Thomas J. Hardman, Jr
thardman [at] thomashardman.com
VaraJet to Weber 38 DGES Swap Notes and Remarks
Here is a diagram of the vacuum and vapor/emissions system as originally sold by Jeep. Click for a huge copy of the same image:
See also 1986 Jeep Comanche.
Please also see Post-Installation and Semi-Post-Installation, and Driving with the new 38 DGES Weber carb at the bottom of this page.
You will probably want to see "addenda"., which covers post-installation, tuning, etc.
Click on the thumbnail images for an 800x600 enlargement. The extremely large original 2180x1440 files are available on request.
The first few photos are a walk-around of the engine compartment, mostly to get a record of where all the original hoses were.
Old VaraJet, Aircleaner
and housing removed.
Extreme close-up of front of VaraJet. Detail of hose attachments.
Extreme close detail of passenger-rear side of VaraJet, linkage detail.
Closeup of passenger-front side of VaraJet, some detail of fittings and
linkage.
Note: Removing the transmission kickdown linkage for this model is done by grasping the plastic end of the cable with a pair of pliers, gently, and pulling towards the front of the vehicle. If, like me, you don't know this, and -- like me -- break the plastic thing, you may be able to use a tiny drop of superglue to get the pieces stuck back together. Once that hardens, a tiny fillet of superglue along the remaining crack will soak into the crack and finish the bonding process.
VaraJet as removed.Note the "primary fuel electric heater" element. This was not re-installed when I installed the Weber 38 DGES.
Removing the VaraJet is not easy; it wasn't designed to be replaced, as best I can guess. You will need some slightly specialized tools. A "bendy extension" or articulated extension is what I used to get at some of the recessed nuts holding the VaraJet to the intake manifold. A angle-end box wrench might also work.
Bottom view of dismounted old VaraJet. Note the complexity and huge number
of attachments.
A note here -- the 38mm throat bores of the new Weber are both about halfway between the size of the throat bores of the VaraJet. This means that the maximum potential airflow of the Weber is probably a bit less than that of the VaraJet.
The new Weber 38 DGES installed, view from driver side. Compare with the
bulky and cluttered VaraJet.
View of Fuel Pump, Fuel Filter, "recirculation splitter" (return flow, for
cooling), and fuel lines.
Really, this isn't quite how you want to do it. And there's no fuel pressure regulator, which you definitely will need.
New Weber 38 DGES installed, with most hoses attached.
Note: the "universal installation procedure" document leaves much to be desired, at least as regards this particular swap and conversion. It's not wrong and if you have some experience and clue about dealing with engine vacuum systems, you will figure it out from the basics. Their "remove single small hose from front of old carburetor" was not applicable to my situation. There were several small hoses attached to an "octopus". However, the "octopus" itself was attached to one fitting which would have had the "one small hose from front of old carburetor" in a different vehicle. I just removed the "octopus" and attached a small hose from that to the front of the new Weber 38 DGES.
Yikes! Things don't fit together on final assembly!
At this point it becomes evident that the factory-supplied air-cleaner "original equipment mounting bracket" does not conform to the reality I have got here. See the photo immediately above, that blue stuff is kitchen-sponge and "forms-a-gasket" used as an emergency gasket. There was a gap of about 1/4 inch between the top place of the carburetor and the bottom of the air-cleaner housing. Not much air will get cleaned by the filter if it all goes around it. Clearly there is some need to do some fabrication.
Also, the original-equipment mounting bracket supplied is too tall; the air-cleaner housing cover has to be mounted upside down to get it to seal to the top of the air-filter. Also, the tie-down bolt is too long; it's poking the hood of the Jeep.
Some details of "below-the-air-cleaner" hose attachments etc.
Sealing the Air-Cleaner Housing to the Carb
View of "emergency gasket".
Needless to say, this obviously doesn't work. Now we will have to get that gunk off of the mounting bracket.
Below are some details of various angles, showing hoses attachments, various manifold vacuum ports, etc.
In the image directly below, the vacuum hose shown running from the bottom of the carburetor is attached to the wrong connection. In the middle of the picture is a rubber cap on the end of a metal tube, near where the carburetor fuel bowl vent hose joins other lines. That rubber cap had to be removed, and relocated to the place where the hose from the bottom of the carburetor was attached. The place where the hose from the bottom of the carburetor must attach to where the rubber cap was, that is a metal tube leading to the rear of the engine, where it fits the hose to the distributor vacuum advance.
Detail of mounting bracket (removed from carb, 4 small nuts with lock
washers hold it to the carb top), Air Cleaner housing, and hand-made
gaskets.
I had to go buy a roll of oil/gas-resistant gasket material and cut these gaskets by hand, in layers. Each layer is a tiny bit different, though all are based on tracing around the mounting bracket and then some eyeball estimations, and a bit of clever work with scissors.
