Stolen from another forum, which appeared to steal it from a book or manual...
In this section weíll deal with the systems in this sequence:
This is the first you deal with, it has to be right, or all the other systems will not function properly. Itís not tuning as such, at least not in the sense that you can try different settings. If people tell you to, donít listen. The floats have to be set as per manufacturers instructions, thereís only one correct setting.
A cold engine (and when in cold air) requires a richer air/fuel mixture, the choke deals with this. There are 3 different types of chokes, choke opening principle: manual, electric or intake heat. We prefer manual because we want to be in control, this requires a cable to the instrument panel. Intake heat chokes are simple but often come off too slowly. Electric chokes work fine when it has the proper voltage, many make the mistake of getting the ignition-on-only 12V+ from the ignition coil, which in many cars incorporate a resistor so thereís only 9-10 volts. This means that the choke comes off too slowly or not at all. Some people take the choke of all together, and you can do that in the sense it wonít hurt performance. But itíll make starting a cold engine less fun, and a properly set choke does not hinder performance in any way.
This is extremely important to get right. This is not just about a decent idle. Your idle circuit controls how your engine behaves 90% of the time. Because the idle circuit provides power up to around 60 mph/100 km/t. And more important for racing, the idle circuit is critical in the transition from stand still to above 50 mph. So your idle mixture is perhaps the single most important item to get right. Usually it consists of a screw per side adjusting the fuel flow, the further in they are, the less fuel passes through.
All carburetors have some sort of Power system. In Holleys it a valve that opens under a certain vacuum (when accelerating vacuum is almost 0), itís an on/off function. Only adjustable item is at what vacuum the power valve opens (canít be adjusted as such, requires a selection of different power valves to tune). Other carbs like the original Autolites and Rochesters have a more sophisticated system, in line with their more ďdaily driveĒ focus, that as vacuum lowers, raises rods out of the jets, making more room for the fuel to pass through. This systemís main function is to avoid a stumble when accelerating mildly (it also helps when accelerating more, together with the accelerator pump).
This is an entertaining feature. Try taking off the air filter and look down the carb when opening the throttle (engine shut down). A clear squirt of fuel is sent down into the intake manifold. On a double carb it will be 2 squirts, on a Holley double pumper itíll be, you guessed it, 4 squirts. Critical factors are volume and timing, this has to be tuned to engine demand. On volume, too little makes the engine stumbles, too much makes your exhausts smoke and you waste money. On timing, too early creates a great start and then it stumbles, too late creates an instant stumble, and then it quickly picks up.
Some carburetors, like most performance carburetors, have a secondary circuit. The reason for having 2 steps instead of 1 step carburetor are twofold. First, as engines got bigger and more powerful 50 years ago, simply having one big throttle would make drivability bad, just like when buying too big a carburetor in the first place. Secondly a 2 step carb that only uses the first step in normal driving uses less fuel. So there are advantages to the added complexity of having a secondary step. The secondary circuit is almost like the primaries (first step). The differences are: Secondaries often have bigger throttle plates, no choke, normally no idle circuit, normally no accelerator pump, come on based on engine demand instead of accelerator linkage position. There are exceptions, for instance the Holley double pumper has its own idle circuit and accelerator pump, and are directly linked to the accelerator linkage, though delayed. So itís almost like 2 carburetors, clearly focusing on performance more than economy.
The jets are the main holes that the fuel has to pass through. The larger jets, the richer fuel mixture (rich mixture always means more fuel, less air). Theyíre in most cases simple to change, most carb require some dismantling though. Jets are probably what most people think about when they talk about tuning, and they may need changing, but you canít tell until the other systems are in tune. Secondly if you buy a carburetor thatís built for the form of driving you intend to do, albeit they are ďuniversalĒ carburetors, they are often jetted correctly or very close from the vendor. Unless you buy a 850cfm double pumper for you 350Ē smog engine, if you find out that you have to change jets more than 3-4 sizes, then thereís something fundamentally wrong with you engine or the parts you have on it are not right for each other or your tuning skills lack something.
Now we start the tuning section, step by step. Just like in the Ignition section this will be the basic tuning that can be done at home, no need to go to the track for most of this.
Tuning the float level
As mentioned above, this is simple in the sense thereís only one correct setting, no tuning and testing, no trial and error. For most carburetors you have to take the top off and measure, and to be able to do that you of course has to know the ideal measure. That measure can be hard to find on an old carburetor, but if you but a rebuilding kit, itíll be included in the package. Youíll need that kit anyway because you need new gaskets, and on an old carb, youíll find youíll soon need many of the parts in such a kit.
