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If I remember right from when I used to overhaul them,they are adjusted by changing the springs behind the diaphram,different spring tensions for different opening speeds.
Here is some info I found for you.
A tip about vacuum secondary diaphragms
Holley makes about 4 different diaphragms. Even though one may look just like another, the link length can vary. This makes your secondary opening rate all messed up, because the spring is not acting on the diaphragm correctly. If your link is too long, the secondaries won't open all the way, no matter what spring you have installed. If your link is too short, the spring may not even touch the top of the diaphragm, which is not good.
How to know if your vacuum secondaries are opening
You can tell if your secondaries are opening by keeping them shut with a spring or other means. Test drive the vehicle. If performance is worse with the secondaries held shut, then they were opening when operating normally. Winging the engine in neutral does not work! Vacuum secondaries operate off engine rpm and load, not just rpm, and there just isn't enough airflow in a no-load situation to open the vacuum secondaries.
Now that you've got the basics down, here is how you tell what circuit needs to be richer or leaner. You put an instrument in the exhaust flow that tells you graphically whether the engine is lean or rich. You then put a vacuum gauge on the engine to tell you what circuit is working at the particular time that you are lean or rich.
The first instrument is known as an O2 (oxygen) sensor. These sensors are used in electronically fuel injected cars as a signal telling the computer whether to supply more or less fuel. O2 sensors create their own electricity in the presence of heat and oxygen. You simply put the sensor in a hot spot in your exhaust flow, then read the electrical output.
There are a couple of ways to put O2 sensors in your exhaust: The trick way, and the cheap way.
The Trick Way
Buy an air/fuel ratio monitor from Edelbrock or K&N. Summit (1-800-230-3030) sells the Edelbrock system for about $130. This comes with an O2 sensor, weld-on bung, and a display panel with yellow, green, and red LED lights representing lean, good, and rich mixtures.
Racer Wholesale (1-800-886-7223) sells several different air/fuel ratio monitors. One has two channels for dual exhausts, and another has rich/lean and injector duty cycle for you FI guys.
The Cheap Way
Buy a single wire O2 sensor at your local parts house, make your own bung, and read the voltage with a digital voltmeter. The sensor that I bought is a Standard brand, number SG-12. The threads on this sensor are the same as a small-block Chevy gasket-style spark plug, so the bung can be made from one of those spark plug anti-foul adapters. Other O2 sensors use the large diameter threads of 18mm big Ford spark plugs. Just cut and fishmouth the adapter so that the sensor sticks into the exhaust flow. You need to put the sensor as close as possible to the engine so it gets hot and stays hot. Just make sure you route the wire so it doesn't get burned by the hot exhaust pipe. Weld the bung to the pipe, then drill and file the hole to clear the sensor.
Since the purpose of this sensor is just a guide to help you tune your carb, not run a fuel injection computer, if you can't get the sensor really close to the engine, don't worry, because it will still work for your purpose. All that will happen is that your reading may go away during periods of idling. On the same subject, don't worry about using a heated sensor, as the expense and complications involved are not worth it for carburetor tuning. Remember, your eyes are using this data, and if it stops for a while, no harm is done!
Sensor installed in Pinto exhaust.
Sensor and welded bung:
Once you have the sensor installed and wiring run up to the inside of the car, attach a digital voltmeter (you really should have one of your own, but you can sometimes borrow these from friends if you don't have one) to the sensor and a good body ground. The sensor is positive. The readings you'll get once the sensor has heated up will be from 1.1 volts (1100 millivolts, or mv) down to about 100 mv. The high readings are rich, the low readings are lean. The perfect mixture for cruise is 400 mv. I have found my car to run well at about 700-800 mv. Once it gets below that, it tends to get into a lean misfire. Your results may vary.
Here is a general idea at what the O2 sensor voltage output looks like. As you can see, the slope around 400mv, which is 14.7:1, or perfect combustion, is very steep. This is why only computerized fuel injection systems can really hold anything close to 400mv. If you're wondering about how a sensor can read oxygen content in rich mixtures where there is no extra oxygen, the sensor begins to act as a temperature sensor above 400mv.
You will need a gauge to read the manifold vacuum on your engine. The more accurate the gauge, the better your results will be. The manifold vacuum is measured at the base of the carb or on the intake manifold somewhere. Don't tap into just one runner, though, as sometimes this will give funny readings. You need to check the signal that sees all the cylinders. Most Holleys have a manifold vacuum port in the front on the passenger side under the primary float bowl.
Start by taking your carb apart and writing down the sizes of the jets, the actuation point of the power valve, and the size of the accelerator pump squirter. Put it all back together, check for leaks, then drive it until the engine is warm. In order to get good readings, you will need to drive at a constant speed of 45-55 mph, accelerate lightly, and accelerate heavily.
The best way to set the idle mixture is to lean the carb out until the vacuum just starts to drop, then richen the mixture by about 1/4 turn. If you have a bit of a stumble in very light, low speed operation, sometimes it helps to richen it up by another 1/4 turn.
