Dipl.-Ing Jürgen Stockmar heeft posities bekleed als hoofd research & development bij onder meer Audi AG, Steyr-Puch, Magna en Adam Opel AG. Hij geeft nog altijd boeiende lezingen en heeft in 2009 in Wenen tijdens de TüV Austria Forum een verhandeling over voorstuwing gehouden en daarbij inhoudelijk veel verteld over inlaat-ontwikkelingen. Het was tijdens deze sessie dat ook Kaimann-constructeur Kurt Bergmann in het zonnetje werd gezet naar aanleiding van Bergmanns 80e verjaardag. "Master" Kurt was in de jaren '60 en '70 naast Formule Vee constructeur ook consultant bij VW Motorsport. Eind 1970 verscheen in het officiële orgaan van Formule Vee Europa een artikel over carburateurafstellingen. Stockmar is de auteur van dit artikel. Onderstaand het (nog) niet vertaalde artikel van 1970.
The regulations for FV and FSV clearly define limits for engine modifications so that good engines only slightly differ in their performance. For the spectators sport-events with cars of well-balanced performance are very interesting, as hereby thrilling fights will develop. Every driver would, however, like to have a few more HP 's available than his competitors. In their effort to obtain the maximum performance the engine tuners do almost anything within their means. However, frequently one possibility to get the maximum output is not completely exploited: the exact carburettor setting.
Every engine will give it's maximum power at a certain fuel/air ratio of the cumbustable mixture (according to the fuel approximately 1 kg fuel must be vaporised in 13 kg of air). We can clearly see that an incorrect carburettor setting may result in a power-loss of more than 10 percent. In order to get possible high engine output over the entire engine-speed range, it is well worth our while to more closely consider a precise carburettor-setting.
The basis for a flawless functioning carburettor is primarely the supply of a sufficient quantity of fuel. Many FV drivers are familiar with the effect that on long straight runs suddenly the engine output drops off, only to recover if for a short period the accelerator pedal is released. This fact clearly points to an insufficient fuel supply by the fuel pump, as the carburettor uses more fuel in full load than the fuel pump can supply. In such cases a new fuel pump or an overhaul of the existing pump will overcome the problem. The correct fuel supply of the carburettor further depends on the correct setting of the fuel level in the float chamber. For setting the fuel level, the standard value may be adhered to, whereby, however, the lower tolerance should be utilised. The level is measured by means of a slide caliper rule with float installed from the upper jointing face of the float chamber. For the Solex carburettors 28 PCI and 30 PICT-2 (FV 1300) the float chamber cover must be removed when the fuel level is measured. This is not necessary for the Solex 40 PII-4 carburettor, as recommended for FSV, as the level can quickly be checked by removing a screw plug in the float chamber. The fuel level may be corrected by washers of various thickness below the float needle valve. With a thick washer the level is lowered, while the use of a thin(ner) washer leads to an increase of the fuel level.
For the engine performance besides the selection of the correct jets the size of the choke tube is a decising factor. For the more recent VW-carburettor 30 PICT-2 the carburettor body may be turned out to a diameter of 25.5 mm without any difficulties. Beyondthis, there is the risk of breaking the wall at the throat section. The largest obtainable diameter should be approximately 26 mm for this carburettor model. A somewhat larger diameter of the choke tube may be obtained by the old carburettor version 28 PCI which is now, however, very rarely installed. For engines of FSV we recommend for the Solex PII-4 choke tubes with a diameter of 34 mm. For engines, which are to obtain peak-performance, choke tubes with 36 mm should be chosen. Additionally, the air flow through the carburettor may be increased by machining the parts protuding into the airstream as diffuser, throttle spindle etc. In principle, hereby attention should be paid to the fact that the strength of the parts is not reduced to such a degree that they may break off by engine vibrations.
