1984 Jeep CJ-7 4.2L Tune-Up Basics and Weber Carburetor Conversion

[forman]

Hello everyone!  Well it is that time of year again when it is too cold to drive around and it is time to work on the few problems that will make the jeep run better.  I think first I should say a little about the jeep.  

 

It has 63,000 miles and the engine is in very good condition.  I rarely drive it over 25 mph.  I don't drive it on pavement, most of its miles are along rough ranch roads, and the stock axles, transmission and transfer case serve my needs very well.  I work in wildlife conservation and the jeep's primary purpose is to travel from point A to point B at a slow rate of speed so I can observe all things flora and fauna.  Most of the time it is a quiet, comfortable ride.

 

This CJ to my knowledge has always been inefficient in fuel economy.  Recently I discovered that the ignition module had been bypassed and all of my attempts to balance fuel, air and a smooth running engine have not yielded good results.  Fuel economy as low as 3 mpg, too fast an idle or a very annoying idle surge event that is just aggravating to say the least.  My evap canister is leaking at times and not in very good shape I would like to remove it.  The catalytic converter has been removed.  My carburetor is an aftermarket replacement.  

 

I would like learn more about my specific problems so I could make a decision to either remove stock components and replace with a simple set up that would allow the jeep to operate in a way that suits my needs, or replace all stock parts. 

 

I honestly don't know the best course to take... So I'll need your help.

 

Thanks!

[Moses Ludel]

Hi, forman, always great to see your posts and topics!  The 4.2L Jeep inline six engine from 1981-90 is an emissions maze.  The fuel/spark/Evap systems overlap, including the BBD Carter carburetor with Sole-Vac and a Mixture Control Stepper Motor, the emissions control circuits, the Pulse Air to the catalytic converter and the ECU regulated ignition.  The tie-in is the ECU or engine control module, also called the MCU.

 

There are a variety of vacuum and electrical devices in the emissions, spark and carburetor circuits.  "Tuning" this engine involves making sure that each of the vacuum circuits is intact and operating properly.  In my experience with the BBD Carter carburetor and the emissions systems on these models, there is only one way to make it "right":  restore the original equipment devices and their functions.  This includes the ignition and fuel systems, the EGR circuit and the EVAP system. 

 

Note: Pulse Air and the cat are legal requirements but optional on the restoration list, as this is a passive, downstream system.  The O2 sensor must function, though, in order for the OEM system to work properly.  Pulse Air involves considerable heat at the cat, which requires a silicone tube connector hose; anything else will melt!

 

What generally occurs is the kind of mess you've inherited.  Some of the system is bypassed or eliminated.  Since these components all interact, there is no clear formula for eliminating subsets.  As one example, there are those who deactivate the Sole-Vac and MC Stepper Motor circuit for the carburetor, which means that ECU controlled fuel enrichment cycling is nonfunctional.  This can lead to harmful detonation if stuck in the lean mode.  If enrichment mode is on all the time, the engine will consistently run rich, carbon up and damage the exhaust valves.

 

You can convert the ignition to a conventional, straight-up module triggered unit without vacuum retard function on the distributor.  Though a violation of emissions requirements, the distributor will function normally with both centrifugal and vacuum advance.  The cat can be eliminated, too, if the Pulse Air injection gets capped off.  As a point of interest, any of these measures violate Federal and some states' emissions laws, whether these laws get enforced or not.

 

The most significant gains and legal way around the system is a 50-State legal Mopar EFI (MPI) Conversion or the Howell TBI conversion.  You'll find the Howell system simpler to install and less costly, its backbone is a stock 4.3L GM TBI with adapter to the BBD intake manifold pattern and a GM-type computer.  Bill Howell adapted this OE system to the 4.2L Jeep engine with impressive results!

 

Either system uses an ignition distributor.  The Mopar kit provides a new 4.0L OE distributor.  Howell uses the stock OEM 4.2L distributor, which can be easily reworked to operate without the original ECU and the vacuum retard function.   Mopar EFI/MPI is basically a complete factory 4.0L MPI induction, sensor and ignition system with a '94-'95 4.0L Jeep Wrangler PCM. 

