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Holes in Holley Butterflys

Posted By: Mopar1970440

Holes in Holley Butterflys - 09/07/09 04:53 AM

Swapping from a 750 holley to an 850 holley that i just purchased. I noticed the 750 had holes drilled in the front butterflys. The car has a larger cam so i understand this was done to assist with idle. Since the 750 had the holes.should i have the 850 butterflys drilled before installing too?
Posted By: D_C

Re: Holes in Holley Butterflys - 09/07/09 06:14 AM

That is one way to solve it, but before you get your drill out, adjusting your secondaries open a bit more (set screw in carb base, access from the bottom) might just solve your problem.

I have an 850 Holley on my 440/'71 Charger, and I went through this same situation around 7-years ago.

The purpose of drilling the holes and/or adjusting the secondaries open more at idle accomplishes the same task, allowing more airflow at idle. This is done to compensate for the weak signal (vacuum) produced by long-duration/high-overlap camshafts.

That said, don't open them up too much either. Here is a quote from Holley:

Quote:

You should not see more than .025"-.030" of the transfer slot exposed past the throttle plate at idle.




The "transfer slot" are the slots machined into the base-plate throttle-bores.

In a "Normal" street application, the idle system works like this quote from Holley.com:

Quote:

IDLE SYSTEM

The idle system supplies the air/fuel mixture to operate the engine at idle and low speeds. Fuel enters the main well through the main metering jet that is screwed into the metering block.

Some of this fuel is then bled off to an idle well where it is mixed with air from the idle air bleed hole. The idle well leads directly to the idle discharge port and the idle transfer system where this air/fuel mixture is discharged




Of course, your situation is "different" and not a "normal" street engine application. The Holley website "used" to have the information on why/when/where to drill throttle plates and/or secondary throttle-blade adjustment, but it was either eliminated, or it's buried in their site.
Posted By: gamagoat6x6

Re: Holes in Holley Butterflys - 09/07/09 11:42 AM

Install it as is for now and try it. The 850 will flow more idle air/fuel to start with and undoing unwanted modifications is a pain.
Posted By: D_C

Re: Holes in Holley Butterflys - 09/07/09 04:23 PM

Actually, that is a better initial approach. Try it out exactly as the factory designed it.

If after running a while, it doesn't work out, try the simplest "fixes" first. Ideally, you don't do any "permanent" modifications.

Usually, component replacements (power-valves, jets, accelerator-pump parts, etc) and adjustments are all that will be required to tailor the carb for your application, and oftentimes, right out of the box works just fine.
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