Carb jetting

[swb85]

I've got an Edelbrock 1405 carb on my truck. I'm getting pretty dismal fuel economy even driving like a grandpa. My buddy thinks edel carbs are gas suckers to begin with, but I've never heard that before. Of the 600cfm performer carbs, the 1405 is supposedly jetted for performance and the 1406 is jetted for economy. Would I stand to gain any noticeable mileage improvement by changing the jets in my 1405 over to a 1406 set? Thanks!

[piecesparts]

Take a look at the book that came with your carb. Inside is a set of graphs and what you should look for to either improve economy or performance. Look at what your base setting is and then decide if you can go the direction that you need to to get to the optimum. I have purchased over the last few years a full box of jets and needles to do my carbs. By the way the Edelbrock is no more a gas hog than a Holley or a Barry Grant. They can be adjusted just as much as any other. The whole package (truck, engine, tires, rear end gears, and type of driving needs to be analyzed. Oh and YES, you can improve, but make sure you don't go so far as to ruin any engine parts (run to lean). By the way the cam that you list on your avatar is probably the real problem, if it is not properly matched toyour gear ratio, since you don't have an overdrive tranny. What are your RPMs on the highway. I have a 383 stroker with an Edelbrock Thunder AVS and I get around 14.5 to 15 MPG with 400 HP and 3.73 gears.

[swb85]

Thanks for the reply! I know the longer duration cam is going to suck some fuel, and that's ok with me. I have 3.42 gears and run ~2500rpm on the highway. The cam was recommended to me for my setup by comp.....three calls and three seperate technicians and this cam was recommended every time. I usually drive like a grandpa, shifting before 2k rpm and never opening the secondaries.....this has netted me 11.75mpg on the last two tanks of gas, 80% highway driving. I know it's running a little rich just by the smell, and a faint black smoke at WOT. I assume all I need to dial it in is a calibration kit for the 1405 carb? That should give me all the jets, metering rods and springs I'll need right? I'll read through the carb manual tonight.

[piecesparts]

The calibration kit is not always the way to go. I looked at what the carb came with and purchased jets on each side of the original and needles for two steps on each side of the original. Now, after a couple of years, I have a complete set of jets and needles from Edelbrock in all sizes. You will have to compare prices and see what you want to do.

You say that you are getting 2500 RPM with a 3.42 rear end--What size tires do you have and what tranny are you running? I had a 3.42 rear end and with the 700 R4 I was running around 2100 RPM, so I am guessing that you are running a 350 turbo or a 350C tranny(lockup converter). That is where your MPG is going. You are most undoubtedly running a little rich abd that can be fixed, but get into a habit of reading your plugs and exhaust tips for proper colors (not to black and not too white)

[swb85]

Where can I order individual needles/jets from? All I found in summit was the calibration kits.

I'm running 235/75/15 tires, so about 29" tall. Transmission is an m21 4 speed. I pulled the plugs this evening, but have a little trouble reading them. The driver's bank looked like the timing was too high based on the mark on the electrode (too far towards the threads), but the passenger bank looked good. Plugs have about 200 miles on them, none of them were fouled or had a lot of carbon on them. I'm going to at least check the floats this weekend and make sure they're where they should be. I'll post some pics of the plugs tomorrow to get some better opinions than mine!

[79BIG10]

Originally Posted by swb85

Where can I order individual needles/jets from? All I found in summit was the calibration kits.

I'm running 235/75/15 tires, so about 29" tall. Transmission is an m21 4 speed. I pulled the plugs this evening, but have a little trouble reading them. The driver's bank looked like the timing was too high based on the mark on the electrode (too far towards the threads), but the passenger bank looked good. Plugs have about 200 miles on them, none of them were fouled or had a lot of carbon on them. I'm going to at least check the floats this weekend and make sure they're where they should be. I'll post some pics of the plugs tomorrow to get some better opinions than mine!

Look here for help on diagnosing your plugs. http://www.dansmc.com/Spark_Plugs/Sp...s_catalog.html

With the driver side like you mentioned you might be running too lean on that side. Check the screw air mixture screw to see if they're both balanced.