 

Cost is involved with an EFI conversion, and the Howell system is easier on the wallet and requires less installation time.  Both systems use a computer-triggered EFI inline fuel pump.  (I've done the Mopar conversion with an in-tank pump on 2.5L TBI and 4.2L YJ Wrangler models.)  The larger performance gains, arguably, would be the Mopar EFI system, although TBI with a stock manifold produces excellent torque and improved fuel efficiency.

 

Here is a link to my magazine article that helps clarify EFI conversion systems.  The MSD Atomic EFI prototype installation is not 50-State legal at this time, however, the install follows guidelines similar to the Howell EFI/TBI system:  http://www.4wdmechanix.com/MSD-Atomic-EFI-for-Jeep-4.2L-Inline-Six.html.  Review this article for insights.  I also have articles on the Mopar EFI system at the magazine, and you will find a detailed installation in my Jeep CJ Rebuilder's Manual: 1972-86 (Bentley Publishers).

 

If the cost of an EFI conversion is too high, I recommend restoring the existing systems.  The catalytic converter is optional, depending upon legal demands and your desire to meet emissions standards.  A performance cat like the Random Technology unit on my XJ Cherokee will meet emissions tailpipe readings if you decide to restore the Pulse Air and catalytic systems. 

 

For an orientation, here is the basic engine vacuum circuit and device orientation for most '81-'90 Jeep 4.2L engines.  If you need specific 1984 CJ Jeep 4.2L vacuum circuit or engine electrical diagrams and parts listings, I can furnish those materials.  This is a start and will help clarify the typical components in these systems: 

 

1989 Wrangler 4.2L Engine Vacuum Auto Trans.pdf 1989 YJ Wrangler 4.2L Pulse Air.pdf YJ Wrangler 4.2L Carburetor Details.pdf

 

Start by reviewing these materials.  If you decide to rebuild the BBD Carter carburetor, I have an in depth how-to with illustrations at the magazine: http://www.4wdmechanix.com/Rebuilding-the-Two-Barrel-BBD-Feedback-Carburetor.html.  This is also covered in detail within my 1972-86 Jeep CJ rebuilder's book.

 

Moses

[forman]

Dang Moses! 

 

Shame on you for telling me about the Howell system, it is very attractive.  I have to assess my problems first I'm sure that to repair and replace the original parts would help my Jeep very much.  I found out from the previous owner that the ignition module or the engine control module I'm not really sure of the name was bypassed because a new one would cost $800.  This is what made me think to ask if removing all original equipment and starting over with simple parts would be the way to go.  I'm sure I'll be asking soon how to test a carburetor solenoid vac and a CTO.

[Moses Ludel]

Forman...The defective $800 item is likely the ECU (control module).  For a complete restoration, the best approach in my view is to find a donor CJ with all of the original devices available and still in place.  This would be a vehicle that is otherwise stock, maybe at a recycler now, or one that is slated for a V-8 transplant or a later 4.0L engine swap. 

 

On that note, there are those who have taken a 1991-up OEM 4.0L complete engine, or just the cylinder head, PCM and peripherals, and waded through the wiring and parts adaption, ultimately ending up with a complete 4.0L inline six or at least a 4.0L cylinder head and MPI system with all its fuel-and-spark components.  If you consider the 4.0L alternative, donor pieces could be a Wrangler, XJ Cherokee or Grand Cherokee with a 4.0L inline six engine.  All pieces from the tank forward to the engine would be needed.  A 1991-95 Jeep 4.0L donor vehicle would most closely emulate the Mopar EFI conversion.

 

Adapting a 4.0L head to a 4.2L block can be done with some relatively simple port block-off changes to the head that match the 4.2L block.  Valvetrain adjustments like valve clearance are also involved.  I dedicated a Vlog to AMC/Jeep inline six valve clearance adjustment.  That video shares the concerns and remedies.  It is available at the magazine...

