Purpose of these holes?

[mickmate]

I know a couple of our esteemed and knowledgeable members referenced in this article.
http://saac.memberlodge.com/resource...209-holley.pdf
I'll see if they have any insight for us.

[dcdoug]

Got the answer from Holley:


"These were early design booster signal control calibrated vents."

Thanks

Shane Whitescarver
Technical
Service Representative

[BAsque1]

Originally Posted by dcdoug

Got the answer from Holley:


"These were early design booster signal control calibrated vents."

Thanks

Shane Whitescarver
Technical
Service Representative

Doug:
Here is the answer:
the two holes in each side of the carburetor are vents (one from each venturi) to the atmosphere.

[Hotfingrs]

I asked my engine builder today and he told me it helps the idle. Instead of drilling a hole in the throttle blade, those holes serve the same purpose.

[BAsque1]

I got my answer from a retired auto tech professor with 35 years of autotech experience, so I feel very strong regarding his opinions.

[CHANMADD]

That answer merely states the obvious.....Professor or not..!!

[Gaz64]

Originally Posted by Gaz64

If it doesn't, then it must be some "engineer's" idea of bleeding off part of the start of the main circuit.

My answer from post 16.

Originally Posted by Hotfingrs

I asked my engine builder today and he told me it helps the idle. Instead of drilling a hole in the throttle blade, those holes serve the same purpose.

Don't think so.

Originally Posted by BAsque1

I got my answer from a retired auto tech professor with 35 years of autotech experience, so I feel very strong regarding his opinions.

Each to their own, not everybody's opinion is correct.

[Hotfingrs]

Originally Posted by Gaz64

My answer from post 16.



Don't think so.



Each to their own, not everybody's opinion is correct.

Just because you don't think so, doesn't make it wrong, without any supporting information.

[lovehamr]

The idle thing makes absolutely no sense. Unless the hole went under the butterflies there's no way that they would affect the idle, the holes are above the entire idle circuit.

This: "These were early design booster signal control calibrated vents." actually makes a little sense. At least they are in a position to affect booster signal. Why they would need it I have no idea. Seems like it'd be better just to use different booster venturies.

[vector1]

I don't doubt anybody's reasons, they can use them for whatever they want. I asked on another forum for more input and here a couple replies:

Some AFB carburetors also had those and I think that the idea was to allow fuel vapor to escape to the outside instead of letting it go down into the intake and flood the engine under a hot soak condition.

They were put there for two reasons:
To vent the venturi above the plate during a hot soak situation and, (in that particular position), to create a small air stream at Wide Open Throttle above the shaft for better atomization at a spot where fuel tends to collect.

The EPA and hydrocarbon air restrictions did away with any of that type venting on later model carbs.

[FatBoy]

Originally Posted by lovehamr

The idle thing makes absolutely no sense. Unless the hole went under the butterflies there's no way that they would affect the idle, the holes are above the entire idle circuit.

I agree. Everything above the butterfly is at atmospheric pressure (or maybe a little higher at high speed) whether inside or outside the carb. I can't see how there will be any flow of air in any direction through these holes because the pressure will always be the same on both sides.

Paul

[Gaz64]

Originally Posted by Hotfingrs

I asked my engine builder today and he told me it helps the idle. Instead of drilling a hole in the throttle blade, those holes serve the same purpose.

And you believed that? You could drill 1/2 inch holes through the sides and the engine would still idle at the same speed.

[Gaz64]

Originally Posted by FatBoy

I agree. Everything above the butterfly is at atmospheric pressure (or maybe a little higher at high speed) whether inside or outside the carb. I can't see how there will be any flow of air in any direction through these holes because the pressure will always be the same on both sides.

Paul

Agree.

[Hotfingrs]

Originally Posted by Gaz64

And you believed that? You could drill 1/2 inch holes through the sides and the engine would still idle at the same speed.

If that was the case, every racer trying to save weight would have 1/2 inch holes in the side of the carb.

I never said it was right or wrong... you did, but offered no proof.

[Gaz64]

I've been a mechanic for 33 years, I don't have an "engine builder" since I do all my own.

I never said it was right or wrong either, I gave my opinion of what it could be in post 16, then you chime in at post 24 with "your info" which would make no difference to the carb idle speed.

I have never seen this done to ANY carb, it makes absolutely no sense to drill holes in the barrel after the venturi to allow unfiltered air into the engine.
To me it wouldn't make a scrap of difference to how the engine runs from idle to high rpm.

As another member has stated, atmospheric pressure would be on both sides or very near to it.

[WardL]

I believe the atmospheric pressure is 14.7 PSI outside the air cleaner. As the air gets sucked into the carb, there is slight pressure drop depending on the air cleaner as the air travels down the venturi. At idle, the pressure just above the butterfly would be pretty much 14.7 and very little flow would come into the carb from these little side holes. At full throttle with the butterfly wide open, the pressure "upstream" of the butterfly would be at the lowest pressure, causing the most flow of air through these little pin holes into the engine. To me, the Holley Tech answer sounded more like they didn't really have a good answer so they came up with a flimsy response. Maybe it was an "improvement" Holley later decided wasn't such a good idea. I say plug the holes if only to prevent dirty air from getting into your engine. Any benefit the holes offer can't be worth the extra dirt in your engine. Pretty interesting little issue!

[Jac Mac]

The purpose of those holes is to reduce the tendency for fuel to be pulled from the main venturi under idle conditions when using a longer duration cam etc that needs a wider throttle plate setting to maintain a decent idle speed, the holes kill off airflow between the throttle plate & main venturi, many production carbs- carter etc particularly 2bbl's use the same trick. Not a really good idea when you think of the crap that can get drawn in at that point. The introduction of 'idle-ezi' by Demon & others gets around the problem and at least keeps the air cleaner.

[FatBoy]

Originally Posted by Jac Mac

The purpose of those holes is to reduce the tendency for fuel to be pulled from the main venturi under idle conditions when using a longer duration cam etc that needs a wider throttle plate setting to maintain a decent idle speed, the holes kill off airflow between the throttle plate & main venturi, many production carbs- carter etc particularly 2bbl's use the same trick. Not a really good idea when you think of the crap that can get drawn in at that point. The introduction of 'idle-ezi' by Demon & others gets around the problem and at least keeps the air cleaner.

That doesn't make any sense to me whatsoever I'm afraid. There is no pressure difference from one side of the hole to the other, which will always be roughly at atmospheric pressure, unless it is a blow-through supercharged/turbo application.

[Jac Mac]

You have to remember that without those holes all the airflow has to pass thru the main venturi, when you have a wider throttle plate setting at idle it should be obvious that the amount of air passing thru the venturi increases which in turn increases the signal at the main venturi and if high enough initiates fuel flow from the main venturi, those four holes & the resultant air leak kill off that signal at the main venturi and allow a more precise idle adjustment of the idle circuit in the normal fashion. In DC Dougs case if his cam & engine build etc is of less duration than whatever the carb was originally set up for then they are probably unnecessary & could be blocked off & or replaced with an idle ezi setup if it still requires help in this idle area.
One of the earlier posters summed it up with ' it delays the initiation of the main fuel circuit' or words to that effect'.

[FatBoy]

I'm sorry, but I still don't buy it. There is absolutely no reason for the air to flow through those holes in priority to the main venture and the air pressure inside the venturi wouldn't change as a result of having those holes there.

Paul