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Thread: reduced air flow in front carb

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    Default reduced air flow in front carb

    Hi everyone. i just recieved my unisyn and have started tuning my engine in preperation for appraisal, insurance and hopefully driving next week. 280 short block E31 head ported with large valves, mild cam. header and SU's. I put kits in the carbs, have set the float level with clear tubing. That alone made a huge difference. I can reach in thru the window and the car starts right up and idles at 800 rpm. The throttle response seem crisp and instant. The problem is when I set the unisny on the rear carb and adjust it so the ball is in the middle then I put it on the front carb it does not move unless I turn in the front idle speed screw and the rpm goes up to 2000. I have tried the carb cleaner around the shafts and gaskets with no change in speed. I have checked the throttle shafts for play and they have zero side to side movement. Am I doing some thing wrong? I have searched this forum and other su sites with no luck. Any help would be great
    Bob

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    Registered User LeonV's Avatar
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    I haven't done this in a while, so correct me if I'm wrong. I thought that you start with the front carb and then use the balance screw to set the rear at idle and then at about 2500 rpm. Start the procedure at the front, maybe that will help.

    Leon

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    Registered User Gary in NJ's Avatar
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    What it means is that the carbs aren't balanced or in sync. There are throttle adjustment screws (air screws) on both carbs. Simply adjust the front carb until it's flow is equal with the rear. A little turn makes a big difference. To get more air into the venturi, you'll want to turn the front screw counter-clockwise. You may find that this change requires that the idle be adjusted.

    Lean, the single balance screw is to set the balance at Fast Idle (like 3000 rpm).
    Gary
    Guardian of HLS30-91415
    Previous Owner of a 10/70 240Z ('83-'85)

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    Registered User Walter Moore's Avatar
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    What Gary means is that you need the synchronizing tool to set the idle speed. You have to adjust both carbs to keep the air flow equal while bringing the idle down to where it belongs.

    It is challenging to those of us who have a hard time thinking in analog mode after years of digital work...
    '71 240Z, Because any fool can drive fast in a straight line.

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    Registered User LeonV's Avatar
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    Ah, that's right Gary. It's definitely been a while!

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    Hi Guys
    I got it sorted out. The idle speed screw on the rear carb was adjusted to high and the front one was backed right off, so the engine was running on the rear carb only. As I tried to adjust up the front the idle went to high. I backed off the rear and then brought up both srews until the unysyn read the same and then procceded from there. It shows the importance of balancing the carbs. It ran ok out of balance but runs great in balance.
    Thanks
    Bob

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