 

This 4.0L upgrade would be a viable option if you're good with wiring, can follow factory wiring and vacuum schematics, and have enough patience to transfer all of the necessary parts.  The Mopar EFI Conversion, based on the '91-'95 4.0L parts, winds up with a harness that only has four wires to tie into the OEM CJ chassis.  The Mopar kit harness includes fuel pump wiring to the tank area, the entire engine harness for all sensors, the ignition, PCM and EFI components.  Without that harness, you would need 1991-up Wrangler or Cherokee/Grand Cherokee harness sections from the donor vehicle to splice into the OEM CJ chassis wiring.  This demands resin core solder connections and plenty of heat shrink tubing to prevent voltage drops in the EFI circuits. 

 

We can discuss this further.  Without the original ECU function, there are a number of concerns for how the ignition and BBD carburetor are currently functioning on the CJ.  We can troubleshoot and evaluate ways to make the engine run adequately if you stick with carburetion. 

 

There are even aftermarket carburetors like the Weber retrofit that will eliminate the BBD's Sole-Vac system and MC Stepper Motor if there is no longer a driver system for the OEM carburetor.  (There are even BBD carburetors without Sole-Vac or a Mixture Control Stepper Motor for that matter, we can discuss those carburetors if practical.)  If necessary, the ignition can be readily wired to run from the Jeep/Motorcraft module without the use of vacuum retard or any ECU involvement. 

 

Note: The Weber conversion kit was not emissions legal years ago and is likely still not 50-State emissions legal. Bypassing the ECU is also not emissions legal in states requiring vehicle emissions inspection. 

 

You have a lot of options.  Budget will dictate the direction...

 

Moses

[forman]

The Howell system is looking better all the time.  You may not recall that I live in the middle of nowhere and some of the parts options are not available here.  All of my projects are hindered by this fact and it is the most difficult part of the project.  The budget is undetermined at this time.  

 

Presently the engine runs and sounds fine except for the lurching/surging issue I spoke of.  After a couple of hours driving your throat will become sore from breathing the fuel rich exhaust is something I would like to remedy.  

 

Photos and questions are coming...

[Moses Ludel]

Jeff...Since the ECU and other devices were disconnected, the Mixture Control Stepper Motor likely runs in WOT or enrichment mode now.  There is also the lack of a cat, which means the downstream HC and CO are not cleaned up.  Cough, cough, surge, surge!

 

The Howell system, if you have budget, is the way to go.  A cat would restore downstream emissions clean-up if you can also restore the Pulse Air system.  The ignition can run without the retard functions and performs well with the Howell system.  I can share simple vacuum routing.

 

Actually nice to live in the middle of nowhere...We miss that...That's where your great wildlife photo subjects live!

 

Best,

 

Moses

[forman]

A couple of photos to show what I mean by a bypassed module, sorry for the out of focus cell phone photos.

 

 

In the above photo a yellow wire was spliced into the purple wire of the module.

 

 

 

From there the yellow wire was connected to a purple wire that disappeared under the distributor cap.

 

This part should not cost very much to replace if I knew the part number I could look it up.

[Moses Ludel]

The concern is "why" the split and splice.  This likely has to do with taking the ECU off-line.  This is the inexpensive Motorcraft module, not the factory ECU.  Confirm by going to the local motor supply/auto parts house and asking for a Motorcraft module for a Ford or Jeep product.  It should look just like this module.

 

Identify the ECU, and see if that's been bypassed.  The ECU, as I shared, would be more like the "$800" device, most likely obsolete at this point. 

 

The common ignition module is under $50.  (The first one that turned up online was $28.95.)  Here's some quick insight into the module, including its fitment within the ignition wiring circuit:  https://www.google.com/search?q=Jeep+Motorcraft+ignition+module&biw=1366&bih=673&tbm=isch&tbo=u&source=univ&sa=X&ei=yhmiVOSxI8mSyATSxoGgAw&ved=0CFwQ7Ak.

