hey just put my new intake on my car and finally got it to work and now im just wondering if anyone can give me a basic lesson on tuning my street avenger. noting precise but just so i can make it work better than it does now bc i messed with it thinking i was mr. mechanic. so just sum basic info would be helpful untill i can take it to a pro.

thanks.

 

well if you have a tach that helps a lot but basically you want the leanest lowest idle. that is what i was always told. now i'm not very good at tuning carbs but some people are.

the best way i can explain it is start with a Base. screw both mixture screws in tell the are snug not tight. the screw both of the out 1-1 1/2 turns (car not running). next start the car turn in one screw at a time like 1/4 or 1/2 turns on tell the engine sound like it is going to die then turn the screw back out about a 1/4 turn or until you hear a higher idle then before. next do the other screw, then mess with the idle.

that is the best way i can explain it if i am wrong some on correct me.

 

That's a good explanation. You just want the air/fuel mixture screws turned out enough so the engine idles the smoothest. Also, Jet changes are usually worth some power when fine-tuning and the best jetting combination of 70 primary/75 secondary which is only two jet sizes smaller than stock on the primary side works very well with the Holley 770 cfm street avenger. Holley has really done its homework on this line of carbs.

 

ok thanks for the help ill try it out and see how it works thanks again.*

 

and one more question after i get the idle screw right am i gona have to re-adjust the mixture screws?

 

Im assuming that your carb is an Edelbrock. (Im a holley guy so I dont know all the fancy names that edelbrock uses). Anyway, Heres a link that I think youll find VERY helpful, it walks you through tuning step by step on an edelbrock. Theyre at the bottom of the page and it takes what seems like forever to download.

http://www.edelbrock.com/automotive_...ech_vids.shtml
-P.

 

You gotta test things and learn to read the plugs. you can get it close but just swapping stuff around, turning screws and changing jets without finding out what it actually did can get you lost in a hurry.


Things you need.

Vacuum gauge. You gotta have one of these, they are inexpensive and help more than most realize when tunning a car. You want the highest vacuum reading at idle. You also need to know how much vacuum you have to decide on a power valve. larger engines make more vacuum, long duration cams, single plane intakes and large carbs contribute to less vacuum. you gotta know how much and where to tune your carb.

Magnifying glass. So you can see the ground strap and white insulator on the plugs better. The white part should be a light chocolate color, and the ground strap should be discolored on the vertical part of it.. more on reading plugs later.

You need a place to test at full throttle. you need to run through first to top gear at full throttle, shut it off, then pull a plug and read it.

Important parts of carb tunning.

Float level. getting the floats set right is the first step because it affects everything else. If the float is too high, you will get a very rich mix at idle and turning the idle screws will have ZERO effect on anything, because the engine is getting enough fuel to run from the boosters. There should be no fuel dripping from the boosters at idle. If there is the floats are too high. If the float is set too low you will get a stumble at light throttle, and when first openeing the throttle. the accelerator pump shot wont be long enough if the floats are set too low. Usually right out of the box floats are set just a bit high, because rich is safer than lean.

Accelerator pump. Holleys have different cam profiles, shooters, and positions on the pump cam. Subtle differences in orifice size, shhoter length and cam profile affect when and how long the pump shot is delivered. You need to have an accelerator pump to take up the time between the throttle opening and the fuel being drawn from the bowls. Without one, you will have a large bog, lag, or stumble everytime you open the throttle. Too short and it will bog later, to early and it will still bog. What you want is the right amount in long enough duration to cover the time it takes for the fuel to move through the boosters into the inlet airstream. With the large amount of vacuum a stock engine makes, tunning on the accel pump usually isnt required. The fuel has plenty of vacuum to draw it from the bowl. Go up in cam duration, run a single plane intake, or a very large carb and you will need a longer bigger pump shot. That is why a 1050 Dominator carb has 50c pumps and a 500 carb has 30cc pumps.

Jets only change the amount of fuel entering the carb during main circuit operation. They cant make a carb smaller since they do nothing to the amount of air coming in. Going to a 52 jet in an 850 carb on a 302 is going to make it extremely lean. You need the right amount of fuel for the amount of air going in. Even if an 850 isnt taxed at all on a 302, it still moves more air than a 500 does. You have to put enough fuel in for the amount of air its using to make it run. How do you know if the jets are right? You gotta read the plugs or have a wideband O2 installed, and check it under WOT operation. if you have gone too lean, you will get a surge at part throttle. Going down the road at 2000-3000 steady RPM you will have a car that bucks, stutters, hesitates, coughs, sputters, and does all sorts of hasty things because it isnt getting enough fuel. It will also tend to run very hot and in the dark you can see a lean engines exhaust glowing cherry red. If it is rich, black smoke, fouled plugs, poor mileage, and if you have a catalytic converter on the car, it will get very hot burning the extra fuel. You will also be down on power, and it will feel sluggish when rich.

