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Thread: Stuck acceleration after adjusting Twin Hitachi HJG 46W Carburetors on Datsun 240z

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    Default Stuck acceleration after adjusting Twin Hitachi HJG 46W Carburetors on Datsun 240z

    Finally managed to adjust my twin Hitachi HJG 46W. I did the balance with the unisys synchronizer and the ball was in the bottom level in both carbs. If you accelerate the ball rises. So its fine. However after setting the idle speed to 1,000 RPM and driving the car a couple of blocks i noticed that the car mantained the RPM between 1,500 to 2,000 and did not lower to 1,000 on traffic lights or stops. The mechanic says that the problem is the carburetor springs and says i need to cut them. I dont share his opinión. THere must be something wrong in the tune up.

    Any advice?

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    Engine off, work the carb linkage by hand, It should feel smooth thru the entire range. Idle then WOT then idle. Push the linkage towards idle is there further movement? If so look for the cause. Lube all pivot points, & try it again. During the fast idle try lifting the pedal to see if the idle drops. I highly recommend ZTherapy's Just SUs DVD. It's the Gospel on SUs. As far as the Uni-syn goes I adjust the U-S(the center of the U-S screws in & out) so the ball is close to centered first, then I check the carb balance. I think you'll need to recheck the carb tuning.The only reason that I can think of to clip the springs is if you wanted more pedal pressure.

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    Rising ball in unisyn is not the technique for balancing.

    Here is how to do it:
    1. Get motor hot.
    2. Set idle to what you want (say 800 rpm)
    3. Briefly stick the unisyn up to one carb mouth (with air box off)
    4. Note where the ball (red plug) is.
    5. Adjust the opening in the unisyn (knurled knob)
    6. Repeat steps 3 to 6 until you get the ball near or above the middle (I like to have the red plug so the top aligns with a line in the clear tube,,,(see photo below where person has the bottom of the plug aligned).
    7. Now move to the other carb with the unisyn and check the air flow.
    8. Adjust the idle stop (air) into the second carb so the ball level matches the same level as carb 1.

    Name:  using unisyn.jpg
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    Tips:

    - Hand blip the throttle often and let the car settle before making a measurement.

    - start with jets down 2.5 turns from top most position.

    - start testing the idle air flow first. To do this disconnect the centre/short ball and socket link/rod of the linkage so that each carb is only resting on its idle stop screw.

    - before starting the procedure, adjust idle stop screws one at a time so that they visually just kiss (start to touch and move) the throttle valve, then give each 2 full turns as a starting point.

    - once the idle is balanced, reattach the linkage then turn the screw with spring that is up high on the linkage so that it opens both throttles to ~ 3000 rpm then adjust the unisyn to measure the higher airflow (knurled knob). Measure the front carb then match the rear by adjusting the off-idle balancing screw that is in the bar linking the carbs at the throttle body.

    - Once the idle and 3000 rpm air flows are balanced, have a look at the plugs then adjust the jet height to get good colour in each side (1,2,3 are coloured by front carb and 4,5,6 by rear). after adjusting jet, repeat steps 3 to this one,
    Last edited by Blue; 06-07-2014 at 10:04 AM.
    Mark Maras and metalmonkey47 like this.
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    Finally i decided to clip both of the return springs attached between the front carburetor and chassis / rear carburetor and chassis. I believe they were already worn due to their age and use. Now the accelerator is not stuck. Where can i purchase this return springs online? Does Black Dragon carries them?

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    Nova Scotia,Canada,Earth Blue's Avatar
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    You can probably find substitutes as a good hardware store.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


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