J'ai longtemps hésité à poster ce sujet parce qu'il a été évoqué souvent, de plus il y a un calculateur et un article sur les doubles carbus.
Malgré la lecture de ceux ci et la plupart des sujets prépa, je me pose encore bcp de questions,...
Je viens de terminer le montage de mon premier moteur à double carbu, la config. est la suivante;
bloc AB
villo origine
arbre à came origine
culasse origine passée en 1600cc, soupapes origine rodées, ressorts berg (les seuls que j'ai trouvé en bonne qualité), rampe de culbu renforcée scat.
Kit cylindrée 87mm, 1641cc.
Pipes 40mm Customspeedparts, conduits alignés et polis.
Carburateurs neufs weber idf 40mm.
009
allumage ignitor pertronics et bobine haut rendement pertronics
gicleur essence 130
gicleur air 200
gicleur ralenti 50
buses 28mm
J'ai monté des gicleurs essence de 130, le moulin tourne assez correctement, deux problèmes cependant, il y a un trou à l'accélération, à peine perceptible en conduite coulée, plus marqué en conduite "moyenne" et à peine perceptible en ouvrant grand.
Mes bougies sont noires de suies, l'échappement aussi et le moteur fume au démarrage (même chaud) et quand j'accéllère (moins facile à voir).
Je consomme pas mal aussi, 12L/100 en roulant assez calmement tout de même.
Donc je me dis trop riche, j'ai remonté les gicleurs de 115 monté d'origine sur mes 40, une vrai cata, là il y a vraiment un énorme gouffre, le moteur crève littéralement, à 80km en 4ième j'ai l'impression qu'il s'étouffe complètement.
Ne pensez vous pas qu'il doit être possible d'obtenir de meilleurs résultats, un peu moins de conso et un meilleur mélange avec de belles bougies marron chocolat / clair?
J'ai utilisé le calculateur de flat4 (régime maxi 4500rpm), qui me donne pour le gicleur essence mini 115 et maxi 135.
Correction d'air mini 145 maxi 160.
Ce que je trouve curieux avec ce calculateur c'est qu'il ne tient pas compte du diamètre des soupapes, levée,... et que viens faire le régime maxi là dedans? Ce qui m'intéresse c'est d'avoir une bonne carburation entre 2500 et 3500 tours... + étant exeptionnel.
Mais ni a t'il pas un lien direct entre le gicleur d'essence principal et le gicleur d'air?
Certains me disent qu'en diminuant le gicleur d'air on enrichit à haut régime, hors il me semble justement avoir un manque d'essence au moment de la transition de gicleur de ralenti vers le principal.
Je m'éloigne de plus en plus des données du calculateur Flat4,... faut il adopter les valeurs mini ou maxi? sachant que je roule tous les jours et préfère un peu moins de perfos et moins de conso,... qu'en est il du gicleur de ralenti? des tubes d'émulsion?
Quel est votre sentiment?
Salut Pedrini67
pour mon expérience le trou que tu trouves là c'est un problème de gigleur de reprise. Je ne sais pas la provenance de tes carbus mais je te conseille de faire cette verif. Pour que l'accélération n'aie pas de trous, les gigleurs de reprise doivent avoir une taille de 50.
La config doubles carbus d'un 1600 cc. d'origine d'un ami
IDF 40 mm.
Buses 28
Reprise 50
Air 200
Ralenti 55
Essence 115.
ça roule très bien sans trous et avec des bougies chocolat.
Komi8
Le seulk hic pour moi est que je n'ai jamais entendu parler de gicleurs de pompe de reprise... où se trouvent ces gicleurs?
Je n'ai de disponible dans le commerce que les gicleurs suivants:
Idle Jet Weber de 040 à 090
Main Jet Weber de 105 à 200
Air Correction Jet Weber de 140 à 245
Emulsion tube Weber F2, F7, F11, F16, F34.
Salut Ivan
pour comprendre mieux tes carbus je te conseille la lecture à la page http://www.flat4ever.com/modules/news/article.php?storyid=161. La tu trouveras aussi une vue éclaté des IDF. Les gigleurs de reprises sont ceux avec le nunéro 16 (on le voi assez mal mais tu peux le trouver au dessous du n. 14 cerclé). Si tu n'as pas la bonne taille (50) tu peux les faires aléser par un prof. Les tubes d'émulsions conseillés: F11.
Voilà!
@+
Komi8
Ce que je trouve curieux avec ce calculateur c'est qu'il ne tient pas compte du diamètre des soupapes, levée,... et que viens faire le régime maxi là dedans? Ce qui m'intéresse c'est d'avoir une bonne carburation entre 2500 et 3500 tours... + étant
Le régime "max" du calculateur est le régime de "puissance max". Il faut qu'on précise...
