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simonf

Carburettor Jet Cleaning

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...in any event, even at idle, the butterflies do not seal the choke tubes....you try running the engine with the butterflies actually shut and see how you get on...(!)  

 

I agree with Moley, the butterflies are actually a slightly different diameter to the choke so even when closed they do not seal them completely, as shown in the diagram where the bottom arrow shows air travelling under the butterfly.  :rtfm:


Cheers,

John W

http://jonwatkins.co.uk

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I agree with Moley, the butterflies are actually a slightly different diameter to the choke so even when closed they do not seal them completely, as shown in the diagram where the bottom arrow shows air travelling under the butterfly.  :rtfm:

 

Yes John,

 

I agree with Moley too. ( I didn't agree with Andy)

However when overhauling the carb, the butterflies must be reinstalled (when removed and cleaned) in a way that they shut as good as possible. When looking  through the carb at a light, you will see it is never completely closed, but almost.

 

There simply wil not be an idle possible, unless you open the four butterflies together slightly by turning them open with the idle screw on the rear carb.

That is shown in the diagram.

At idle, the air travels at the bottom and top of the butterflies. The more you open the butterflies with the idle air screw, the higher the idle rpm gets.

If the idle air screw is backed completely, the butterfies will close and the engine should stall. If this is not the case and the idle can not be set below for example 800rpm with the idle air screw backed, then the two carbs are not ballanced well, or the butterflies are not installed well and do not close down enough, or there is an air leak somewhere in the system.

 

Geert

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In post #47 it even states it very clearly.

The fuel air mixture is delivered to the barrels DOWNSTREAM of the throttle valve.

 

The throttle vales (aka butterflies) are not the route through which idle mixture is delivered.

 

I agree (partially) with Geet, in so much that you adjust the air vacuum level with the air bleed screw but it does not open the butterflies, it opens up more of a small hole.

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Andy, mate, if the butterflies are fully shut...the engine will not run. Honest. Try it yourself.....stick your hand over the choke tube and see what happens. The air going into that small hole is used to produce a fuel/air emulsion that is supplied to the progression holes and idle mixture screws, and then mixes with the air coming past the butterflies to give the final mixture that goes into the engine. As Geert says, the idle mixture screws enable you to balance the two chokes on each individual carburetter. The main jet assembly in it's emulsion tube, delivers a richer emulsion to the choke tube for high power running. Carburetter jets always supply a fuel/air emulsion...the only time they supply neat fuel is when the accelerator pump is operated and it's jet squirts the stuff in like a fire hose. You can't run the engine on the idle jets alone, without the air that comes past the butterflies. I suggest you make the experiment and satisfy yourself....every adjustment affects all the others, it's an iterative thing...and the idle adjustment on the butterflies forms part of this.


Scientists investigate that which already is; Engineers create that which has never been." - Albert Einstein

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Andy,

 

i have to answer with a question and give the answer:

What is then the purpose of the idle speed screw on the side of the rear carb, that actually opens or closes the 4 butterflies of the two carburators by turning the throttle shaft a little?

The answer : You adjust the idle rpm with it.

 

I never stated that the 4 bleedscrews are opening the butterflies. But the idle speed screw does.

 

Now the fuel air mixture screws are indeed used to adjust the richness of the idle mixture.

When opening the butterflies (with the idle speed screw), the emulsion is sucked and mixed with the passing air. If the mixture screws are opened wider, more fuel/air emulsion is sucked in and the overall idle mixture becomes richer.

 

And it is right that to create the basic emulsion the fuel from the idlejet is mixed with air from the well. But that mixture will be used to mix with the passing air .

 

idle speedscrew more open > more passing air through butterflies > higher idle rpm

idle mixture screws more open > more air/fuel mixture sucked in > richer idle

 

 

Geert

Edited by gvy

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I've decided that I'll just agree with Geert in future...concise, accurate and to the point!


Scientists investigate that which already is; Engineers create that which has never been." - Albert Einstein

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Moley,

 

Thank you,

I have to be concise anyway.

English is not my native language ( third language actually), so sometimes a bit difficult to be detailed

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Andy, mate, if the butterflies are fully shut...the engine will not run. Honest. Try it yourself.....stick your hand over the choke tube and see what happens. The air going into that small hole is used to produce a fuel/air emulsion that is supplied to the progression holes and idle mixture screws, and then mixes with the air coming past the butterflies to give the final mixture that goes into the engine. 

No it doesn't

Read the manual, look at the carbs, anything, you'll see that the exit for the air fuel mixture is engine side of the butterflies.

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No it doesn't

Read the manual, look at the carbs, anything, you'll see that the exit for the air fuel mixture is engine side of the butterflies.

Andy, I think you're confusing air & air/fuel mixture.

The mixture flows through the jets & out of the orrifices, the air flows past the butterflies. :-)


Cheers,

John W

http://jonwatkins.co.uk

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The butterflies need to be open a little (at idle) to create the vacuum - otherwise as has already been stated the engine will not run.