Once cut out and made nicely to size, the layers are made into a laminate gasket by very lightly coating the surfaces in a continuous bead of that blue silicone forms-a-gasket stuff. Do NOT put this stuff on so that the gaskets are glued to the carb or to the air-cleaner housing! You'll be sorry if you do! Just make sure the layers of gasket stick to each other and to nothing else, and don't laminate until you are sure you have enough layers to make the seal. Now place the mounting bracket back on the carb but don't secure the hold-down nuts. Then laminate as you place each layer onto the mounting bracket, but do not stick the gaskets to the carb or mounting bracket, only to each other. In this way you get perfect fit and placement of the gasket layers. Then bolt on the air-cleaner housing and let the goop dry for maybe 10 or 20 minutes.
In my case, when tightening down the mounting bracket, I didn't replace the 4th nut, which was on the passenger-rear side of the carb assembly. It was most of the problem. With the combination of hand-made gaskets and omitting that nut, I could get a tight seal. Please see a close-up.
Measuring the offending oversized mounting bracket.
Bottom of the air-cleaner housing.
"Re-shaped" air-cleaner cover.
After sufficient poundings with a hammer, the central dimple of the stock cover has been "adjusted". This took about 40 whacks with the hammer.
Comanche with a
Load!
Cold Idle Kick-Up Vacuum Switch.
This is a strange little electrically controlled vacuum switch. The coiled hose, attached, goes to a diaphragm on the VaraJet, where it is located just in front of the linkage. In extremely cold weather, this would raise the idle. This electrical circuit may (or may not) be what should be adapted to operate the electric-choke assembly on the Weber 38 DGES. I had wanted the 38 DGAS (water thermostat choke) carb but this is what they shipped. Having waited for months to get this carb, I figured I'd just work with what I had.
Fully installed and ready to tune!
Post-Installation and Semi-Post-Installation
FUEL FEED
First, I have to admit that I didn't do this "properly". I didn't have all of the tools on hand, and I didn't have all of the parts in place at the right time. I also made some other mistakes, and perhaps the worst of those mistakes was doing this with extremely limited cash on hand for incidental "wasn't expecting that at all" purchases.
The carburetor arrived first, before the pressure regulator. That pressure regulator was a Holley 12-803 Low Pressure Fuel Regulator ("LPFR"). The day after the carburetor arrived (a week after payment to Carbs Only) I was told that the Holley LPFR was in and that I could come get it. Unfortunately, it shipped without the fittings. The fittings I needed were obtained from Applied Technologies in Gaithersburg MD, where they had them there one day after order. These were NTP fittings, which were very hard to find in the proper sizes. They have to be 3/8ths on the threaded end, and "barbed" on the smooth end which must fit 5/16ths interior diameter fuel hose.
For my setup I already had a "recirculation splitter" to keep fuel flowing through the pump and filter at all times. This has two main effects: the first is that gasoline doesn't stay cooped up and stagnated in hot hoses in the hot engine compartment, which prevents vapor-lock; the second effect is that before long all gasoline in your tank has been circulated through the fuel-filter. This results in clean gas, dirty filter, and the filter will have to be changed regularly.
Because of this recirculation splitter, I needed a block-end NTP fitting, also known as a "plug". One of the "hose barb" NTP fittings takes gasoline into the LPFR and one takes it on to the carburetor. Be sure to be extremely careful to have no gasoline leaks at all, and test your fittings for leaks by turning on the fuel pump. Short across the oil-pressure safety switch if you have to do this, but be careful that your short across the safety switch cannot spark or you could go up in flames along with your vehicle. If the fittings remain non-leaking after a few minutes at a time of running the fuel pump, turn off the power and remove the safety-switch short-wire so that the fuel pump will operate only when the oil-pressure switch indicates a turning engine and operating oil-pump.
N.B.: USE a pressure regulator. The carb will not run properly until you have a LPFR in place, and will not be usefully tuned until the fuel pressure is properly controlled. Set for 2.5 PSI, as per Weber's advice.
Eventually the fuel distributor system -- the regulator and related fittings -- started to leak a little around the edges. Careful application of Type I gasket sealer (or type 1, same thing) sealed the leaks. You might want to get everything into the final configuration and then go ahead and seal it once you are satisfied.
TUNING THE CARB
Follow Weber's advice on getting the fuel mixture correctly set. Keep in mind that on my vehicle, this takes two pairs of screwdrivers, and a lot of patience and a lot of caution. The rear-side idle mixture adjustment screw requires a screwdriver that has a short handle and short blade, altogether measuring about 2 inches long. Any longer and you can't get to it because it's too close to the distributor cap. On the front side, you need to be really careful and also need a fairly long-shaft screwdriver, because you have to get past the hot-water outflow pipe and thermostat on the intake manifold. This screwdriver also has to be short enough so that you don't get caught in the radiator fan, which will of course be turning as you make these adjustements. I might add that you probably don't want this screwdriver to have too broad a blade when trying to adjust the idle-speed throttle screw.
I should add that probably you should tune the carb once, drive it for a few days or a week to let it "settle in", and then tune everything again.
DRIVING
These notes should be considered as applying only to the upgraded GM Performance Parts "crate" 3.4L engine. If you still have the original 2.8L stock engine, "your mileage may vary".
