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Edelbrock/Carter Idle Settings after PCV fitment

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    Edelbrock/Carter Idle Settings after PCV fitment

    After some very sensible advice from members I've fitted a previously absent PCV (positive crankcase vacuum) to my 64 289, using an Edelbrock valve cover cap piped to the PCV vacuum port on the front of my Edelbrock/Carter 4 barrel carb. Got the cap at very reasonable price from John Woolfe Racing in the UK. No more whisps of smoke/fumes from the valve cover cap But the engine idle speed has risen markedly since doing the job. Would be grateful for any explanation - and advice on rectifying this please. Thanks guys

    #2
    This increase in idle speed is to be expected. The PCV system has introduced a measured air leak to the manifold to enable fumes from the crank case to be sucked into, and burnt by the engine. You really need to readjust the idle settings which is quite simple. You need to adjust the throttle stop screw where the throttle linkage joins the carb and the two mixture screws sticking out of the front of the carb. First run the engine until it is fully warmed up. A trip around the block would be best. Then unscrew the throttle stop screw until the engine idle speed returns to normal. With this done slowly screw either one of the mixture screws clockwise (noting the fractions of a turn) until the engine starts to go lumpy then screw it anticlockwise 1/2 turn. Do the same with the other mixture screw turning it the same amount as the first screw. If the engine starts to go lumpy as soon as you start turning the mixture screws try unscrewing them anticlockwise. What you are looking for is the smoothest idle you can get. The other way to do this is with a vacuum gauge fitted to the engine where you are looking to get the highest vacuum possible at idle but I am assuming you don't have one of these. My suggestions will get you the smoothest idle with the engine running as lean as you can for lowest emissions at idle.
    Allan

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      #3
      Originally posted by ALLAN R View Post
      This increase in idle speed is to be expected. The PCV system has introduced a measured air leak to the manifold to enable fumes from the crank case to be sucked into, and burnt by the engine. You really need to readjust the idle settings which is quite simple. You need to adjust the throttle stop screw where the throttle linkage joins the carb and the two mixture screws sticking out of the front of the carb. First run the engine until it is fully warmed up. A trip around the block would be best. Then unscrew the throttle stop screw until the engine idle speed returns to normal. With this done slowly screw either one of the mixture screws clockwise (noting the fractions of a turn) until the engine starts to go lumpy then screw it anticlockwise 1/2 turn. Do the same with the other mixture screw turning it the same amount as the first screw. If the engine starts to go lumpy as soon as you start turning the mixture screws try unscrewing them anticlockwise. What you are looking for is the smoothest idle you can get. The other way to do this is with a vacuum gauge fitted to the engine where you are looking to get the highest vacuum possible at idle but I am assuming you don't have one of these. My suggestions will get you the smoothest idle with the engine running as lean as you can for lowest emissions at idle.
      Thanks Allan - makes perfect sense. I did back off the throttle stop screw completely and it still ran fast - so I'm guessing the idle mixture is WAY too rich ?? Will adjusting those mixture screws do the job or is there something else going on here ?

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        #4
        Ok just to see where those mixture screws are set screw them fully clockwise until they bottom out counting the turns. A good starting point is for them to be around 1 1/2 turns out. If they are a lot more out than this then this could be your trouble. They should also be set about the same. Start with this and see if you can get the engine to idle at a sensible speed. It does sound as if the engine is running rich. Another issue could be that the secondary throttle plates are being held a little too far open but I can't remember what the throttle stop arrangement is for these, you need to look around the carb. Don't fiddle with this until you have tried adjusting the mixture screws.

        I found this, scroll down to Figure 8 where you need to check that the secondary's are closing properly. Someone before you may have been fiddling with the carb.

        https://static.summitracing.com/glob...rbsrebuild.pdf
        Allan

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          #5
          I always find a video is useful to work out what to do with anything and there are a couple of useful Edelbrock carb tuning videos that I watched before first stripping down and then setting up my 4 barrel carburettor..



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            #6
            Thanks muchly fellas - will have a go at this as soon as I can !

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              #7
              Allan, from reading your posts I think you have only one component of a PCV system the cap with a vent tube. This should be connected to the air cleaner, You also need a PCV valve this is usually fitted with a grommet in the other valve cover, a pipe connects this to the vacuum port on your carburettor. Watch this video it explains the system well especially the operation of the valve at the end of the video. https://youtu.be/DTlCep4_jZA


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