On Holleys this adjustment is very easy as it can be done without dismantling. With the engine idling take out the sight plug on the side of the fuel bowl. Be ready with a rag to catch the fuel that might run out if the float level is too high. With the sight plug removed, use a big screwdriver and a 5/8Ē/16mm wrench to adjust (the screw should just be loosened, adjustment is done on the nut). Adjust so that fuel is up to the level of the sight plug hole and just slowly trickles out. Note that if the level is too high and you adjust down, you have to wait until the fuel is consumed or trickled out, which can be an issue especially on the secondary fuel bowl. Also note that if you adjust down too low the nut loses its grip on the float bolt and when you try to get it back up it wonít work. In that situation, stop the engine, take off the screw and nut entirely and screw the float bolt up with your fingers. You may need a new gasket after doing this.
When both floats are adjusted as specified above, you can screw in the sight plugs.
Tuning the choke
So you decided to keep the choke to improve drivability. Our choice as well. Engine should be off during this session. Chokes are tuned by loosening the normally 3 screws, and then turning the round choke housing. Which direction is rich and which is lean is normally seen marked with arrows, if not, try turning it, and note when the choke plate closes (you may need to slightly open the throttle for the choke plate to move), thatís the rich condition, since closing for the air richens the fuel/air mixture, just what a cold engine likes. Try placing the choke housing in the middle of the scale. Then start the engine and drive a normal trip. Note that the engine has to be totally cold, this means not started for 4-5 hours. This means that testing several settings of the choke normally takes days, we often try a new setting when we come back from a trip, then leave it until next morning, and then test.
How does the engine behave. Is there a lot of smoke from exhaust itís much too rich. If it stumbles, spits, and is not easy to drive, itís too lean. If it runs fine, then youíre close, but you should try the next leaner setting until it starts to stumble, spit etc., because having too rich a choke setting costs fuel and fouls your spark plugs. We like a choke setting thatís a little on the lean side. Why? We know it costs a little on the drivability some cold mornings, but remember that an automatic choke, electric or intake heat controlled will take some time to heat up after a normal 1 hour shut down. So if you go to the shops, do your shopping, goes back to the car and starts it, then the engine is still so hot it doesnít need choke, but the electric and also to some extent intake heat controlled chokes take a few minutes to heat up and pull the choke fully off. So youíll be driving a few minutes with too much choke, which uses gas, fouls your spark plugs, and maybe even most critical to us, it makes your car send smoke out the exhaust which indicates the engine is out of tune! Embarrassing!
So choke tuning is really trial and error approach. Set it as lean as you can live with.
Tuning the idle mixture
Start with engine off and warm, the choke must have heated enough to not be affecting the fuel/air mixture. Slowly and softly turn the idle screws all the way in, noting how many turns. Make sure you down force them in too hard, itíll ruin the finely tapered ends, and then you need new idle screws. Were they a similar amount of turns out? In the future they should be. Most carbs we dealt with should start the tuning 1-1Ĺ turn out. If your carb ran fine and it was 2 turns out previously, start there. But in general, turn out both screw 1Ĺ turn.
You now need to hook up your vacuum gauge. If you donít have one, buy one, see section Vacuum gauge, the wonder tool. Most people that tune cars and donít have a vacuum gauge, use the tach instead, but tuning based on rpm is a lot more difficult, inconclusive and inaccurate than tuning with vacuum. The vacuum gauge hose should be attached to a permanent vacuum port on the intake or on the carburetor. Do make sure itís not ported vacuum, or on cars from the 70-80s with plenty of vacuum hoses, some were affected by temperature. Do use the correct port.
Start the engine and let it idle. Automatic transmission cars need to be in drive, so have a friend in the car with a foot on the brake, the parking brake is not sufficient. Look at the vacuum gauge. It should be high and steady. If the needle is floating back and forth more than 2-3 itís rich. If itís steady but occasionally drops a little, itís too lean. If itís totally steady youíre close to the right setting. If the needle does strange things, troubleshoot your engine in the section Vacuum gauge, the wonder tool.
Now turn the screws 1/8 of a turn. If the vacuum gauge indicates too lean, turn out and vice versa. If the needle is steady, turn in. Do both screws at the same time, then give the engine 30 seconds to settle with the setting. If you got a higher or a more steady vacuum, youíre going in the right direction, try once more. If not, try the other direction. Turn only 1/8 at a time, itís a lot for the carb. When you get close to the absolute right setting, go to 1/16 increments.
