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Forum Vet
Wiki Edit 1
Threads 46
Posts 1492
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| Fri 27. Oct. 2017, 14:03:19 |
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Bernhard;
...weitere Komplimente fuer Deine Studie!
Ob sie von der fruehen oder spaeten Ausfuehrung sind, sieht mann auch gleich an der Zahl LuFiloecher...
Ich diskutiere dieses Thema auch zZ mit meinem SU Expert hierzulande [Tom Bryant, der schon die (wohlgemerkt: erfolgreiche!) Restaurierung von hunderten von SUs hinter sich hat. Siehe: https://thosbryant.wordpress.com/2014/01/02/su-carburetor-rebuilding-hs-6 ]. Als erstes hat er zugestimmt, das zuloeten von dem Overrun Valve keinen bemerkbaren Flussunterschied ergeben wuerde, dann hat er hat auch auf die von Dir angezeigten Unterschiede mit dem Vakuumanschluss gedeutet...aber auch noch dazu meint er das mann die DKlappen NIE und NICHT mit dem anderen Styl tauschen sollte, weil dann die Position und ansprechverhalten des "Fast-Idle-Cam" nicht mehr stimmt und richtig einstellbar ist!...und dies ist warscheinlich das wichtigste!
Seine ausfuehrlichen Anmerkungen zum Thema (mit Erlaubnis wiedergegeben):
Hi, Ron.
Yes, I'm well aware of the difference in throttle plates. I frequently rebuild carburetors where the previous rebuilder screwed up and installed incorrect throttle plates, and I have to replace them with the correct ones.
Basically, the difference is the angle at which the throttle plate sits in the bore of the carburetor. The later carbs have a nearly round throttle plate that sits closer to the perpendicular than the throttle plates in the earlier carbs, which have a more oval throttle plate. The the difference in angles of the edge is just a reflection of the difference in the angle at which the throttle plate sits.
AFAIK, *all* "two-hole" Volvo SUs use the more oval throttle plate, that is, unless someone changed them, and I have encountered some that had been changed. In fact, I just mailed a two-hole pair back on Monday where one of the pair had the proper throttle plate, and one had the later (more nearly round) throttle plate. Of course, I had to replace that throttle plate before the carb could ever be expected to work properly.
In addition to the difference in throttle plates, the throttle shaft hardware (the thing with the fast idle screw in it, and which rests against the idle screw in the carburetor body) is also different to accommodate the differing angles of the throttle plates. If you put an earlier throttle plate in a later carb, the throttle shaft will be turned more toward the open position, and the idle screw will not be long enough to work properly. The fast idle screw will also be too short to work properly.
If, OTOH, you put a later type throttle plate in an earlier carb, the throttle plate cannot close completely because the throttle shaft hardware will fetch up on the carburetor body, right beneath the idle screw, so it will be impossible to get the idle speed down to where it belongs.
In addition, the early two-hole carbs, the ones that have a vacuum advance tap on the rear carburetor, cannot work properly with the later type throttle plate installed, even if the throttle shaft hardware is also changed. That's because the throttle plate will be too far away from the hole for the vacuum advance tap, which is supposed to be just *upstream* of the throttle plate at idle.
The "three-hole" carbs come in both types; earlier ones have the more oval throttle plates, and later ones have the nearly round ones. Typically, if the throttle shaft hardware includes a lever for the auxiliary throttle plates (in the intake manifold), then the throttle plates will be the later type, and 3-holers without the levers will have the earlier type throttle plates. But, this does not seem to be a hard-and-fast rule. Also, earlier type throttle plates generally do not have the poppet valves, whereas the later type throttle plates often do. Once again, however, this is not a hard-and-fast rule.
As for why SU (or Mother Volvo) chose to go from the more oval throttle plates to the more round ones, I don't know. But, I can speculate: Gasoline engines tend to have a "nasty" throttle response at low throttles, that is, a slight movement of the throttle plate off the idle position will often result in neck-snapping acceleration, or unpleasantness, at least. I personally find this quite annoying when switching from driving my Diesel cars to driving one of my gassers, or someone else's gasser. Now, just think about the implications of having a more nearly round throttle plate..... Slight movement of that throttle plate off the idle position will result in less of an opening, and therefore less air flow, than the same angular movement of the more oval throttle plate. The result will inevitably be a smoother throttle response when coming off idle. So, that's my theory, and I'm sticking to it.
Ende der Wiedergabe.
Dann haben wir auch den Grund dieser Aenderung durch Verstand der Auswirkung versucht zu Verstehen (Anderungen haben meisten ihren Grund...und es waer schoen diesen zu wissen!)...Tom hat, auf das Ansprechverhalten der Benzinmotoren gedeutet und wir beibe mussten darauf kommen, das die Klappen durch eine Cosinusform sich Bewegen. Sprich das soll heissen, das die (spaeteren) naeher zum 90 Grad Winkel stehende Klappen sich verhaeltnismaessig durch eine weniger linearen Region gleich nachdem LL bewegen, als die (frueheren). Dadrauf deutet der aufmerksame Kay auch...
Gruesse
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