Hey guy´s!

On my 1968 BB Vette I have a Holley carb mounted, where the secundaries are moved by vacuum.
So when the secundaries open, I can steer this with a spring. Low, medium or hard.
The mileage was not verry good so I tried to change the spring to the hard one, because it could be, the secundaries opens too soon maybe and do I need a lot of fuel.
Now the question is, how can I check up, if the secundaries open or not?

Thanks, Günther

 

Hi Gunther,
You should be using one of these:

http://www.holley.com/20-13.asp

That kit allows you to choose from a nice assortment of springs to control the secondaries. There is a chart inside to show you opening rates, etc.

If you're doing this often, I'd also suggest one of these:

http://www.holley.com/20-59.asp

It makes the change a lot easier.

When the car is parked, it's pretty difficult to get the secondaries to open by just revving it up. You really need to load the engine so that there is enough airflow to warrant opening the secondaries. If the spring is too light and you punch it from low RPM, you'll probably notice a bog. If the spring is too stiff it will feel like the engine is straining to get to higher RPM or may just feel "limited".

What size is your engine and which model carb are you using?

When you say mileage was not very good, can you give some numbers?

 

Take a clothes pin and clip it to the rod that opens your secondaries right below the diaphragm housing.
Now go drive the car. If when you come back the clothes pin has slid down the rod, it is because the secondaries opened.
If you manually open the secondaries until the clothes pin is right under the housing again you can see how far they opened.

 

Same concept, but I use a paper clip on the rod to record its movement and travel - a paper clip does not clamp the rod with much force, so it will not impede the movement of the rod.

Changing secondary spring won't change your mileage - the secondaries aren't going to open unless you have the primaries nearly wide open.

 

Exactly these spring from the link you gave me I´m using.
I have a 427ci, but I don´t know which Holley exactly I have at the moment.
The mileage is about 30+ litre per 100 kilometers - but I´m not a racer!

Do the secundaries open only per vacuum? I thought there is a linkage to the secundaries from the accelerate arm?

 

The only mechanical link from the throttle prevents the secondaries from coming open.
The only thing that opens the secondaries on a vacuum seconday carb is venturi vacuum.

 

Here is where you look for the part number:



30 liters per 100km is about 8 mpg - my old 440 Six Pack Super Bee got 6mpg, so 8 isn't a surprise, but it could be better.

With a Holley carb there are a lot of variables. You could have a bad power valve - that will dump a lot of extra fuel. You need to check the vacuum while at idle. The power valve will be open at any point below its rating. The lower you go the longer before it opens. A backfire can rupture the valve and casue it to leak fuel all the time. To find out which one is in your carb, you'll need to remove the front float bowl and metering block. The power valve is gray and about 25mm in diameter. There should be a number stamped into the edge like 2.5, 8.5, etc. (They area ll in ".5" increments except for the 5.0 version.) Here is a great site for converting US and Metric measurements so you can go from inches of vacuum to millibars:

http://www.convert-me.com/en/convert/pressure

The pwoer valve looks like this:



You'll also have the fun of scraping off gasket material. Do yourself a favor and get a set of the re-usable gaskets from Moroso or one of the other companies that offer them. http://www.summitracing.com/search/?...20holley&dds=1

While you have it apart, see which jets are in the front and see if that is what is supposed to be in the carb stock. A complete listing of carbs by part number is here: http://www.holley.com/data/TechServi...%20Listing.pdf

Hope that helps!

 

Hi!
Where can I find the power-valve and for what is it good for? How can I check the power-valve?

Here is a pic from my carb:




Thanks, Günther

 

And for what is this for? I made a pink circle around it.

 

That's the vent tube for the fuel bowl. The purpose of the vent is to keep the fuel bowls at atmospheric pressure. If the vents are obstructed (generally, nothing including the air cleaner lid closer than around 15mm), the fuel bowls will be slightly below atmospheric pressure and the fuel flow through the carburetor will be restricted. The way fuel flows through the carburetor is a pressure differential is created in the venturi (the idle circuit is drawn from manifold vacuum and not by venturi pressure) and fuel is pushed by the higher pressure in the bowl through the emulsion circuit and into the boosters. By the way, the power valve is in the fuel bowl. If you remove the four screws on the end of the bowl, you will see the backside of the valve. If you remove the metering plate (the thing the power valve is screwed into) you will see the big end of the valve with the wrench flats.

 

Aaahh ok, I know what you mean.

Thanks, Günther

 

And how can I check the power-valve it works correctly?

Thanks, Günther

 

This should help: http://www.chevyhiperformance.com/te...ems/index.html