Vacuum gauge Idle Action
Steady, high Good Try either direction for higher steady needle
Steady, high, occasional drop Lean Try 1/8 out until needle is steady with no drop
Floating more than 2-3 Rich Try 1/8 in until needle is steady
When this is done, youíll end up with a strong steady idle and probably at a higher rpm. So you need to reduce the idle speed to the required level by adjusting the idle screw (not the idle mixture screws we just tunedÖ).
Some engines with long duration camshafts will not be able to obtain a high steady vacuum. On those engines you need to settle for the highest vacuum you can get, and it wont be 100% steady. Thatís the price you pay for a long duration camshaft (but the idle sounds great).
Now you have tuned not only your idle, but also your 0-60 mph driving circuit, and if done correctly, the car will feel much stronger in that area. But as we see in part 2, weíre not satisfied with ďfeelingĒ faster, we will test and verify it..
Tuning the Power valve
As mentioned there are many ďpowerĒ-systems, many different constructions. On Holleys itís separate valves you have to buy, install and test. On a Rochester itís a small spring that can be interchanged. In this section weíll focus on the Holley set up, partly because of the volume of Holley carbs out there among enthusiasts, and partly because Holley are so nice that the stamp the specific vacuum with which it opens on each valve, this makes tuning easier.
Holley power valves are not tunable with regards to volume of fuel, only when itíll start flowing, at which vacuum.
If you car feels strong at part throttle, also up a long hill, then thereís no tuning needed. The power valve is not the main player at fast acceleration, thatís more the accelerator pump, idle circuit and jets. The power valve is to cover for when you take that long hill or overtake on the freeway. If you in those situations feel a stumble or just lack of power, then you should go to a power valve that comes on sooner, at a higher vacuum. Letís just reemphasize, when you press the pedal down you decrease vacuum, and when the vacuum gets below the vacuum stamped on the power valve itíll open for extra fuel. Most Holleys come with a 6.5 power valve as standard. Rule of thumb is that you should have a power valve that is at half the vacuum at idle. So if you have 15 vacuum at idle, you should have a 7.5 power valve. The 6.5 power valve will work fine (unless you feel the stumble and lack of power as mentioned) in most cars. But some hi perf engines, especially those with a long duration camshaft, will idle at maybe 10-12, then you need to go a 4.5 or 5.5 power valve.
Some ďracersĒ think itís smart to take out the power valve and install a plug instead, and then richen up the carb with jets to get to the same end result. Only problem with that is that the carb will be rich all the time, also when itís not needed, so fuel economy goes out the window, and spark plugs foul. Not recommended
In the old days Holleys were rumored to blow the power valve when the engine backfired (a well tuned engine never backfires). Holleys has added a checkball to avoid this to as far as we know all carbs produced in the last many years, so it shouldnít be a problem anymore.
Tuning the accelerator pump
Again here we would say the set up when buying would in 80% be perfect. But if you bought too big a carburetor as many people do, then thereís a bigger ďholeĒ to cover when you smash the pedal to the floor, so more pump shot is needed. There are many things you can change on the accelerator pump. The total volume, standard is 30cc, a bigger pump can be bought pumping 50cc. Weíve so far never seen an engine needing more than the 30cc though. Secondly the speed of the ďarmĒ can be adjusted by changing or moving the colored plastic cam on the throttle shaft. It determines the volume in relation to throttle position. A Holly kit contains the following cams (least pump shot->white, blue, red, orange, black, green, pink, brown->most pump shot). Thirdly the size of the holes in the shooter determines how fast the fuel is squirted into the bores. So adjustments are infinite. Which makes things easier but also more complex. This is not just a screw you can turn.
One thing before we start. Regardless of which colored plastic cam and size of pump (30/50cc) itís critical to adjust end play when throttle is fully depressed. If end play is too much you lose pump action, if itís to little you end up destroying the pump. End play is determined by opening the throttle fully (engine off) and then seeing how much further you can press the accelerator pump at the are situated on the fuel bowl. You should be able to create a gap of 0.4mm, adjust spring just above to obtain that.
You need to tune the accelerator pump based on two sets of info. Is it too little or too much? Is it too soon or too late? This must be based on experience when flooring the pedal. First though, letís get rid of a myth. People that experience a stumble when suddenly accelerating often describes it as ďit wont take the gasĒ. It is in fact the opposite that happens, the engine doesnít get enough gas to cover the ďholeĒ when the pedal is pushed down, throttles open and vacuum drops. So whatís your experience? Does it stumble or is there smoke out the exhaust? And is that condition immediate or after few seconds?