 

Moses

[60Bubba]

I wish, wish, wish I had read this a few days ago.  I pulled my old ECM out when I rebuilt my heater box, but it went to the scrap heap because we're cleaning up in anticipation of an upcoming move. I can't actually guarantee it would have worked since mine has been disconnected for 15 years, but the price would have been right!  FYI, if you're still looking for the factory ECM it is behind the glovebox sitting on the plastic heater box.

 

I know budget is always an issue, but I've owned a '79 CJ-5 with a bastardized stock carb setup, and it was no fun.  My current Jeep has the MOPAR EFI swap and the difference in response is amazing. When I was shopping for my current Jeep, I researched the Howell system.  I decided I was going to buy a previously fuel injected CJ-7, or budget for an immediate FI upgrade after purchase.

 

I have no experience installing one, but I did just replace my entire vehicle wire harness, so I can say with the MOPAR kit the electrical part of the job is pretty straightforward.  If I'm not mistaken, I think the Howell kit is mostly Chevy parts, so I bet that would be easier to source replacement parts if needed.

 

Good luck with your decision.

[forman]

Thanks for thinking of me Bubba and yes I would love to go with fuel injection but given how I use the Jeep and the amount of missing emission parts... I'm going with the Weber replacement carburetor.

 

As for the bypass mystery it looks like a Nutter bypass was performed before I got the Jeep.  The three cut wires in the photo below correspond in color and lead to the ECU.

 

 

I believe the stepper motor should be where the block off plate is on this rebuilt carb.

 

 

Moses a couple of questions before I order the Weber carburetor.  The install guide I downloaded mentioned that the recommended fuel pressure be 3.5 psi,  Do you think I will be able to attain that pressure?  What is the best way to check the fuel tank liquid check valve?

 

 

[Moses Ludel]

Forman...The pressure to the Weber is only 3.5 psi and requires regulation because the OEM fuel pump pressure is high enough to unseat the float needle in the Weber carburetor.  You can easily find an aftermarket fuel pressure regulator for this purpose. Try Summit Racing and other sources.  

 

The regulator goes between the fuel pump and the Weber carburetor.  I like to use steel double-flare end fuel grade tubing when forming lines.  You can hard line the regulator in place or install it at the OEM fuel filter.  We can discuss safe attachment practices further.

 

Run a good inline fuel filter.  The Jeep OEM fuel filter is a three pipe, the third pipe returns fuel to the tank.  I like this, as the continual fuel flow reduces risk of fuel vapor lock.

 

The fuel tank "rollover" valves stop fuel pouring out of the tank in the event of a vehicle rollover.  Atop the tank, they are not that easy to reach.  You might be able to test the valve(s) with a simple vacuum hand pump: test at the tank vapor line.  Applying vacuum at that line should close the valve(s); if the valve is seating, the pressure will rise and hold at the vacuum gauge. 

 

Here is a 1989 Wrangler explanation from Jeep that makes virtually no sense at all from a testing standpoint:  1989 Jeep Rollover Tank Valve.pdf.  The CJ uses similar valves and configuration.  In the factory workshop manual, there is no mention of testing the rollover valve system.

 

Here is a useful diagram of your exact vacuum circuit and the underhood devices: 1984-86 CJ Jeep 4.2L Vacuum Diagram.pdf.  This can be our blueprint when discussing what needs to remain intact.  You need EVAP, EGR, the closed crankcase with a ventilation valve and ported vacuum advance to the distributor. 

 

The Weber will eliminate Sole-Vac and the Mixture Control "Stepper Motor" features of the BBD.  (You share that the Stepper Motor was already eliminated on the BBD.)  We can break it down from here.  The CTO still has a function, that's a device to maintain, I'll explain how you can keep it intact and useful for several devices.  The ECU or MCU (Micro Computer Unit in Jeep language) will be out of the loop.

