Purpose of these holes?

[dcdoug]

Originally Posted by 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'.

My "B" cam is pretty close to what was originally in these cars.

[CHANMADD]

I would agree that their would definitely be a pressure difference between atmosphere and in the throttle chamber. When the motor is sucking at idle there is for sure a vacuum above the plates ......seems pretty obvious.....I think Fatboy just wants to argue....

[Jac Mac]

Originally Posted by 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.

Gary, you should have started 10 years earlier.
Most if not all of the carbs fitted to 6 & 8 cyl Holdens, Chrysler Valiants, and Ford falcons produced in OZ during the 1970's & earlier have similar holes drilled above the throttle plate area. In this particular case with the standard production carbs/engines it was to help prevent fuel pullover from the main venturi when the choke was in operation with the throttle blade in the fast idle position. Strombergs, Carters & Email? carbs, both single & 2bbl had this feature, I will have to have a look at some Autolite/Motorcraft carbs to check, but I think some of them had an internal drilling/passage from above the choke horn to the throttle plate area that performs the same function, plus uses cleaned air. You have to remember that in this instance the carb airflow ratings and venturis were quite small .

[Jac Mac]

Originally Posted by CHANMADD

I would agree that their would definitely be a pressure difference between atmosphere and in the throttle chamber. When the motor is sucking at idle there is for sure a vacuum above the plates ......seems pretty obvious.....I think Fatboy just wants to argue....

Maybe, but all I can think of in his case is that he must be an interesting passenger anytime he flys in a planeto use his train of thought it wont matter if there is a hole in the wing..........

[dcdoug]

Soooooo....

Leave as is, or plug with a small bit of RTV in some unnoticeable way?

[FatBoy]

Originally Posted by Jac Mac

Maybe, but all I can think of in his case is that he must be an interesting passenger anytime he flys in a planeto use his train of thought it wont matter if there is a hole in the wing..........

Okay, good analogy. I'll need to think about that one.

Paul

[Jac Mac]

Originally Posted by dcdoug

Soooooo....

Leave as is, or plug with a small bit of RTV in some unnoticeable way?

As I think ERA Chas suggested, I would cover each hole with a small piece of duct tape for starters, then use a piece of perspex over the carb top to observe whether any fuel is being pulled from the main venturis at idle. If not and the idle quality does not change then a dab of devcon or epoxy on each hole should be a permanent fix ( RTV would swell & break down over time with exposure to fuel vapours etc ). If it still requires the holes for idle quality then you have to decide whether the road conditions etc that you normally drive on are going to cause any problems long term-dust-grit etc and if so look into conversion to later idle ezi system etc.

[Gaz64]

Originally Posted by Jac Mac

Gary, you should have started 10 years earlier.
Most if not all of the carbs fitted to 6 & 8 cyl Holdens, Chrysler Valiants, and Ford falcons produced in OZ during the 1970's & earlier have similar holes drilled above the throttle plate area. In this particular case with the standard production carbs/engines it was to help prevent fuel pullover from the main venturi when the choke was in operation with the throttle blade in the fast idle position. Strombergs, Carters & Email? carbs, both single & 2bbl had this feature, I will have to have a look at some Autolite/Motorcraft carbs to check, but I think some of them had an internal drilling/passage from above the choke horn to the throttle plate area that performs the same function, plus uses cleaned air. You have to remember that in this instance the carb airflow ratings and venturis were quite small .

Is that right. Let's see some photos.

Sounds like b/s to me.

"help prevent fuel pullover from the main venturi when the choke was in operation with the throttle blade in the fast idle position"......

The choke system, so aptly named, because it strangles the engine for air, it causes the engine to pull fuel from the main discharge nozzle at low airflow, ie: cranking speed.
The choke flap is offset, the flap opens a predetermined amount once the engine fires. The engine may still be partly on the main circuit at high idle.

[WardL]

For what it is worth, I got my story backwards, at full throttle the vacuum would be low in the intake manifold and at idle the vacuum would be high. I still agree with most that they should be plugged temporarily to see what what happens, then when nothing noticeable happens, plug them permanently. Think about it, 99% of our carbs don't have the holes and they run fine. What else could be different with this carb to require the little holes?

[CHANMADD]

I know..!!! .....I know!!!!!1..........they're for Nitrous oxide jets..........