Autre question: Combien d'avance as-tu au total ?
__________________
Philippe LEVIEZ - Staff Flat4ever
http://blog.flat4ever.com/cox
J'ai vu à quoi ressemble le gicleur de pompe de reprise, on dirait une sorte "d'arrosoir" à l'envers. Je démonte ce week end pour vérifier si c'est bien du 50 et checker le tube d'émulsion.
Merci Pleviez pour cette importante précision, je peux donc supposer que la puissance max devrait être atteinte à un régime comparable à une configuration d'origine (entre 4000 et 4500rpm).
J'ai calé le 009 à 10° d'avance et vérifié l'avance totale à la lampe strobosco, 32° (soit la valeur maximum me dit on, les 009 variant de 28 à 32°, tolérances de fabrication).
Qu'en est t'il de l'influence des gicleurs d'air? Ceux ci permettent t'ils uniquement d'enrichir à haut régime. Comment interpréter les données maxi et mini du calculateur flat4?
Resalut à tous!
Quelques nouvelles sur le réglage des IDF pour ceux qui s'en préoccupent!
Je continue à faire des tests avec différentes combinaisons de gicleurs essence et de correction d'air.
La meilleure combinaison que j'ai trouvé pour l'instant est #130 pour l'essence et #180 en correction d'air.
Je continue à trouver ca étrange parce que je suis supposé utiliser plus petit, j'ai trouvé la règle suivante:
gicleur essence = diametre de buse x 4.1 à 4.3
ce qui donne:
28 x 4.1 = 115
28 x 4.3 = 120
Pourquoi diable est ce que mon moteur refuse de tourner avec du #115 malgré des éssais avec une correction d'air de #160?
Je me suis aussi livré à d'autres tests en ce qui concerne le gicleur de ralenti et suite à la lecture du catalogue général weber sur les conseils de Tof (merci à lui).
Et j'arrive à la conclusion que je suis trop riche au ralenti et jusqu'à 2500~3000 tours pour les raisons suivantes:
- mon moteur fume au démarrage (même chaud), quand j'accélère et que je conduit la cox j'ai plus de mal à voir si ca fume ou pas :'(
- j'ai nettoyé mes bougies et fait un tour du bloc de 3/4km en restant sous la barre des 2500 tours, vérification des bougies, elles sont noires de suies!
- j'ai mis de l'essence dans un flacon de parfum :-) et vaporisé dans le corps du carbu au ralenti et le moteur perd des tours (il en gagnerait si il était pauvre) même test à 2000 tours, il perd de nouveau des tours.
- j'ai bloqué la tringlerie à 2000 tours et refermé la vis de réglage de richese, le moteur gagne des tours! Je suis trop riche.
Je vais tenter des gicleurs de ralenti de #40 (les plus petits dispo) en lieu et place des #50 actuels, on verra bien.
Ce qui me fait hésiter est l'extrait ci dessous que j'ai trouvé sur
www.redlineweber.com
et concernant tout particulièrement le tuning des IDF:
“Simple Rules for Calibration”
If your mixture screw is out more than one turn like 11/2 turns then your idle jet is too lean, go up one half size on the Idle jet.
If you mixture screw is not out one full turn, something like only 1/2 turn out from seated then your Idle jet is too rich, go down one half size on the idle jet.
This is all based on the important fact that your speed screws are not open more than ½ turn if they are then that is also an indication that you have a lean Idle circuit. You are cheating by opening the throttle plates and exposing additional progression holes in the transition.
Je ne connais pas votre niveau d'anglais,.... en gros, mes vis de richesse sont ouvertes de 2 tours et d'après le texte weber, le bon réglage est de 1 tour d'ouverture, ce qui indique d'après eux que mon gicleur de ralenti est trop petit!!!! J'y comprends plus rien.
Vos conseils ou expériences, à ce stade, me seront précieux!
Pour ceux que ca interesse,... j'ai trouvé sur
www.aircooled.net
un article sur le choix des gicleurs, pour ceux qui comprennent l'anglais ;')
How to Jet your Carburetor(s)
People often go right ahead and spend $500+ for a set of carburetors, bolt them on, and seem "pleased" with the results. Others are upset by a backfiring, or smoke belching monster. In either case, spending the time to properly jet a set of dual (or single) carburetors is a very worthwhile task, since recieving a "perfect out-of-the-box" carburetors is one of the biggest myths in the VW industry!
For emissions and power, the engine has to have a certain amount of VAPORIZED fuel (liquid fuel doesn't burn) for a certain amount of air. The carburetor is designed to meter out a mixture of air and fuel in a form that can be burned quickly and completely by the engine. This is rarely done properly! For complete combustion, the air/fuel mixture must be supplied in a VAPOR, and not with liquid droplets (remember, liquid fuel won't burn). So, besides metering how much fuel the engine gets, the carburetor is also responsible to atomize the fuel and mix it with the air entering the engine.
