Adding a Power valve to a full race carb

660 center squirters and many other Holleys did not come with an enrichment circuit. Due to this they are very effective race carbs but often not suited for street usage.

Ideally you want a rich no load idle, with a transition that leans out as vehicle and engine speed increases. As the main circuit starts to contribute more to the running of the engine, the carburetor should seamlessly add fuel without being overly rich.

As the light load continues the engine tends to want a leaner mixture.

If the throttle is applied more vacuum drops in the intake manifold and the Power valve will open (determined by it’s opening point).

This is how most Holleys handle main circuit enrichment as load increases. Simply put, lack of vacuum allows the power valve to add more fuel via the Power Valve Channel Restrictions (pvcr).

If you do not have a PV on a street car you are jetted for max throttle at all times.

If cruising with a light load this can make for an overly rich condition and subsequent bad running and potentially fouling plugs.

So let’s add a Power Valve.

here the metering block is set in a vise on a milling machine.

If you are the exacting type, by all means use a dial indicator to assure you are perfectly aligned. I have done hundreds of these and have not found the need.

Yes you could probably do this on a drill press, but it would likely be more difficult to get set up.

Drill chosen. I normally just move the chuck by hand and observe the light cut to figure it being centered. Again, there are much better ways to do this task but I have not found the need.

Drilled badly with my potentially dull drill bit. Tap magic is used on the drill and tap.

I don’t bother using the machine under power to tap this hole. I am really just using the mill for alignment reasons and to hold the tap. I spin chuck by hand to tap this hole.

Test fit new Power Valve. All good, no issues.

Now we have to drill the pvcr and assure vacuum is getting to the power valve airwell.

I don’t do anything fancy, just knock a punch to where the pvcr need to be drilled. They look crooked in this photo, they are both in line with the idle feed ports.

I won’t really go into what size you require for the pvcr, that will depend on a million factors.

For 2x4 usage a good starting point is .038-.043

For single usage you may need a considerably larger pvcr, .043 is a good starting point, but some carbs require .068 or larger.

Obviously you will have to jet down considerably.

In the case of 2x4 660s you may be dropping the jet size from 74-76 down to 67-69.

Again there are a million variables, you really need to determine your light load cruise fuel requirements first.

After that is established you can enlarge pvcr to determine your wot fuel additions.

If you really want to make these adjustable, that isn’t an issue just drill and tap your pvcr for set screws and drill the set screws as needed.

Obviously do not tap all the way into the main well, just enough to insert your set screws firmly.

Some carbs come with the manifold vacuum hole drilled. This provides the vacuum required to keep the PV closed when not at heavy load.

If it is not drilled, easiest way to find where to drill it is to use the baseplate as a guide.

Yes you 100% can do it in a more sophisticated manner, my goal here was to show how a home builder can do this easily with basic tools you have in the shop.

Good luck.