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Tony Bal

Idle adjustment 425

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Hi Tony,

  I`m assuming you are dealing with a dual quad setup? The idle adjustment on the rear carb is the large brass screw in the front center of the rear carb....IF you have the original carb. This is an idle air bypass adjustment so instead of the primary butterflies being slightly opened by a typical idle adjustment screw on the throttle linkage the brass screw is opened until it bypasses enough air to satisfy the correct idle speed. Make sure BOTH the front and rear carb butterflies are completely closed. After you adjust the idle air volume go to the mixture screws and adjust those for highest idle speed and turn them in until a very slight drop in idel RPM is sensed. If you need more idle RPM go back to the brass screw and turn out to provide more air and repeat the mixture screw adjustment.

Tom Mooney

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1 hour ago, Tony Bal said:

Tom thank you for your help good info I do have a large brass screw with spring. Never would have guessed!

Tony

Hi Tony,

  Yes, the throttle plates should be fully seated (very often the front carb plates will hang open due to bearing surface/friction in the linkage trunnion so make sure they are seating properly) and all idle air consumption should be regulated by the brass screw. It is very common to find that someone has added a typical idle speed screw to the rear carb either because they dont understand the adjustment procedure or there exists some other factor, like a vacuum leak or incorrect ignition timing, which necessitates extra idle air volume to acheive an acceptable idle quality/RPM.

Tom Mooney

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Tony,

 

   I'll add a little more tech info to Tom M's post since I rebuild & re-bush these carbs. all the time for people all over the world.

1st. the dual quad (2x4)set-ups have ALL EIGHT throttle blades (front & rear carbs.) cut at a 15* angle. What this does is CLOSE OFF ALL THE AIR going into the engine past the throttle blades & is TOTALLY CUT OFF (again both front & rear carbs,) so the big screw in the center of the rear carb. controls ALL the air entering the engine, NOT FUEL.  Thru this big screw controls the idle speed by introducing a controlled vacuum leak. The reason every time you adjust the idle speed with the big screw then it adds more or less air in the idle position & the smaller mixture screws add more or less fuel & the reason you have to go back & forth with the adjustments. 

   A single four (1x4) has the front throttle blades are at 10* angle & the rear are cut at 15* because you DON'T want to add air thru the rear throttle blades.  IF you wanted to close the front throttle so NO air would pass, the engine would of course stall, BUT also at 10* the throttle blades would stick because of the throttle spring pulling the throttle blades too tightly closed the reason an idle control adjustment screw is used & seldom do you have to go back & forth between adjustments.

   Of course the throttle shafts in the body will cause a vacuum leak IF there is too much play between the body of the carb. & the throttle shafts. The reason bushings are installed is to remove the play in the body from the shafts resulting in a vacuum leak.  Another thing is the bases of both carbs. (same with a 1x4) are often warped because the carb. attaching bolts are tightened too much which causes the warping needing to be milled to produce a flat surface again.

   I can't tell you how many carbs. that have been messed with before I receive them had the front & rear venturi's are mis-matched, both primary & secondary, between front & rear carbs.  YES, they are different. As well as metering rods & jets, accelerator pumps not operating, rear secondary weighted flaps, etc. etc . etc.  This is just a few. There are MANY differences between front & rear carbs.

 

Tom T.

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