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Thread: Drilling a Holley

  1. #1
    I seen a number of posts regarding the drilling of air bleeds in the butterflies of Holleys.
    Engine is a 502 with a vac sec 750 Holley. Legend pump.
    Rich idle, soots up transom.
    Another curious behavior is that after cruising around 3-4K RPM and sidesteping foot pedal, the engine simply dies. RPM goes to 0. Restarts OK. Other than this, no other symptoms.
    Pulled carb off and throttle plate showing .08-.09 of transfer slot. Idle mixture screw set a 1/2 turn out.
    Readjusted throttle plates to .03-.04 slot showing and 1.5 turns on mixture screw.
    Restarted engine and won't idle, as the idle speed is too low. I think I need more air.
    I am looking for specific info:
    1. Where to drill, near the edge or mid-way on the butterfly, or near the shaft?
    2. Diameter to start with. .093, .125, or ??
    3. Any other recommendations.
    thx for your help.

  2. #2
    gstark, I drilled mine .125 (1/8") inline with the slots about 1/4" to 3/8" in from the outside of the butterfly. It really improved the idle and you can blip the throttle at idle and it wont die. Hope that helps.

  3. #3
    sounds just about like what I had to do with my first boat. It helped a lot -- much better idle.

  4. #4
    WAIT!!! look at the carb from the bottom, see the little threaded hole to the left of the secondary blades on the outside of the plate? that is a stop screw. turn it clockwise maybe a half turn and see how it idles. The air mix screws should be turned in (clockwise) until the engine begins to choke up and die, then back out slightly until idle smooths. (I understand this is relative to the cam profile) This is something of finesse. Use this method of adjustment before you drill holes in the blades.

  5. #5
    1stepcloser, that makes a lot of sense. I think the vac sec also have the transfer slot but it is a ways from being uncovered.
    It would guess that opening these up a little at a time should do the same thing as drilling the primaries.
    Cam is stock for 415HP 502. Not big at all.

  6. #6
    gstark The reason you only have the idle screws out 1/2 turn is because of how much transfer slot you have showing. You need about .020 - .030 of slot showing. If opening the secondary thing does not work out. You drill the existing hole in the primaries to 3/32" and see how it idles. step it up a little bit until you get the correct amount of transfer slot showing. You definately have a lack of airflow problem at idle. This is common with a good performance engine. Do not overlook the other causes of excessively rich idle, float level adjustment and powervalve leakage.

  7. #7
    The curb idle mix screws were set at the factory (Marine Power) and I have never touched them. Once I found that they were 1/2 out, I knew there was a problem, especially with the fine info shared from this and other threads.
    I reset the curb idle to 1.5 turns open and set the primary butterflys to about .03-.04 and set the secondary butterflys to about .02 visible.
    Fired the engine and it idled, but needed some more. I added more of the primary and a touch of secondary. Definitely on the right path.
    I am going to remove the carb again, using these settings as a baseline and increase both sides to a common opening level, as they are probably not equal at this point.
    I spoke to Holley tech services and this is exactly what they suggested. They advised against drilling - in their opinion this is not required. Just keep working airflow with both sets of barrels. If not successful, then drill.
    FYI - power valve, needle/seat, and float levels are spot on - not just my opinion, but that of my dealer as well.
    To all - thank you for the invaluable info you have shared. I've worked with Holleys for over 30 years and thought I knew a whole lot about them. Not enough obviously.

  8. #8
    Racing Ray
    Before drilling holes in the butterflies a good tuning method that is little known is to orfice the circuts on the metering blocks. This will cut down on the amount of fuel supplied to the transfer slots. This trick is mostly used on Tunnel Ram set-ups to limit fuel and clean up the idle characteristics. A picture is worth a thousand words as they say I hope it helps.
    Barely visiable I have already put a wire from 1-2 you also want to put one in position 3-4. Wire size can be experimented with but usually a single strand about 5mils thick will do the trick. Bend it "U" shaped and install as shown.

  9. #9
    Thanks Racing Ray. I'm glad you decided to keep posting and ignore some of the stupid stuff the comes up on the forums. I tried that trick of drilling your butterflies and fiddled with it long enough to get pissed and weld the holes back up. This was on a Wieand tunnel with 6109's. Best I ever got was about 900 rpms with an Ultradyne flat tappet 108, 251 259 @.050 with .600 and .625 in a 427 12 to 1 Howard runner. I'm going to try the wires. Are they just a piece of automotive wire pulled from a chnk of say 16 guage. Does this work with blower motors also? I have this 427 torn down now. Should I keep the same cam?

  10. #10
    Racing Ray
    Try to find one just a bit thicker than a strand out of 16 ga. wire. If that is all you have then try 2 or 3 strands twisted togeather. Are you asking about that cam with a blower?

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