In addition, proper jetting is different depending on the size engine, elevation, cam overlap, etc. I have even seen the SAME engine combination require different jetting (same carbs) on two different engines! So, for someone to say "this is what you need" is ridiculous. They can get you CLOSE, but not exact. To get exact you need to spend time and patience, and dial it in. It IS worth it too!
Carburetors are simply devices that meter fuel depending on the amount of air that is entering the engine. There are a couple problems though! Air and fuel have different viscosities, and since air and fuel do not flow the same, the metering of fuel is NOT LINEAR. What this means (if this didn't make sense to you) is that you can have a correct metering for a while, and then at certain RPMs it goes off. So, you have another set of jets to "correct" it and one more set to correct the correction!
Idle jets operate from idle to 2500-3000rpms or so. Main jets are primarily from 2500-4500rpms, and Air jets affect the engine from 3500/4000 on up. Lastly, you have emulsion tubes to change "where" it's rich, lean, etc. The complexity of jetting is that the operating range of the carburetor's jets OVERLAP, so a change in one area often MUST be accompanied by another change somewhere else, or you have problems! It makes it seem very complicated if you start at the middle. It also makes it imperative to start at the right point (our beginning), and then it's a straightforward process and will yield top-notch results!
I'm not going to bore you with too many details on carb theory (I've done enough boring you already). What I am going to spend time on now is what YOU can do to optimize what you have!
One last check: you MUST have your engine in top mechanical shape (no leaky valves, etc), no vacuum leaks, and a good ignition system (no fouled plugs) or you are simply wasting your time messing with the carbs. Read our How to Synchronize your Dual Carburetors too, before doing jetting!
JETTING PROCEDURE
Sizes: One "size" change on an idle or main jet is "5". So to go richer one size from a 55 idle jet, we would choose a 60. When I say "one size" on the air jet, it means 15-20 if you look at the numbers! So, one "change" in air jet size would be from 200 to a 180 or 220.
First, we need to figure out if you have the proper sized carburetor to begin with! Bigger is NOT necessarily better. Carburetors come "out of the box" with specific venturi (also called a choke tube) sizes depending on the size carb BODY you have. Example: 40mm (signifying a 40mm BODY) Dellorto carbs usually come with a 34mm venturi. The venturi is stamped with a number, and it's the narrowest channel in the body of the carb. You can see the number on it (it's usually even) if you look down the top of the carb throat. Ideally, you would use a chart to find out if the venturi size is correct. It's dependent on max RPMs, and engine displacement. I'll leave this topic here, since venturi changing is not cheap. If your engine (after jetting) has no power at the top end ("lays down"), or has no power until 2500 RPMs (no low end), there is a good chance your carbs are too small or too big for your application (respectively). If this is the case, then you should consider changing venturi sizes or go to larger/smaller carbs!
Why Jet? If it's too rich, the extra fuel will wash the fuel off your cylinders and wear out your pistons, cylinders, and rings quickly (no oil), in addition to building up deposits on your valves, heads, and piston tops. If it's too lean, the engine will run very hot, misfire, and can eventually lead to a burnt valve(s). Get it right!
Idle circuit: The idle circuit is the device that controls how much fuel you have at idle and it also sends fuel to the progression circuits. Where many people run into problems is that they don't understand that the IDLE JET IS THE CIRCUIT THAT THEY DO MOST OF THEIR DRIVING ON. So, this jet is critical in engine flexibility and economy. The idle jet affects engine operation up to about 2500-3000 RPMs!
Almost every dual carb application I have seen with VW engines use a 55-60 idle jet. Center-mounted carbs need a RICHER (larger) idle jet, since the carb is so far from the cylinders and the fuel condenses on the way there (keep your manifold pre-heater).
What you want to do is this: get the engine fully warmed up. Now, with synchronized carbs (remember?), turn the idle mixture screw on one cylinder in until you hear the engine misfire. It means THAT CYLINDER is too lean, and it doesn't have enough gas to run properly. Turn the mixture screw back out until the engine idle smoothes out again, and then go out another ½-1 full turn. Repeat for the other 3 cylinders.
Idle jets ARE available in small increments. Normally, shops stock them in round sizes of "5"s, like 50, 55, 60, etc. You can find 57s, etc if you look around to REALLY get it right on! If you have a significantly larger idle jet (48 IDAs usually come with a 70!), I recommend going down until you can NOT get a good engine response with the mixture screws anymore. Idle jet sizes of 70 or so will "run" on anything, but they don't run WELL (unless fouled plugs or 12mpg are your idea of "well"). In addition, if you have a problem with a 55/60 idle jet, and you have the right main jet, and STILL have a hesitation regardless of accellerator pump adjustment, you need to play with your idle air bleed (what the idle jet fuel mixes with)! This is a much-needed modification to 48 IDAs on the VW street engine, and is beyond the scope of this article, and you best leave that modification to an expert. Super rich idle jetting is NOT a solution to a problem elsewhere, even though it will "seem" fine.