First, let me point out that if you have an A-904 TorqueFlite as I do, that you will definitely want to reconnect the kickdown cable. If you don't, and shift manually, probably you will burn out your transmission in short order.
With the old VaraJet, and the stock engine, the way you got speed or power out of the engine was to hold down the gas pedal and let the rather large secondary throttle open. This would occur when the flow through the main throat was so restricted that it opened a vacuum diaphragm which previously locked shut the secondary throttle. The kickdown and full-throttle upshift points were set so that you could drop down into first at up to 25MPH or so, and first gear would wind out up to around 40MPH. Mostly you would be just working the engine severely and probably this contributes to the classic failure modes of the 2.8L V6.
With the new Weber 38 DGES, if you are sitting at a line and apply full throttle, you simultaneously open both throats. I think that the Weber 38 DGES doesn't have the full flow capability of the wide-open VaraJet, but you can get the full flow of the 38 DGES from a dead stop. Of course, this means that you will instantly lower the vacuum at the heads until the engine RPMs catch up.
The Weber 38 DGES is probably at its best at fairly low engine RPMs, and because it doesn't seem to have the top-end high-RPM flow rating of the VaraJet, you are much better off, and are strongly advised, to not just give it wide-open throttle. Where the VaraJet-equipped V6 works most powerfully when you use a combination of kickdown to lower gear and winding out the engine, the Weber 38 DGES works best -- for me, anyway -- when you give it about half-throttle. The linkage connection to the kickdown cable seems to be different from that of the VaraJet, so that you have to depress the pedal a lot harder to get a kickdown to the next lower gear, and the limiting speeds that govern whether or not the kickdown is possible seem to be set lower; this means that you can't kick down to first unless you're going less than 15MPH or so, in my experience. Also, in normal driving, it upshifts at about the same, but slightly lower, speeds as it did when the VaraJet was still in place. Taken all together, this means that you'll be doing a lot less winding out, and a lot more high-torque low-RPM rapid acceleration.
Low-flow low-RPM operations is where the Weber 38 DGES is a major improvement over the VaraJet. It seems to me -- I haven't made a huge statistical study out of it -- that with the Weber 38 DGES you will go just about as fast up to freeway speeds at about 1/3rd-throttle to half-throttle as you would with the VaraJet at full throttle. Indeed, starting from the line with the Weber "floored", I wasn't that much impressed with any sensation of major improvement. But starting from the line with the Weber at about 1/3rd or half throttle, I was very much impressed. The improvement of power at lower RPMS was such that you just let the transmission do the work in higher gears, rather than with the engine screaming in a lower gear.
This may be the single "drawback" of the Weber 38 DGES, and it's probably more particular to my specific situation. Simply stated, I may now have too much low-RPM torque for my old A-904 TorqueFlite transmission to handle it. In first gear, if I'm not careful, I start burning off my drive-wheel tire, and then when it drops into second, the over-torque seems to be more than the trans can accomodate quickly. The shift from first into second under any conditions should be instant and unambiguously sharp, which isn't presently the case. A transmission overhaul is probably overdue (170,000 miles or so), and might solve this problem. But the shift up to third-gear under heavy throttle is pretty clunky when it occurs. Usually that's a sign that you've got too heavy a load or far too much power. I am going with the theory that it's far too much power. When the truck is at freeway speeds and with the torque-converter locking clutch engaged, opening up the throttle to about half-way results in very nice acceleration, considering that after all I am driving a truck.
[Note: As of November 2007 I did get the transmission rebuilt at BCC Transmission on Gude Drive in Rockville, MD. This cost me about $1500 but so far it is all good... other than the fact that the trans is probably underrated for the output of the engine.]
Recently, I took a job hauling which so loaded my pickup bed that the vehicle was audibly creaking, which is a first for me. Yet with the 3.4L "crate" engine and the new Weber 38 DGES carb, I had no problem moving that load down the road, and had to be fairly cautious about giving power to first gear after the vehicle accelerated rapidly enough to somewhat shift the load.
If you are looking for a new carburetor to replace your crappy old VaraJet, and you want something that will let your engine breathe in huge amounts of air-fuel mixture at screaming RPM revs, you might find the Weber 38 DGES to be a bit small and constrictive at the high end with the rated flow of 330 cfm (cubic feet per minute). The original VaraJets, remember, have flow ratings of 375-390 cfm or so. Consider one of the larger Webers, or perhaps something along the line of a Holley 500 2-bbl. But keep in mind that these are probably more trouble to install than is the Weber K490 Jeep "outlaw performance" kit.
If you are looking for a new carburetor to replace your crappy old VaraJet, and you want something that will give you considerable increase in low-RPM torque and will definitely improve a light truck's load hauling capability, the Weber K490 kit should be at the top of your short list.
April 30, 2007, by Thomas Hardman
Please see:
Addenda I which gives additional detail on
tuning, subsequent modifications, general remarks "as time has passed", etc.