Holley Brawler Headaches

[Jay Don]

I put a 454 in my 77 GMC with a "Mother Thumper" Cam from Competition Cams. Providing fuel is a Holley Brawler 750CFM carburetor. Here's my problem:

I can't get the carb set where it will idle good while in gear, which makes loading it on a trailer a real problem. I have watched YouTube videos and saw a suggestion to drill a 3/32 hole in each of the front 2 throttle plates. Supposedly, this cam set up is not giving me enough vacuum to make the idle circuit provide fuel to the engine at idle.

Does this sound like a good fix? Any other suggestions?

 

[legopnuematic]

First is get a vacuum gauge, that way you have some way to get a baseline on tuneup and changes.

What is your ignition timing set at? Square up your transfer slots if they are not already, if the throttle is open too far it will be idling on the transfer slots/transition circuit, not the idle circuit, this will cause lots of tuning headaches as the idle mix screws will basically not respond at all.

I wouldn’t take a drill to throttle butterflies unless I was 1000% sure that is what will correct it.

 

[Jay Don]

Don't know how to "square up" the transfer slots. I know what they are and I understand that if the throttle is open too far it will be idling on the transfer slots/transition circuit, not the idle circuit, but I don't know how to square them up. And you are right--the idle mix screws don't change anything. I can screw them in all the way and nothing happens. So I think it is definitely idling on the transition circuit.

 

[legopnuematic]

Squaring up the transfer slots is simply adjusting the idle speed screw to move the butterflies to make the transfer slot square in appearance.

On the left is representative of being over exposed, right is how it should be.

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There should be (you’ll need the carb off and upside down for this and squaring the slots) a small set screw around the secondary butterflies that will adjust the rest position of the secondary butterflies. This will allow for some idle speed adjustment without overexposing the primary transfer slots.

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[Jay Don]

Thank you very much for the detailed help--I really appreciate you helping teach me on the carb problem

 

[Jay Don]

Squared up the transition slots and it seemed to help a little. i pulled the plugs and they look terrible--black and soot covered--is it possible that the carb is pumping too much fuel? The timing is set up around 13 degrees advance--if i try to lower the timing, it won't run at all, just bogs down and dies. Any other ideas would be greatly appreciated.

 

[Beast496]

When you say "13 degrees advance" do you mean 13 degrees initial timing? For comparison, I run 34 degrees of total advanced timing in my built 496. Maybe our terminology is off.

 

[Rustisbest]

Hook vacuum advance to manifold vacuum if it isn't. It'll want the extra timing with the overlap that cam has.
Transfer slots are squared now readjust mixture screws equally to highest vacuum.

 

[Jay Don]

Thanks guys!!!