Dhla Throttle Plate And Progression Hole Alignmment

[olNick]

hi from new guy,

 

AFAIU, the throttle plate.butterfly should be covering the first progression hole completely when closed/idle.

My dhla40, old no suffix, 4 small holes, partially exposes the first hole when plate is cracked a 1/4 turn.

Plates are 7481-2 with the milled sweep slots. Why slots? I dontknow...

 

 

Is this normal to have progression hole partially exposed?

Also seems one barrel exposed more hole than the other...

 

trying a pic...

[Triumph-V8]

The progression holes are the most effective remedy to bring

part throttle engine response to perfection.

 

Unfortunately you can easily ruin a carb when you do the wrong things.

 

In history those carbs often had only two holes and that expanded

during the years up to seven.

 

In history WEBER carbs supplied their dealers with a set of fine drillers

starting at 0.45mm up to somewhat 2.5mm stepped in 0.05mm.

This was for fine adjustment of the carb bodies and I could get such a

original set which I used very often.

Today this knowledge is fully lost and nearly nobody drills on the progression holes.

This might have the reason in the large number of holes that are easier to set up as they are.

 

If you have few progression holes it is good to have the first hole fully closed

and enrich mixture at idle.

When opening the throttle the mixture than gets a little bit leaner

until the progression hole is reached and than gets a kick to become rich.

That continues to the next hole and if availiable to the third.

 

So a lot of progression holes smoothes the AFR curve over the opening of the butterflies.

It is not unusual that the first hole comes open early because you have a lot that come in continously

and mixture has not to be set that rich at idle but other butterflies can be obtained if that does not fit.

 

If the butterflies have to be opened at idle too much what might be necessary with wilder cams

the butterflies might be drilled with a little hole at the bottom.

Many modern carbs have a bypass system to set idle mixture with butterflies closed.

Did not see if this carb is such a system.

 

So the final conclusion is to use the carbs as they are and look how they behave on the engine.

A lot can and must be done first with the idle jets for fuel and air.

Only at that late point you can make the final decision if the progression hole adjustment

is good for your engine.

 

If one hole is earlier set free than his brother the spindle might be twisted.

That is often the case and can be cured by bending the spindle

which again has the chance that it will be destroyed.

Keep in mind that the spindle is sliced and only a very small area

is connected between left and right butterfly.

[olNick]

Thanx for the response.

 

As stated this carb has 4 small holes, and no idle bypass.

Coming to my original question, was/is it common to have the first progression hole partially uncovered at idle?What are we trying to achieve?

It would seem intentional, thus the slotting of the butterflies.

 

And I'm not understanding the implications. As the throttle initially opens, vacuum drops and since the first hole is already feeding, maybe the delay is not as bad????

Nice bit of history on drilling progressions... could you expand a bit on the theory practice/approach? I'm a sucker for old, "useless" knowledge....

 

Yes, I will fit the carbs and see how things go

[Triumph-V8]

As I said if you have enough progression holes it is not unwise to open the first hole soon.

It should not be opened more than half at idle and normally should be hidden from the butterfly.

So that is normal.

 

If you are in trouble you might change the butterflies from left to right

to get the slit on the side where it does not make trouble.

 

To get closer to the carbs there is a Haynes book for the Weber DCOE

which are from principle the same as DHLA.

 

For understnading the progression holes it is helpful

to think in constant depression on engoinen side.

If depression drops much the main system clogs in and that makes it more complicated.

We watch the part throttle system where the whole amount of fuel is delivered by the idle jet.

You can cut off the main system if you want and still can drive with part throttle.

 

When the butterflies are opened a bit some holes are on engine side

and some are on air filter side.

The fule sucked into the engine goes through the idle fuel jet

and is mixed with the air from idle air jet and the air from the progression holes

on air filter side.

 

So to get the mixture at this point richer gives a lot of possiblities:

Make idle fuel jet bigger

make idle air jet smaller

make progression hole on engine side bigger

make progression hole on ait filter side smaller.

 

As all belongs together and has effects vice versa

it is so difficult to do the right things not to end in a single laine.

 

For the start the rule of thumb is that fule level has to be set properly,

idle air and fuel jet has to be set properly what can be noticed by

turning idle mixture screw about 1.5 turns +/-1 to get proper idle.

 

Next rule of thumb is that row of progression holes needs extension towards

air filter if too large venturis are in use because main system starts later than.

 

If all is in limits you will have an air/fuel ratio curve over opening degree of butterflies.

That may be felt but better should be measured by wideband lambda control.

 

If you than still have too lean mixture at some butterfly position you may drill

the hole before the butterfly (towards engine) bigger.

If you are too rich you may enlarge the hole behind (towards filter)

to get more air into the mixture.

[Triumph-V8]

Sorry for the mistakes, did want to read but posted already.

 

Just had a look at the picture.

Changing the butterflies does not help,

the slit is too wide and would cover holes also on the other side.

Maybe it was produced to have only one type for the two sides.

Remedy would be to buy other type of butterflies if needed

but on the "closed" side it seems to be pretty well.

 

Also the spindle maybe twisted, one hole seems to be much more open.

With care you can bend them back into position.

As stated they brake very easy but anyway

in this case of twisting I would bet they will not work properly.