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The butterflies need to be open a little (at idle) to create the vacuum 

If the butterflies were closed to such an extent as to be air-tight you'd end up with a vacuum, one much stronger than normally experienced there. So the butterflies don't "need to be open.... to create a vacuum".

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Andy....I think you are now simply poking at us with a stick!!! And we are all responding..... All your declarations cannot alter the facts. As I suggested....try to get the engine to run with the choke tubes blocked. It won't. Were your assertions correct, it would idle happily...this will not happen.

 

Since there is no point in continuing to discuss this, I now withdraw from any further chat on the subject.


Scientists investigate that which already is; Engineers create that which has never been." - Albert Einstein

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Andy....I think you are now simply poking at us with a stick!!! And we are all responding..... All your declarations cannot alter the facts. As I suggested....try to get the engine to run with the choke tubes blocked. It won't. Were your assertions correct, it would idle happily...this will not happen.

 

Since there is no point in continuing to discuss this, I now withdraw from any further chat on the subject.

Blocking the inlets of the choke tubes means the vacuum is then formed on the outside of the butterflies as well as the inside, that has implications for the whole routing of fuel and air.

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Andy,

 

i have to answer with a question and give the answer:

What is then the purpose of the idle speed screw on the side of the rear carb, that actually opens or closes the 4 butterflies of the two carburators by turning the throttle shaft a little?

The answer : You adjust the idle rpm with it.

 

I never stated that the 4 bleedscrews are opening the butterflies. But the idle speed screw does.

 

Now the fuel air mixture screws are indeed used to adjust the richness of the idle mixture.

When opening the butterflies (with the idle speed screw), the emulsion is sucked and mixed with the passing air. If the mixture screws are opened wider, more fuel/air emulsion is sucked in and the overall idle mixture becomes richer.

 

And it is right that to create the basic emulsion the fuel from the idlejet is mixed with air from the well. But that mixture will be used to mix with the passing air .

 

idle speedscrew more open > more passing air through butterflies > higher idle rpm

idle mixture screws more open > more air/fuel mixture sucked in > richer idle

 

 

Geert

 Andy,

 

Could you please answer my question, because I did give my answer.

 

Question 1

What is then the purpose of the idle speed screw on the side of the rear carb, that actually opens or closes the 4 butterflies of the two carburators by turning the throttle shaft a little?

 

And because I am eager to learn , I have a second question.

I do check and syncronise my carbs and set the idle every six months.

I already did explain in short how this is done.

 

Question 2

How do you syncronise your carbs and set the idle at an exact 950rpm ( or the exact speed you choose) ?

 

 

Just a tip : Question 1 is explained in the Dess Hammill book on " how to build & power tune weber&Dellorto carbs" page 89 in the section throttle (linked throttle arm type)-final synchronisation.

But hey , really , you don't need the book to find out what that screw does , when you turn it.

 

 

Geert

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Awaiting Andy's reply and to keep the topic informative,I d' like to post the following article on tuning Dellorto DHLA carburators.

I find it very informative and understandable on how the balance procedure and idle speed setting of dellorto DHLA carbs is done. step by step.

Enjoy.

 

Geert

 

 

foto010_0008_zpsc635d446.jpg

foto010_0009_zps9f76760b.jpg

foto010_0010_zps6eb66038.jpg

Edited by gvy

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Hi Geet, sorry, I'd misunderstood which screw you meant, had I realised you meant the speed screw then yes, I agree it does open the butterflies. I'd mistaken thought you meant one of the other screws.  

I synchronise them the same as you, getting the vacuum on one car even across the venturi by opening the air bleed screw of one venturi, and bringing the other carb to the same state by opening/ closing the butterflies further and the air bleed screw on the one venturi of the second carb.

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The disagreement is where the fuel air mix goes for idle, I still maintain that the basic mixture at idle is via the top route of the carbs, not via the chokes and butterflies, only joining the main part of the carb after (downstream/ engine side of) the butterflies.

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To return to this...the idle jet and the progression jets deliver a fuel/air emulsion downstream of the butterflies. The emulsion they deliver is far too rich for the engine to run, and has to be mixed with air coming down the choke tubes past the butterfies. As you apply more throttle, the mixture they deliver changes to keep the ratio more or less correct at the inlet valves. The old line that a carburetter is an instrument scientifically designed to deliver the wrong mixture at all times applies....as opposed to fuel injection, where you can get the right mixture at all times!


Scientists investigate that which already is; Engineers create that which has never been." - Albert Einstein

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Afraid not, take a look at the diagram, the arrows show the airflow.

 

dhlaoperation3.gif

Idling

Fuel from the float chamber is metered through the idle jets (20) and mixes with air from the well (3) through the channels (19). Mixture through channels (21) reaches the idle mixture screws (22) and, when regulated by them, reaches the main barrels (10) downstream of the throttles (13).

Super diagram John. 

 

What keeps the fuel mix from dribbling through the progression holes before the butterflies move past them?  Does the air rushing past it keep it in place?

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