Immediate 1) 2)
After a few seconds 3) 4)
1) Here the accelerator comes in with too little to late. This can be solved by either changing the cam to one that provides more shot earlier, or a shooter with bigger holes. Notice the spring on the accelerator pump arm? Fluid canít be depressed, so when the pedal is floored, the spring depresses and only extends as fast as the holes in the shooter allow. So if you need to get more shot faster, itís the cam or the shooter size. The cam comes in kits, normally with data on each cam. If not, put them on top of each other, you can clearly see which has the most aggressive profile. Shooters are starting around size 0.025Ē. Go up approx 0.003Ē at a time until the problem is cured. Whatís best, cam or shooter. Well a cam kit is a lot cheaper than various shooters, and in 90% on the cases it solves the problem.
2) This is the reverse of 1). Except that here itís our experience that decreasing shooter size is more effective than cam changes, maybe because there are no cams radically less aggressive than the standard cam. Anyway, 2) is a rare occasion
3) So initially itís OK, but after a few seconds it stumbles. This is because all the fuel in the accelerator pump is used up. This results either from too big holes in the shooter or too little total volume of the accelerator pump. If you see smoke on the immediate throttle opening, your shooter is too big, squirting too much fuel in too soon and running out of fuel. If thereís no smoke on the immediate throttle opening, that part is perfect, then thereís just not enough fuel for your engineís demand, and you need to increase form 300cc pump to 50cc pump. As mentioned this is rare, never happened to us. But then again, we never buy our carburetors too big for the engine, that would necessitate the bigger pump.
4) This is also a rare condition. The shooter size is OK, the immediate reaction shows that. But then it just pumps on and on and thereby too much. It would be logical to decrease pump size from 50cc to 30cc, but we never seen anybody go that way, against human nature maybe. We suggest a milder cam, one that gives less arm travel per throttle movement. Find a cam where the initial lift is the same but the lift after that is less.
If all this doesnít solve a stumble, it could be because the secondaries are opening too quickly, so letís look at that.
Tuning the secondaries
First letís talk about secondaries on double pumpers. You canít (shouldnít) adjust the linkage, point of primary throttle position that the secondary throttle start to open should not be changed. Only thing to check is that when the primary throttles are fully open (engine off), the secondary throttles should also be fully open. If there is a problem with the secondaries on a double pumper, itís usually the secondary accelerator pump that needs adjustment, see previous chapter. We havenít ever had a need to adjust anything on the secondaries on double pumpers except the mentioned full throttle opening.
Letís discuss the other type, the vacuum secondaries. In fact some models open based on vacuum, others on flow through the bores. Holleys are the latter, but we still name them vacuum secondary carburetors. Some carburetors keep the plates closed until demand is there with springs, others use weights. Common is that they can be adjusted as to how soon and how much they open. If they open too late, you lose power. If they open too early you get a stumble. On some carbs changing the spring/weight is easy and aids tuning, but on others like the Holley itís tricky to do on the engine and the choke has to be removed. Fortunately Holleyís made a kit to avoid that, a quick change kit so you can change the spring in 1 minute.
Regardless of whether itís springs or weight, the procedure is simple. First you should adjust the manual linkage from primary throttle axle to secondary throttle axle. Yes, there is a linkage, also on a vacuum secondary carb, but it doesnít open the secondary throttles, in fact the opposite, it ensures that they close when the primary throttles are being closed. This is to avoid the engine running on when you release the pedal. What you need to adjust is so that at idle itís closing the secondary throttle all the way.
Then you can start tuning with the springs/weights. If your car has no stumble you try softer springs/lighter weights until it does, then you go one spring/weight heavier. If you have a stumble, you try heavier spring/weight until that stumble is gone. So you aim for the softest spring/lightest weight that doesnít make the engine stumble.
Just to clarify, what is a stumble here as compared to what we talked about when tuning the accelerator pump? A stumble here is something that is felt when cruising and the quickly pressing down the pedal slightly, like an overtake manoeuvre, since the extra angle of the throttle axle is small, the accelerator pump doesnít help much, and maybe the power valve doesnít even come into play. But the engine will only stumble like this if the secondaries open too early, which in fact more than doubles the size of the carburetor.
Tuning the jets
When everything above is perfect, only then can we start to look at changing the jets. But since we have to do that at the track, the guidance can be seen in section Tuning a carburetor, part 2. But there are some things we should discuss first.
Obviously when the carb is perfectly tuned as described above and smoke comes out the exhaust when the engine is warm, itís probably too rich. And if the spark plugs are too dark, that would indicate the same thing. Are the spark plugs too white, it indicates too lean condition, and you should fix that a.s.a.p.. See section Spark Plug reading.