Power valve. They open under high demand and allow more fuel to enter the engine, if the PV opens too soon, you get an overly rich condition that changing jets wont help. It works on engine vacuum and opens under low vacum conditions. Some engines with long duration cams will have so little vacuum at idle that a stock pv is open the entire time. It continually dumps fuel in because the vacuum never reaches the point where it closes. A stock tpe engine with a race type PV could never have it open, since it still pulls more vacuum at WOT (wide open throttle) than it takes to open the PV. If your PV is a 2.5 and your enine pulls 3in of vacuum at WOT, the PV isnt gonna open. leaving all the fuel requirements to the jets. You will read that as rich. If your engine has 3in of vacuum at idle and you have a 5.5 PV in there like a stock carb does it will be open constantly. There isnt enough vacuum to close it.

Vacuum vs mechanical secondarys.

Vacuum secondarys only alow the amount of air the engine needs into the engine. They use a spring (holley) or a weight (Edelbrock/carter) to limit the opening of the secondaries. Holleys limit the throttle plate itself, where as the others have an air flap over the secondary throttle bores. Changing the spring changes the amount of vacuum needed to open the throttle blades. If you have a Holley vacuum carb, some come with the spring kit, and the others its a simple matter to install one. Then spring changes take less than one minute and you can see the difference if you are at the track. For streetability, the vacuum carbs are much more suited to the low rpm on off driving demands than a mechanical carb.

Mechanical secondary carbs have another accelerator pump in the rear, because the throttle opens as fast as your foot can open them. You know about the lean condition when the throttle opens before fuel arrives in the booster, hence the need for the second pump. Carb sizing is vastly more important with a mechanical carb, because you need to keep the velocity of the airflow up for the carb to work. A small hole flows air at a higher speed than a large hole does, provided there is a vacuum on one side and pressure on the other. Instant opening means you need enough fuel to keep the engine running with the added airflow, without it, you will have a huge bog. Lose velocity in airflow and you have a bog. A 1050 dominator on a stock 289 will have no velocity when you mash the gas, hence an enormous bog, the engine might actually stop running. it is a balance between airflow and velocity when you choose a mechanical carb. Also everytime you come close to opening the secondary, the pump will shoot fuel in preperation of them opening. That is partially why mechanical carbs get worse mileage.

In a situation where you have high vacuum high rpm, like cruising at 75mph around 3000 rpm. The vacuum carb will only have the primarys open while the mechanical carb will have more throttle opening and be flowing fuel from all four venturi. If there is little demand the vacuum carb will only use what is needed, while the mechanical carb will have it all there even if its too much. We have run both on the street, and once you know how to tune them, a vacuum carb is much better for day to day driving than a mechanical carb. Of course most of the ones we run are 850s, and the smallest carb is a 780 holley, but the engines are much larger than most of yours.

Choke. Remember how you get higher vacuum and higher velocity with a smaller opening? Add to that knowledge the fact that cooler air is denser than hot air. So on a cold day, there is more air to add fuel too. When the engine is at operating temperature, it is heating the air coming in. When it is first started, there is no heat for the air, and its calibrated for hot air operation, because cold air needs to run richer than hot air does, you dont want to do it the other way. once it gets hot it would be very very rich. So when its cold, you limit the amount of air coming in, create

 

not to hijack this thread or anything but man THUMPIN when ever i see you post something it is a lot to read. don't get me wrong it is great info but do you type it all out or do you have this typed out some were so all you have to do it copy paste. i think you should wright a book! you know more about every thing then i will ever know.

 

I am writing a book, its all online.. Just search for my name..lol..

In reality, other people have written the books. I have read a few of them and learned more by doing it myself. 27 years of owning V8s and keepin them running is enoughtime to learn the right ways and wrong ways to do things.

I type them all out, its rare I copy and paste anything. but then again, I type kinda fast. if I didnt have such large fingers that dont work so well rom all the scars, and beatings they have taken from workin on cars and with USAF cargo, I think I could type really well.. but I am not a secretary type..lol.

not all of my posts are books. But a carb is an indepth, misunderstood, and highly technical piece of equipment, using mechanical, chemical. and thermodynamic engineering, plus physics to get the job done. They are simple but they way they work is somewhat complicated, hence the huge post to put it in laymans terms that almost everyone can understand.

The trick is to write it so your mom could rebuild a carb as well as she bakes a cake. if we could do that, we could make billions... lol.

 

Thumpin, can I get you to send me some of it. I would like (with your permission) to put some of your excellent info on my own site, http://www.oldschoolhotrodder.com I am trying to make it a resource of some of the stuff I have learned, collected etc. in my own 40+ years of messing with engines of various types.