If you are starting your adjustments fresh, begin at about 2 turns out. Depending upon if you have a regular carb or a smog carb, your adjustment can be lean when screwing in (normal) or rich going in (smog style). The way to tell the difference is the smog carb will have a little sticker on the metering block telling you it goes opposite of normal, and the smog carb will have blunt screws rather than the pointed ones on a regular carb. If you have a carb with a normal system where the mixture leans as you turn the screws in, then when you screw them in almost to a light seating, the engine should die.
On a regular, standard idle circuit Holley, turning the idle mixture screws all the way in should kill the engine. If your idle mixture screws do not respond to adjustments, you may have several different problems. The most common is using the wrong carburetor for the application. Smog carbs with reverse mixture screws (these are the ones with the little decals, telling you to turn the screws in for richer mixtures) only have a small band of adjustment built in. Smog carbs work well on stock engines only, and you can't expect them to work well on a modified engine, especially one with a lopey cam.
Another common cause of non-responsive idle mixture screws is having too much (more than .040 inch) of the idle transfer circuit exposed. The idle circuit allows a very small amount of finely metered fuel into the engine. By exposing the transfer circuit too much, the gross feed of fuel coming out of the transfer circuit overpowers the fine amount coming out of the idle ports. This is akin to adding a squirt gun's flow into a garden hose's flow...the fine amount of the squirt gun is so insignificant compared to the garden hose, that there is no way that adjusting the flow of the squirt gun will make any difference. There are several ways to fix this problem:
1. If your engine is worn, or has a big vacuum leak, you should fix those problems before trying to crutch your carb. Sometimes, with worn engines or one with a vacuum leak, the carb must be opened quite a bit just to get the engine to run.
2. Buy the right sized carb! If you're trying to use a 600 cfm carb on a lopey-cammed 460, then you must open the primaries too far into the transfer slots just to get enough air into the engine to get it to idle. A larger carb on this example would have a larger throttle plate, which would need to opened less to allow the engine to idle.
3. Open the secondaries a bit, and close the primaries a like amount. This will allow more air in, without exposing quite so much of either primary or secondary transfer slots.
4. If opening the secondaries doesn't work, then you may have to drill small holes (1 per plate) in the primary throttle plates to allow air in while the plates cover the transfer slots. This is a trial and error procedure, so start small, about 1/16 inch. You should not have to go much larger than 3/16 inch. Drill on the side opposite of the transfer slots, to help keep this added airflow away from the idle ports and transfer slots.
5. Check for a blown power valve. See the section on power valves below.
If you notice a lean surge or throttle tip-in stumble due to an excessively lean idle, and you can't get the idle richened up, you may have a clog in the primary OR secondary idle systems, or both. Most people don't know that there is an idle system on the secondary side of the four barrel Holley to prevent the fuel from getting stale. If the secondary idle system is clogged up, no amount of cleaning on the primaries will get the thing to idle correctly. You must clean BOTH the primary and secondary idle systems. And since those circuits are very small, small amounts of debris or even varnish from disuse will clog the tiny passages.
Going down a flat road at a rate of about 45 mph or higher will give you a good indication of your main jet sizing. Shoot for between 400 mv to about 700 mv. Since carburetors are not as exact as computer controlled electronic fuel injection, keeping at the perfect 400 mv will be tough. You always want to go a bit rich, as excessively lean mixtures will cause damage to your engine, create pollution, and give you bad gas mileage and performance.
You can do this without a meter, but it's a bit tougher. Start with the factory jetting and proper float levels. Drive it around for a while, noting if the engine surges at highway speeds. Take a look at the plugs. If they are sooty, you may need to lean out the main jets. If the plugs are white, you may need to richen the main jetting. Start stepping the jets up or down, one step at a time, and drive the car around for a day or two. Do not make large changes on your jetting. You can't just throw any jets in there because you think it needs to go one way or the other. It takes time to get it right, and you must change jets one number at a time.
This is a much more difficult task to tame, due to the acceleration most vehicles provide when the secondaries are open. When the car is acclerating hard, it is difficult to read any gauges, therefore, I can only recommend the trail and error, or dragstrip method to jet secondaries. Go by the seat of your pants or take the car to a dragstrip to work on the secondary jetting. If you, or a spotter, can see puffs of black smoke when the secondaries open, then it's too rich. That's about all the advice I can offer.
If your carburetor has the small metering plate inside the secondary bowl, you can change to a secondary metering block, with screw-in jets, using a secondary metering block conversion kit, which does not come with provision for a secondary power valve. Secondary power valves are really not necessary anyhow, and just add complexity and failure points.
Once you have the main jets set, it is time to play with power valves. Going up a slight incline at highway speeds, or accelerating slowly at highway speeds, you will notice the vacuum reading falling. As it falls, it will come to the opening point of your power valve. You can tell when the power valve opens because the meter will go lean for a while, then the valve opens, and the meter begins to show rich. You'll probably notice the power increase right when the power valve opens. I can tell exactly which power valve I have in my car by the vacuum reading when the lean misfire goes away under light loads. About 700 mv to 900 mv is a good reading for light loads with the power valve open.
If your PVCR's are too small, the meter won't go up high enough, and power will suffer because the engine is still too lean. You can drill out the PVCR's with a small drill bit in a fingertip drill bit