 

Moses

[forman]

Thank you so much for the diagrams and advice about the fuel pressure question.  I'll be ordering parts today.

[forman]

The Weber carburetor kit came in and I had a fun time installing it on my 4.2 engine. I just knew that once the install was complete the engine would start and run better and I could make minor adjustments and enjoy the nice ride. Remember the main issue I had with the old carb was horrible fuel mileage and my inability to make the right adjustments. I started and drove the jeep on a cold morning from the garage to the shop allowing it to warm up, total time approximately 15 minutes. The cold start took 10 seconds.

This installation requires that a fuel pressure regulator be used to regulate the pressure to around 3.5 psi. I originally installed the regulator between the fuel filter and the carb... This fuel filter has two outlets the other goes to the fuel tank return. I didn't think about this until the engine wouldn't start. I pinched off the line that goes to the fuel tank return hoping that this would force the fuel through the regulator and into the carb. It didn't. So in an effort to force the fuel into the carb I bypassed the regulator. I could smell and see fuel entering the intake manifold... I could feel vacuum as I cranked the engine over... I pulled a plug to verify that I had spark and yes I had it, but she will not start. I'm lost please advise.

[Moses Ludel]

Spark and fuel...Need both to run...See if the accelerator pump in the carburetor is working by the usual means: opening and closing the throttle with the engine shut off.  Watch for the typical spray down the carburetor throat.  If you see fuel, that should be enough to start.

 

Is there a possibility of a large vacuum leak?  Any disconnected hoses from the BBD carburetor now open to atmosphere, especially in the closed crankcase system?  If so, the mixture could be too lean to run.  You're trying to start an engine that needs enrichment when cold.

 

Moses

[forman]

I re routed the fuel pressure regulator and discovered that it had a broken spring causing it to not regulate. So I was sending over pressurized fuel to a carb that requires 3.5 lbs.

Later with repairs made and a few adjustments the little engine ran better than I can remember. The Weber carb is easily adjusted, cleans up the engine compartment, and most important to me stopped my engine surge issue.

Moses you mentioned that I would be able to use my EGR and CTO in an earlier post?

My fuel regulator issue is probably not resolved as I will need to make sure the pressure is 3.5 lbs. I don't have a fuel pressure gauge but I do have questions... It seems most gauges measure high pressure systems these days and are sold in ranges of 1 to 100 psi. I was wondering if you could recommend a set up that I might only use this one time? I ask because I use your spark plug leak down tester set up.

[Moses Ludel]

In the 'sixties, when I was teething on all things automotive, a popular tool was the combination vacuum gauge/fuel pump pressure tester for mechanical fuel pumps of the era.  A high output muscle car V-8 pump put out a whopping 8 PSI to fuel dual quad carburetors!  These gauges read accurately down to the 3.5 PSI you desire...

 

RAC, Sun and others made these gauges in the day, and I bet if there's a used tool or even a yard sale in your area, you can find one of these relics for a few dollars.  I have such a gauge still, they were inexpensive when new.  There are modern versions like the inexpensive Actron for less than $15: http://www.summitracing.com/parts/anm-cp7802.  (This gauge measures to 30 in. hg. vacuum in one direction and 15 PSI maximum pressure in the other.)  You might find this item at the local auto supply or Auto Zone. 

 

I suggest using the EGR and CTO as originally intended.  The vacuum source for EGR is simply "ported" vacuum from the carburetor, the same as a distributor vacuum advance uses.  You can curtail the maze of vacuum connections and focus on just a closed crankcase, EGR and CTO.  The EVAP system must remain intact and functional to prevent fuel supply issues on these AMC/Jeep CJs.  EVAP is passive like EGR and won't impact performance if the parts are still functional.

 

Glad the engine is responding well to the carburetor swap.  This Weber is proven.  They are sensitive to high altitude and often need a jetting change if you hang out for any length of time at the Colorado 'Fourteeners!

