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{NOTE: This is currently a work in progress, but I wanted to post what I had already done so far. It may continue to grow as more time allows. (Or as people here point out my obvious screw-ups...lol) Bookmark it, come back to it as often as you want to, and please feel free to copy/paste it, print it out, and pass it around.}
OK....this is going to be the Short Course in what you need to know to tune and jet your Weber 38/38 DGES Carb successfully. This is not specific info about your specific truck/engine combination, modifications, etc. - or to give you magic jetting numbers.(We have a free file here in the Information Section that you can get to from the front page of this site for that and if you haven't downloaded it yet - SHAME ON YOU!!) Weber Weber 38/38 DGES carburetors are one of my favorite modifications on Luvs because they respond so well over factory POS Hitachi carburetors. Yet they are one of the most difficult modifications to tune and set up right without proper basic knowledge of how they actually work and function.
Most of the time you can not just bolt on an after market carb and expect it to work just fine. The carb will most likely have to be tuned and jetted to your engine application. Jets are small brass screws that set inside of the carb and have a hole in the center of them. They have a number stamped on them indicating the calibrated size of the hole. Depending on the type of jet, they regulate the amount of fuel or air passing through the hole. Sometimes you can get lucky and the carbs will work fairly well, but you will never know the true performance potential of a Weber carb until you try different jet settings to see if you can get it to run stronger throughout the entire rpm range.
But please don't worry - I promise this is an easy process and there is no "magic" to this. It is just a simple step-by-step process - that once you get the hang of it - you will be able to make your carb work right every time. It involves more than just jetting the carbs, and in fact, that step is the easiest. Following these directions will also make it much easier when you are purchasing jets for your carb.
This is a guideline only to get you started or to make sure you are not totally out of range which makes it very difficult to tune. There may be some of you running trucks successfully with a modified carb outside this range - so please don't e-mail or PM me about it.
First things first as people always say:
The main difficulty people have with tuning these carbs is that there are many combinations that work well depending on their engine's condition and/or modifications - and the state/condition of their ignition and it's settings. That is why everything is tunable. You can custom-tailor these carbs to suit your needs, driving style, engine, location, weather and altitude conditions, etc....
And before you blame the carb, make sure you have the fuel delivery right and the spark timing correct. If you do not have enough timing advance, you will have a lot of problems, and it will have nothing to do with the carb tuning.
If you are having a problem with how the engine runs..poor idle quality, stalling, etc. - then this is the first thing to do. CHECK FOR VACUUM LEAKS! This is particulary true of new installations where you just purchased and installed a conversion kit and you are having problems.
You have to remember that most Weber conversion have been around for many years and have been installed on hundreds of cars and trucks. They work. The possibility of having a defective carb out of the box is about is about the same as being hit by lightning indoors on a sunny day...lol. If you are having a problem on a new install, it is most likely this problem or one of the steps below. DO NOT OVERLOOK this as a problem...This is the #1 problem with any Weber carb conversion.
Do a compression test. If the compression is off by more than 10% per hole you will have a really bad time tuning the carb.
Always start with a fresh set of spark plugs. No don't just clean the old ones - start with a fresh set of brand new plugs. This will give you a good reading of fuel mixture and enable you to make a good judgement of what you need to do.
Set the timing. Generally speaking, you need a little more advance with a Weber than with the stock carb. A good place to start for most Luv's with Webers on pump gas is is in your manual or on a sticker under your hood. This is not cast in stone and you need to make sure your car does not detonate (spark ping) at these setting. If you get any detonation, then obviously you need to back off the timing.
Know what is in your carb NOW. Do not rely on a list on a website or a book that says what your carb might have come with. Actually remove the jets, check the sizes, and WRITE THEM DOWN. You can not make any jetting decisions unless you know first what you are starting with.
Make sure your ignition system is working properly. Remember...a Weber carb is always a performance upgrade and is designed to deliver more fuel and air and make more HP then the stock carb did. This means you need to have adequate spark to burn the extra fuel, or you will have problems tuning and jetting your carb. This is especially true of older engines that have point type ignition systems. A lack of spark or weak spark will make it impossible to tune the carbs properly or to their maximum potential. A good electronic ignition distributor/ignition module combination to replace the old stock points-type distributor/ignition and the addition of a good high performance coil like an Accel plus high quality plug wires are IMHO the best way to go.
Make sure that your fuel delivery and/or fuel pressure is right. Once again, you can not tune carbs that do not have adequate or proper fuel delivery. DO NOT ignore this step.
It is also very important to verify all linkage and levers are installed without binding and the linkage opens to full throttle and closes to the idle speed screw. The number one and two reasons for tuning errors are improper linkage installations and an over tightened linkage nut, causing a binding in linkage assembly.
