a wide band O2 sensor. Also check out

http://www.innovatemotorsports.com/f...isplay.php?f=7

and read all the posts on tuning Holleys. There is tons of info I have found here that I have seen no where else...