 

Moses

[forman]

Moses I'm having some issues with my new carburetor. This past week I had the opportunity to guide hunters on a safari style hunt and ran the Jeep through our back country, and to be honest I was a little embarrassed at times,

I plan on giving the guys at Weber a call but of course I'm interested in your opinion first.

To begin with the engine will just not start up when cold. I purchased a manual choke and it is functioning properly as far as I can tell.

It seems that not matter what technique I use or how long I crank the engine will not start, typically I just walk away for a period of 20 minutes to an hour then when I try to start again it will start with or without enrichment. The rest of the day the engine starts within a few seconds regardless of how long the engine ran previously.

The Jeep idles and accelerates fine,with no chugging or missing at any time until I climb a hill, it doesn't matter how long the climb is the engine will flood and die then of course I have starting issues.

This is a new Weber carb that came completely built. I adjusted the idle and have adjusted the fuel:air mixture 1/4 turn lean.

Any ideas?

[forman]

I should add that I have a fuel pressure gauge installed inline just before the carb and the pressure is set to the recommended 3.5 psi.

[Moses Ludel]

Forman...Check the float level in the Weber carburetor.  Sounds like one extreme or the other, or both:  1) the needle is sticking, starving the bowl of fuel or 2) the float height is off, either too high or too low. 

 

As a quick test for a stuck needle, try gently tapping on the carburetor float bowl cover (at the air horn/fuel inlet to the carburetor) with the handle of a screwdriver.  The needle may be sticking and keeping fuel from filling the bowl.  The flooding on a hill could be a float level set too high. 

 

Good practice to carefully remove the air horn/float cover while preserving the gasket.  Do the float height check and adjustment.  If there is a float drop setting, make sure you set the float drop as well...Also inspect the needle and seat function for sticking or not seating. 

 

The fuel pressure is correct, so that's not the issue any longer...Shipping a carburetor can disturb the need and seat or float settings.  You can check the fuel pump volume if necessary, though it should be okay if the OEM BBD received adequate fuel.

 

Moses

[forman]

There was no stuck needle this morning as I tried a cold start.  Ample fuel available as I looked into the carb as I pumped the linkage.  The first attempt produced a sputter, this has never happened before.  I decided to pump the gas pedal then apply the manual choke as I turned it over and it started!  This hasn't happened so quickly since installing the new carb.  

 

While the engine warmed up, I backed the Jeeps rear wheels off of the shop floor placed a set of vehicle ramps in front of the front wheels and drove up on the ramps. This would simulate the incline that would cause the vehicle to flood and stall.  In just a matter of seconds the motor chugged and smoked and stalled.   I called the good folks at Weber and spoke to a knowledgeable fellow about the flooding issue I spoke of in my last post. He gave me some measurements to check and when I compared the distance from the float to carb housing I indeed needed to make adjustments.  After the adjustments I parked back on the ramps and tested the new float level and there was no stalling this time.  A trip to the back country for a test drive and once again I'm happy with the motor performance.

 

 

 

 

[forman]

I think this is a photo of a CTO switch.  Can you tell me how it works?

 

[Moses Ludel]

The CTO is very simply a thermal vacuum switch that opens by coolant temperature.  At a preset temp (typically 120-140 degrees F) the CTO switch "opens" (usually, a wax pellet melts in a thermal switch) and allows the source vacuum to pass through to the other port(s) on the switch. 

 

On the CTO system, the source of vacuum should be ported carburetor vacuum.  (See the vacuum circuit diagram.)  This is not manifold vacuum, which applies all the time with the engine running.  Ported vacuum is sourced just above the carburetor throttle valve's opening position.  When the throttle is closed, there is little or no ported vacuum (check with your vacuum gauge).  As you crack the throttle slightly, the ported vacuum jumps way up, and this is optimal for the distributor's vacuum advance and the EGR valve.  Neither of these devices should operate with the throttle closed or at an idle position.

