Adjusting an Edelbrock Carb.

[skipper]

Does anybody know how to properly adjust an Edelbrock carb. Mainly, I just want to know what the two screws on the front of the carb do and how to make the carb run, or at least idle, with a leaner mixture. What woulod be a "normal" adjustment. My exhaust smells really gassy. I am running an Edelbrock 750cfm on a built 460 if that helps. Thanks in advance.

[vettehauler]

[updated:LAST EDITED ON 11-Jul-02 AT 09:11 AM (EST)]Edelbrock makes several 750 carbs.(squarebore, spreadbore, etc...) Here is a link to the site, they have detailed info on your carb. I hope it helps.....

http://www.edelbrock.com/automotive/index.html

regards
Drew

ps the two front screws allow you to enrich/lean your mixture.

[teschwab]

catch up with torqueking at this site and rattle his cage about edelbrock carbs. i would say that he could be a big help.

[skipper]

My carb is just the basic 750cfm squarebore Performer series carb. Which way do you turn the screws for rich or lean (in or out), and why is there two screws for the rich/lean mixture? Sorry if these question sound kind of dumb but all my carburator experience has only involved a Carter 1bbl. carb. on a straight six. I bought my truck with the Edelbrock already installed, so I have absolutely no idea how to adjust it.

[TorqueKing]

Sorry I didn't chime in sooner, I don't usually check on this forum, but I will from now on!

the two front screws are your IDLE mixture only. They control the metering rods for the primary idle circuit. If you're exhaust smells bad, then I recommend going to AutoZone, or you favorite parts store, and picking up a $15 vacuum gage. Hook it up to your vacuum tee on the back of the intake manifold, where the power brake vacuum line is. This will give you a very quick responding reading of how well the engine is operating at idle. notice where the gage reads. If the needle bounces severely, then you probably have a dead cylinder, or at least one that's leaking vacuum. It's okay to bounce around a little bit, this is normal. take a flathead screw driver, and turn one of the screws CW and CCW until you get the gage to read higher. This will maximize your idle. The engine's RPM's may increase, this is desirable. Once you've found the best setting for the first one, do the same with the second one. The engine should now idle a little higher, and the exhaust smoke should diminish, it may not immediately dissapear, this is okay. The engine's RPM's will have increased if your idle mixture was bad, so set the new idle speed on the little screw on the right side of the carb's body, on top of the throttle linkage, it has a spring around it's threads. I believe CW will increase the idle speed, an vice versa, but you'll find out as soon as you touch it anyway. If you accidentally stall the engine from setting it too low, then turn it a half turn or more the other way, until you can get it to start, then resume tuning. Once you have the idle speed set where you want it, remove the vacuum gage, and plug all vacuum sources not occupied by accessories (so there's no vacuum leaks). Drive the truck for a while, preferably on the highway for at least 15 minutes at a good, healthy speed, to heat up the spark plugs and burn off the carbon that got loaded on them during idle tuning. As soon as you get off the highway, pull out a spark plug (warning, it will be VERY hot). Notice the color, if the ground electrode (the little arm) is white, then you're running lean (other things can cause this, we can get into that if necessary). If it's black and sooty, you're running rich (again, other conditions may cause the same thing, but this is generally true). If it's a tan light brown color, then your jetting is perfect, and you're done tuning.

If you are in a rich or lean situation, then you'll need to adjust the primary metering rods, as it's unlikely the jets will need to be changed, these things are usually perfect out of the box with little or no adjustment at all. This is remarkably easier to do on an Edelbrock than anything else you've ever used. This will require a little more work, and a little bit more sophisticated gages to get it perfect, but I'll walk you through that if need be, let's get your idle set up, and a good baseline of the primary circuit first!

There are other ways of tuning the idle, the other most popular one is to listen for the engine's RPM to rise when you hit the sweet spot. This is very unreliable, as the actual carb tuning has less impact on RPM as it does engine vacuum, and the vacuum will respond to everything you touch. Some guys even try to use the tachometer as a gage. This is unwise, because 1) how accurate do think that tach really is? and 2) do you think you're gonna be able to notice a 50 RPM increase? The answer is no, at least not very well, so I always use a vacuum gage. The most primitive way I've ever heard of is to look at or even smell the exhaust. This has ovious health drawbacks while being even less reliable that the RPM method, and it takes an expert ear, and unfortunately, a nose. It is wise however to visibly monitor the exhaust as a feasability check, as a well tuned idle should produce no visible vapors at all. If yours is still puffing a little, don't worry, it will dissapear as soon as you put a few hard miles on it. The reason for this is that when the engine is idling, there is not enough heat to burn off the carbon deposits that form in your combustion chamber, and they leak out slowly, causing that tiny puffing out of your tailpipe. Loading the engine, as in driving it hard, places massive pressure, and therefore heat (reference: ideal gas law, Boyle, 1725). This will purge everything but metal out of your engine, given that you have the correct spark plug heat range. If your plugs are still black after highway use, then I'd recommend going one heat range hotter, and retesting. Motorcraft P/N 450's are a good median for most Ford V8's.

Let's get you rolling on this, and check back in with me when you get a chance, and sorry again I was late on the response! -TK


'77 F100, 302 (the aftermarket Prodigy), C4
Cadet Second Lieutenant John F. Daly III
South Carolina Corps of Cadets, The Citadel
The TorqueKing

[skipper]

Thanks for all of your help.

[skipper]

Hey Torqueking, I adjusted the screws with a vaccuum gauge, the only thing is that the carb seems to run even richer. Which way is richer CW or CCW? I ended up turning each screw 'bout one turn CW. Should each adjustment screw be turned to about the same amount or does that have nothing to do with anything as long as the vaccuum is at it's peak?

[TorqueKing]

To lean it out, do the screws CW, for rich go CCW. When you have peak vacuum, take the screws slightly CW to find what's called "lean best idle". Looking at the vacuum gage, this should be only a slight if any drop in vacuum, and the RPM should drop approximately 20 RPM from peak. Then you're ready to run! -TK

[TorqueKing]

BTW, the screws do not necessarily have to be in the same position, they are independant idle circuits. TK

[skipper]

Thanks TorqueKing.

[DailyDriver]

[mechageek]

Just wondering where i recently started work at we use an 85 f-350 truck with dump bed and it has an edelbrock carb and a new 460 engine. the question i have is that it seems to be using to much fuel it might be getting 5 to 10 mpg. what kind of adjustments can i make to get better mileage? I think the tank size is 18 gal. would a bigger tank size do better?
thanks mechageek

[fmc400]

Welcome to FTE. Honestly, 5-10 MPG on that setup isn't too out of the ordinary. You can try dialing it in with a rod and jet kit from Edelbrock and adjusting the idle mixture with a vacuum gauge to try and keep it a little closer to 10 mpg, but you'll probably never beat 10 mpg. 8 mpg is a pretty reasonable expectation with this setup. A bigger gas tank won't help, it will just take longer between trips to the gas station, and cost more to fill it up!

[mechageek]

thanks for the help if that the best it will get than i won't mess with it, and not worried about gas trips if i put in a bigger tank. already filling up about every 3 days now denping on where i have to go. i have a 86 bronco with 351w ho and 33gal tank and it get 10 mpg that is about as good as 460 gets. thanks again for all the help

[1966Tbird]

i've always thought a trashcan was the best way to adjust an edelbrock carb