[piecesparts]

I went into the Edelbrock catalog or even on the computer and got the part numbers for the edelbrock jets. I then went to the Summit computer catalog and type in the number. Be sure to use the prefix for edelbrock (example: edl-xxxx) Summit sells way more than you redily see in the catalog and just expanding your search and ensuring that you use the right desriptors, you can get many items for your engine.

I would say that your fuel issue is still in your gearing and the amount of RPMs you use on the truck. The motor will tell you how it is running and what you nedd to do for it, but the rest is up to you. What is your timing set at? That will affect your performance greatly, also.

[swb85]

Initial timing is set at 10 degrees BTDC.

So you think I'm running too many rpms at my cruising speed? I told comp my entire setup and they recommended the cam I chose 3 different times. They said my cruising rpm would be close to peak efficiency for the cam. I wasn't gonna argue with them after hearing the same thing from 3 different techs. I checked out some plug reading sites, none helped very much and just showed extreme cases. I'll try and post some pics of my plugs and see what you guys think, I plan on at least checking the float levels tonight. Thanks for the suggestions/help!!

[piecesparts]

No I don't think you are running to many RPMs at cruising speed, but that is why you are running into economy problems, to some degree. The manual tranny will give you acceleration problems at any RPMs lower than that and anything higher than that will just gobble fuel.

First let me ask---what are you runnig for a distributor? Is it still connected to the computer or is it a HEI unit from an older motor. This might be part of the problem, the computer is not letting you run where the motor is performing right. The HEI would eliminate the computer (if you still have one and give you a lot of options) If you do have an HEI unit then you might consider getting an adjustable timing light and set your timing to about 35 degrees total advance,(at around 1500 RPM, vice doing the initial timing at 10 degrees. The motor runs well at 10, but your economy will be more prevelant when your timing is set to get the max you can out of the motor. Listen for any dieseling or detonation when you shut the truck off or when you are driving. If it does, then retard the timing just a degree or so. (I have my 383 running at 36 degrees, and the intial timing is about 18 degrees. Also, when you adjust your carb's idle mixture screws, do it with a vacuum gauge rather than by ear. Connect a vacuum gauge to the intake manifold (a good reference port) and get your initial vacuum reading at idle. Screw one of the idle mixture screws in until the motor starts to slow and falter, then turn the screw outward until the idle is good again and you acheive the maximum vacuum reading that you can get. Now do the other screw the same way. Once you have done this, turn each idle mixture screw back inward an 1/8 th of a turn. You are now set to the optimum idle your engine can do.

At what RPM do you have to set the carb, so that your motor will stay running at stoplight. Since you have a clutch, it is easier than an automatic. Some cams cause the motor to idle sloppily around the 700 to 800 RPM range and the idle speed screw is turned in to keep the motor idling effectively. This is taking the primary butterflies out of the idle circuit and into the power circuit of the carburetor. This will cause the idling motor to smell rich and run a little rough when you first accelerate. That is common for a lumpy cam profile, so you will have to get used to it. My truck has a fair amount of rough idle and the carb is adjusted slightly into the power circuit, but with my 2400 stall converter I have solved some of that and the truck is a lot better than when I first put the motor into it.

If your plugs and exhaust pipe tips are a moderate light brownish color, then you are relatively close in your adjustment of the carb. Get the timing set and the idle mixture set and then you might be doing good.

[swb85]

The distributor is just a run of the mill HEI with vaccuum can, no computer intervention. Don't know what springs are in the dizzy, probably whatever came with it. My timing light isn't adjustable....I need to pick one up. Let me just say that the truck runs pretty good. It never stumbles or coughs/spits at me. I'm still in the idle circuit of the carb and am idling at 750 rpm. I tuned the idle by ear & rpm, so I really need to get a vaccuum gauge on it this weekend to dial it in. I'm starting to get some pretty nasty oil smoke at startup, so I need to change my valve seals pretty soon before I get too deep into tuning. Thanks for all the help, I'm sure I'll need more!

[piecesparts]

The valve seals are one of the most common things GM motors have problems with. Look into a spring set for the dizzy andlook ahead to a unit that has been built with am an adjustable vacuum advance on it. Hook the dizzy up to the direct vacuum connection on the carb, stay away from the ported orifice, it allows the advance to come in to slow.

As you advance the idle the RPMs on the truck will go up and the throttle response will be better. It will take some time, but the pay off will be worth it.