 

The purpose of CTO is to not apply distributor vacuum advance or EGR when the engine is cold.  This is an emissions concern for the ignition.  For the EGR, it is very undesirable to have manifold vacuum holding the EGR open at an idle, so the source is ported vacuum in each case.  EGR does not operate with a cold engine.

 

Your Weber carburetor should have a ported vacuum source for distributor vacuum advance and EGR functions.  Route that ported vacuum to the vacuum source side of the CTO first.  From the CTO, a hose can go to the distributor vacuum advance canister and the EGR valve.  Check these functions cold and with the engine warmed.

 

Moses

[forman]

I have a question about the idle fuel:air mixture screw. The two screws on the carter carb were located in front where the fuel enters the carburetor. The one fuel:air mixture screw on the Weber carburetor is located on the back of the carb on the opposite side of where the fuel enters.

On each carb what am I adjusting fuel or air?

[Moses Ludel]

Forman...I can see where you might have questions here.  Automotive and motorcycle carburetors differ, and this can be very confusing.  On some motorcycle carburetors, the mixture screw controls air flow, on others, it controls fuel flow.  On the two automotive carburetors in question, the mixture screw controls air/fuel flow volume. 

 

For the BBD Carter two-barrel (OE carburetor on the Jeep 258 inline six), the two idle mixture screws control the volume of pre-mixed air/fuel flow that originates in the idle circuit feed and moves downward to the mixture screw(s).  When you turn the mixture screws inward, the volume of air/fuel available (ratio already determined by the low-speed jet and economizer) is reduced.  As you open the screws (counterclockwise), the volume of air/fuel entering the manifold from below the carburetor's throttle plate will increase. 

 

So you're really not adjusting the "mixture" but rather the volume of air/fuel entering the engine with the throttle plate closed.  This does enrich (turn mixture screw outward or counterclockwise) or lean out (turn mixture screw inward or clockwise) the tailpipe emissions reading because the throttle plate (i.e., the idle speed setting) remains in a fixed position while more or less fuel/air flows into the manifold from below the throttle plate at the mixture screw opening.

 

The throttle plate setting is the "curb idle" on the BBD.  There is a balance between the amount of throttle plate opening (idle speed) and the volume of air/fuel entering the manifold through the idle mixture screw opening with the throttle plate backed off yet still open enough to keep the engine running.  As the throttle opens, there is an additional idle port exposed that increases the volume of pre-mixed air/fuel to provide a smooth throttle transition as you open the throttle to move the vehicle in gear and under load.

 

As for the Weber, and I'm assuming you have the Weber 32/36 DGEV carburetor, the adjustment of the idle speed screw and the idle mixture screw are closely intertwined also.  Weber wants the carburetor idle screw as slow as possible while adjusting the "lean best" setting of the idle mixture screw.  This usually means backing out the idle speed screw (engine warm, choke off) to the slowest idle speed possible.  Then adjust the idle mixture screw and continue to back down the idle speed screw to the slowest setting possible.  When you set the idle mix screw for a smooth though leanest (clockwise) setting, you can then raise the idle speed screw to a normal curb idle speed.

 

At this point, I would again set the idle mixture screw for lean best.  This means backing out the idle mixture screw (counterclockwise and enriching) very slowly to a point where the speed increases slightly.  Let the engine stabilize there, then turn the idle mixture screw inward to just the point where the engine begins to stumble or miss slightly.  Back out the idle mixture screw just enough to eliminate the miss.  This is the lean best setting.

 

The confusion with setting up a Weber is that the idle mixture setting process is supposed to begin with the throttle plate closed.  The engine may not want to idle there, of course.  The concept is simple to understand though:  If you open the throttle plate before setting the idle mixture screw, you will add additional air/fuel as the throttle plate begins drawing from the progression ports (think of the idle ports on the BBD, above the idle mixture screw opening). 

 

This is the transition I described for the BBD.  It's the "progression" enrichment on the Weber.  For either carburetor, this is the added fuel to enable a smooth transition from a stone curb idle to moving the vehicle.  A distributor vacuum advance does the same thing, using that same "ported vacuum" signal to advance the ignition timing substantially just as you tip-in the throttle.