OK...Now that you have everything else sorted out, (Right? You did do all that other boring stuff didn't you?), you can actually start tuning the carb. The good news is that you probably made your truck run much better by doing those other steps that there is not a lot else to do. If your truck is still not running properly, or you feel that you should have more power, (Be realistic about that. A stock 4 cylinder Isuzu G180 1.8 with a even with a Weber installed and set up properly is only going to make about 80-100 HP max - so don't expect miracles.), then it is time to jet and tune the carb.
One of the keys to tuning a carburetor is learning how to read color on a spark plug. The spark plug will give you a basic idea of what kind of fuel/air mixture ratio is being inhaled into the motor. The part that you need to read on the plug is the insulator cone - the white part (when new) that surrounds the center electrode (where the spark jumps from) - not the outer metal ring or any other part for that matter.
You need to establish if you are running lean or rich. There is no way around this. There are a few easy ways to establish this and it is important to know otherwise you can not get different jets to cure the problem. You can be running rich at idle and lean on the main circuit and vice versa, and that is why you need to do a plug check. Unless you have access to a 3 Gas or 4 Gas analyzer or Smog Machine to do your tuning, then reading the plugs is the next best thing. White plugs are lean and black plugs are rich. Ideally, you want the ceramic part of the plug a nice dark tan-to-medium brown color with a slightly darker ring right at the base of the threads of the plug. And BTW...a new set of plugs may take a few minutes to get some decent readable color on them.
Some fuel additives may make plugs tint a slightly different color, maybe a bit more orange. But for the most part you want to get it somewhere between the two extremes. Also, make sure the car just ran at the rpm you want it to run best before you read the plug. Do not read the plug after sitting at idle speed for ten minutes or so, unless you are just tuning for the idle. The best way to get an accurate read at say four or five thousand rpm is to take the car out for a run. Get it to the rpm you want (to run strong) in the gear you want to be in. When you reach that point, quickly turn the ignition off and coast over to the side of the road. And no - you do not want to do this on a major busy highway! (Duh....lol.)
The plug will be very hot, so take gloves if necessary. Read the plug and take notes. Always make notes of all the adjustments you made and what the results were, because the combinations are endless, and you can not remember everything. I personally use Autolite plugs non-resistor type for this as they last and read well. I stay away from platinum plugs because they actually reduce the voltage of the spark. Platinum plugs are for people who want to get the most mileage out of a plug between changes. They will last longer but they do not necessarily give peak performance for a high-energy racing ignition.
That all being said and done...let's just get on with it, shall we?
There are a lot of theories about setting idle mixture adjustment on a Weber carb to determine rich or lean jetting. A Weber will run correctly with the mixture screws from 1/2 turn to 3 turns out, so you can not say that because it is 1/2 turn out that you need smaller jets. You need to find what is right for your particular engine. For example: an engine with a strong vacuum signal will draw more fuel in with less turns of the screw than an engine with a weak vacuum signal will. This is the kind of thing that throws the "exact certain number of turns" theory right out the window.
The mixture screw and idle circuit is CRITICAL to the overall driveability of your truck. It does not just control idle - but the entire low speed running and part throttle transition. The mixture screw lets in an ALREADY MIXED volume of fuel and air to the engine. This is not an Air Screw. The more you open it, the more mixed fuel and air enters the engine. Clockwise is LEANER and Counter-Clockwise is RICHER.
Set the idle speed screw by backing out the idle speed screw until it is not in contact with the throttle stop lever. Cycle or snap the linkage again to be sure that the linkage comes to close without any assistance. (Checking for linkage bind) Now bring screw back into contact with the lever and continue to open or screwing in, no more than 1/2 turn in, maximum.
Set the mixture screws by turning each screw in until it lightly seats. DO NOT FORCE OR BIND AS THIS WILL CAUSE DAMAGE TO THE SCREW AND IT’S SEAT IN THE BODY OF CARBURETOR. Now back out the screw 1 1/2 full turns.
Start the engine and let it warm up, the engine will run very slowly more like a tractor. As long as the engine stays running, idle speed is not important at this point.
Remember: DO NOT set “up” the idle speed, set the idle mixture screw to lean best idle setting. First, turn in the mixture screw until the engine dies or runs worse, then back out the screw (I recommend turning it 1/4 turn at a time). The engine should pick up speed and begin to smooth out. Back out 1/4 turn more, or until the screw does nothing or runs worse then turn back to the point where it ran its best.
USE YOUR OWN TWO EARS, not a scope or tuning instruments at this point. You want to tune the engine by sound. Adjust it to it's best, fastest and smoothest running point.