For the main driving range of a racing car the most important jets are the main jet and the air correction jet. The main jet may greatly influence the fuel/air mixture over the entire engine speed range. Large main jets result of course in a rich mixture, while smaller main jets have have a leaning effect. The effect of the air-correction jet is particularly pronounced in the higher rpm-range. With smaller air-correction jets the mixture at high rpm cab be substantially enriched, whereas larger air-correction jets will lean the mixture. Before modifications are made to the accelerator pump and enrichment system in any case the main jet system should be set, so that the performance over the entire engine speed is optimum with the existing possibilities. When an engine stand for carburettor setting is available, the setting of the carburettor is very simple, as after each modification of the jets it may be immediately be determined whether a a positive or negative influence on the performance results is the case. If the jet setting has to be determined in a driving test, greatly incorrect jets may be determined as follows: An excessively lean mixture results in shock-like stalling of the engine, which is similar in its effect to faulty functioning of the ignition system. Furthermore, engines with excessively lean setting respond only very sluggishly to accelerations and also in the lower gears they do not pick up revs. With such a lean setting the exhaust pipe will not show any colour, the insulator remains white. As particularly in air-cooled engines an excessively lean setting may damage the engine, we should always try to begin with an over rich mixture, when setting the carburettor, which is then progressively leaned till the maximum performance is obtained.
An extremely rich mixture gives the impression of a sluggish engine with a heavy engine sound. If the mixture is much too rich, the performance breaks down when the throttle butterfly is in the wide open position. Often the engine runs noticeably better, when the throttle butterfly is slowly returned to part load position. In addition to this, of course, a sooty exhaust and black spark plugs point to an excessively rich mixture. In this connection it should be noted that also with a correct carburettor setting the exhaust end pipe may often have a black colour. An evaluation of the appereance of the spark plug is only possible if a longer distance has been driven with the same load position and the engine is immediately stopped. Particularly when increasing the choke tube size, a faulty transient may occur. This is caused by the fact that at a certain throttle butterfly position a "flat spot" will be noted. The very lean mixture at this point may be enriched by increased the idle jet. Hereby the idle jet should not be larger than 10% of the initial value, as beyond this point a constant idle may be very difficult to adjust or even it may be impossible to do so. If the flat spot cannot be eliminated by means of the idle jet alone, a somewhat larger main jet may additionally be employed to improve the response of the main jet. Should, owing to the increased main jet, the carburettor setting be too rich at full load, it should be brought to the approximate optimum by installing a larger air correction jet. Only if no correct transient may be obtained with the idle and main jet systems, the injection volume of the accelerator pump may be increased. In some cases the calibration of the injector tube must also be increased.
Modifications to the enrichment system should only be performed if an engine stand is available, as only here the influence of this system may be clearly determined. Only if with the main jet and the air correction jet the mixture, which is correct over a wide rpm-range for maximum performance, is too lean in the top range, the calibration of the discharge of the enrichment system may be increased. In most cases the standard enrichment will prove to be sufficient. Lik any mixture formation for Otto-engines (four-stroke engines) also the carburettors for racing cars supply a slightly differing mixture according tot he temperature of the intake air and the ambient air pressure. With top performance engines this minor variations of the fuel\air ratio are sufficient to noticeably influence the engine performance. In this case a correction of the main jet according to the weather conditions is necessary. Exact particulars may be obtained by request from the Deutsche Vergaser Gesellschaft, Sportabteilung, D-4040 Neuss, Leuschstrasse 1. This is very important for racing in higher altitudes.
Frequently, drivers run tests with various emulsion tubes for FV engines. The hereby obtainable modifications are so slight that it is recommendable to use standard emulsion tubes. In general, it is a good advice not to change a once found good carburettor setting for each race, hoping to come to still better results. Also recommended jet settings from other engines do not necessarily lead to better results as particularly the carburettor setting for racing engines has to be individually adapted. Owing to this reason we cannot give any general recommendations for jet sizes, as according to te engine the optimum sizes vary so greatly that nobody would be helped by them
This article is not complete yet. More will follow soon (september 2010)