 

Moses

[forman]

Awesome answer!

 

It doesn't help that I am working on two Jeeps each with the carbs I mentioned.  For simplicity sake in dealing with the Weber could I have an over rich   idle mixture screw mixture that allows the engine to run well (by my perception) but gets really lousy fuel mileage?

[Moses Ludel]

Forman...Try adjusting the Weber in the way I suggested...I'm hesitant to suggest, however, that the lousy fuel mileage is the result of the idle mixture screw setting. 

 

Some clues about a rich idle mix would be an odor of running rich.  A quick additional test is to pull the manual choke out very slowly with the engine warm.  If very little choke sets the engine into a blubbering state, the idle mixture could be too rich.  Be sure that the choke consistently opens completely.

 

Unless you're idling the Jeep a lot, richness causing poor fuel mileage would be more aptly due to rich main jetting.  The jetting provided with the Weber should be for sea level.  What is your elevation?  The Weber can be sensitive to altitude.

 

I would look at one other possibility:  the EGR valve.  This should not be opening with the throttle closed, and if it is, you have a manifold vacuum line going to the EGR and not a ported vacuum line.  When the EGR valve unseats, does not seat, or is opening with the throttle closed, you will have a rich idle and an incredibly difficult time setting the idle mixture.

 

The quickest way to check the EGR function:  Hook a hand vacuum pump directly to the vacuum pipe on the EGR valve.  With the engine idling, apply vacuum to the EGR valve diaphragm.  You should hear a noticeable change in engine speed and smoothness.  (Also, the vacuum gauge should hold steady when applying vacuum to the EGR.  If the vacuum gauge will not hold, and there is no buildup of vacuum as you pump the vacuum pump/gauge, the EGR diaphragm leaks.)  For the purpose of getting a stable idle, you should be concerned about the EGR valve seating properly.  A carbon encrusted EGR plunger will not seat, and this causes a rough idle, too.

 

Note: I've cleaned EGR valves in carburetor cleaner.  Do not submerge the diaphragm, only the metal parts, and make sure you clean the plunger and seat carefully.  This can often salvage a sticking or unseating EGR valve.  Many EGR valves get replaced when the diaphragm is still good; there is simply carbon on the plunger, casting seat and in the passageways.

 

If the engine is applying vacuum to the EGR at an idle, the manifold and air/fuel stream are being "diluted" with spent fuel or exhaust.  The EGR valve should open with ported vacuum.  This means it should only operate as you tip in throttle, through the mid-range throttle settings and gradually taper off as the throttle opens widely—just like ported vacuum does!  Ported vacuum is very low with the throttle closed at an idle; manifold vacuum is at its highest during idle...There should be a distinct ported vacuum pipe on the Weber carburetor, just like on the Carter BBD.

 

As a related consideration, is the exhaust system in good condition?  Is there a clogged cat or muffler?  Does exhaust at the tailpipe end feel restricted?  This could cause all kinds of trouble...What is the manifold vacuum with the engine idling, and is it steady on the gauge?

 

Moses

[forman]

I plan to tackle the Weber and am confident your suggestions will make adjusting the carb very easy.

Let's see my elevation is 2600 feet, exhaust is in good condition but the cat has been removed (part of the reason I chose the Weber) and the EGR is functioning properly.

I'll get on this and get back to you as soon as I do.

Thanks

[Moses Ludel]

Forman...Cat removed will increase tailpipe emissions but have little impact on performance.  Lessened exhaust restriction should improve performance some.

 

Listening for your results with the Weber adjustments...

 

Moses

[forman]

You give great instructions Moses, I made the changes and except for a near inaccessible idle screw all went well. I'm fairly certain that my idling for long periods down some of our flat roads and a rich condition were the reason for my poor fuel mileage. I just need to put some miles on it now.

Thanks for everything!

Jeff

[Moses Ludel]

You're welcome, Jeff...

 

Moses