IMPORTANT: If the mixture screw is more than 1 1/2 turns out then the idle jet is too lean (too small). When the mixture screw is ½ turn or less out, then the Idle jet is too rich (too large). These assumptions are based on the fact that the idle speed screw setting is not opened more than 1/2 turn in. If the speed screw has to be opened more, then this is also an indication of a lean condition usually requiring greater change. At times it may appear to be showing signs of richness or flooding - it is really a lean condition. Please understand the need to keep throttle plate as near to closed as possible so as not to prematurely expose the transition holes. This is what causes the visible rich condition, and confirms the need to increase the jet size.
(For example: With the idle speed screw set at no more than 1/2 turn in after contact with the idle stop lever, and the best idle occurring with the idle mixture screw set at 1 1/2 turns from lightly seating, this indicates the need for a larger Idle jet. Achieving the best idle at less than 1/2 turn indicates the need for a smaller idle jet.)
Turn both mixture screws in until the idle starts to stumble and gets rough. Do BOTH OF THEM a little at a time, and then back them out until the best idle quality is acheived. This is a very simple operation. The engine should idle well and small adjustments leaner (turning them in) should make the idle drop off. Opening the screws more should make it a bit richer, but it should still idle. Assuming you have no vacuum leaks this is a very simple process. If you can not get a good adjustment on these screws and you have to open the idle speed screws quite a bit to get the engine to idle, then there is a good chance you have a vacuum leak and you need to fix it before you go any further.
Again, if you have to open the mixture screw more than 2 turns on a 38 DGES your idle jets are probably too small. Simply put…the bigger the number the richer the jet. A 50 Idle jet is a .5mm fuel hole. If you have to shut them below 1/2 turn they are too big. Also, the 38 DGES can be a little tricky because you are always idling on both barrels at the same time.
You have 2 mixture screws and they will not be set the same on most carbs. This is because the typical POS stock Isuzu plenum type manifold that they are on distributes fuel unevenly. By having 2 mixture screws you are delivering fuel from 2 places in the intake manifold. The mixture screw closest to the engine will no doubt need to be in much farther than the outer mixture screw. This is OK.
I DO NOT recommend stagger jetting the idle circuit on a 38 DGES In other words, DO NOT use 2 different size idle jets - even though you need to adjust the screws differently. This can cause part-throttle driveability issues.
These carbs have fixed venturis so the choke size is not an issue. The pump jets are also not a problem. Do not mess with them. If you have a flat spot or hesitation when you first take off - then it is likely that the idle circuit is too lean.
If you know you have the idle circuit right, then the main jet is too small. Increase the main jet size 2 steps at a time. 150 main is a 1.5mm hole. Main jets are in steps of 5. eg: 150, 155,160,etc. Keep checking the plugs after driving it for a few minutes. DO NOT let it idle when checking the spark plug color for the main jets.
The bigger the idle jet number - the richer the jet. 1 step in idle jet size can make a HUGE difference. DO NOT EVER go up or down more than 1 step at a time when tuning the idle circuit. Idle jets are in 1/2 steps...50,55,60,65 etc.
Now that the mixture screw is at its best running location, you can adjust the idle speed by it's screw. The idle speed screw will be sensitive and should only take ¼ turn to achieve the idle speed you like.
Check and set the idle to your driving preference. Apply a slight load, (turn on your lights, heater fan, stereo, wipers, etc. all on) and set the idle as you like it. Don’t set it too high, as this will cause causes excessive clutch and brake wear.(Not to mention it can lead to over-heating and/or cost you gas mileage.) The idle only needs to be 800 to 900 RPM with a load on it.
Now go drive your truck.. It should come off of idle good and transition to the main circuit smoothly. If it falls on its face and will not take any throttle or runs better if you back off the throttle, then you could have a few different other problems.
Next re-check your ignition timing and vacuum hook ups just to be safe and insure all is well.. Re-check your mixture screw to lean best idle again. If it all is still giving you your engine's best and smoothest idle, then confirm and WRITE DOWN the final settings.
To confirm or check your carb's settings in the future, with the engine running and warmed up, start by screwing in the mixture screw and count the number of turns it takes to bottom out and note if the engine dies. If the idle mixture screws are still with in ½ turn of base line setting, then all is well. Also check the idle speed screw and note how many total turns from initial contact. You may have opened (turned in) the idle speed screw. Your final setting should be under a 3/4 turn in. Re-set the screws (back in) to the best final settings (from your settings you wrote down before) and go on a short test drive. If the settings are other than I just described, then you may want to still re-calibrate the idle circuit (low speed circuit) better to your engine's specific needs. If not - congratulations! You are good to go!
Good job! Now go enjoy driving your truck!!
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