Once you find the idle jet size (around 55-60), you are done there! They are easy.
Main Jets: The next jet to dial in is your main jet. As a starting point, choose a main jet size about 4.1 - 4.3 X Venturi Size (mm). Simple! We are only trying to get in the ballpark! If you live at a high elevation, go down one size ("5") from what you come up with. If I have a 34 venturi, my main jet size should be around 34 X 4.2 = 142.8. A 140-145 is where I want to be. It's usually best to start rich and go leaner as we learn, so we'll choose a 145. 4-5000 foot elevation would choose 140 (one size down).
Now, you need to DRIVE the car. Drive it hard! You really have to pay attention to how the engine is running (turn off the radio).
Spitting/sputtering/cracking is an indication of being too LEAN. Now, what RPMs that it's doing this tells you which jet you need to change! 2000 RPMs and lower, and you need to richen up your idle jet (go bigger). 2500-4000 RPMs and you need to richen up your main jet!
Do NOT confuse accelerator pump adjustments with main jet adjustments! If the engine stumbles when you open the throttle, it could be a pump adjustment, but it could also mean a main jet change. Pay attention to STEADY throttle position when testing in these areas! It's easy to confuse the two. If the main jet doesn't solve it, try the pump. If it runs good at 3000-4000 rpms and stumbles elsewhere, your main is right and the problem is somewhere else. Try to do main jet testing from 1000 RPMs up to 4000 or so. This eliminates the accelerator pump issue from your testing!
If the engine pulls "OK", but doesn't sound CRISP (it feels "heavy"), it's too rich! Try to go DOWN on your main jet one size, and try again. They are very easy to change, and this is one of the beauties of Weber/Dellorto carburetors! Once you get used to it, it goes very quickly! Whatever you do, do NOT drop a jet down the carb throat, or you have to remove the carb or fish it out (they are brass, so a magnet is of no help!). To get the engine EXACTLY right, I recommend going down on your main jets until it sputters/spits, and that means it's too lean! Go back up on the mains until it smoothes out, and you are done on the main jets!
Air Corrector Jet: The air jet jets AIR, not fuel! So, if you want to lean it out, you go BIGGER on the air jet, and smaller on the air jet if you want to richen it up! The air jet affects the engine from about 4000rpms up. You really have to wind it out to use this one, so be careful! Start with a baseline jet of 200. If it starts spitting and sputtering at high rpms, you need to richen it up, so change it to a 180 air jet. You are following the same procedure as the main jet, but your "operating window" is now 4000 RPMs up instead of the main jet's 2500-4500. I have seen air jets in the range of 160-240. If you are out of this range, you need to go back to the 200 air jet, and change emulsion tubes.
Emulsion Tubes: The Emulsion Tube mixes air from the air corrector jet with fuel from the main jet, and controls the mixture from the discharge nozzle at all engine speeds that the main jet is working! Basically, the Emulsion Tube changes the diameter and location of small holes in it's body. For Weber IDF carbs, the F7 or F11 is usually what you need. The F "series" has no rhyme nor reason to the numbering system, so you can't tell anything by the number unless you look it up! But, with these two examples, the F7 richens the mixture up at the bottom of the RPM range and under acceleration, while the F11 leans the mixture out on the top end a little bit, and is usually used (high RPMs) when you use air jets larger than 200.
Accelerator Pump: Remember from the beginning of this article, that air and fuel have different viscosities. They also have different densities! Since fuel is much heavier than the same "amount" of air, when you open/close the throttle, the air will change speed MUCH faster than the fuel will (think of a heavy car vs. a light one). When you open the throttle quickly, the air INSTANTLY speeds up to fill the engine. The fuel responds TO the air volume, so it has to react. In addition, it's heavier so it can't change "speed' as quickly. The result is that the engine has a lot of air, and NOT ENOUGH FUEL. It leans out! So, almost all carburetors have an accellerator pump that adds fuel when you open the throttle. This gives the main jet time to start metering the right amount of fuel to the engine.
We want as little accellerator pump squirt as possible to remove the stumble from the engine. Too much gas as it's washing our oil off the cylinder walls (wearing out our pistons and rings) as well as lowering our MPG. Basically, turning the pump screw in adds more fuel shot to the engine as the throttle is opened up. Be conservative!
Hopefully you now have tools and confidence to jet your carburetors, and can stop listening to guys who are only tuning carbs by doing what everyone else is doing, instead of LISTENING TO WHAT THEIR ENGINE NEEDS.
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