Dellorto jet covers

[vxah]

Can anyone explain to me what the “fluid dynamics “ if that’s the correct wording, of the DHLA jet cover fitted or removed is please?
Bit of an oddball question I know but I’m trying jet up the 45E’s on my Elan and found that it makes a massive difference but, not the way round I was expecting!
With covers off I needed a 58 idle jet with a .9 air jet to get somewhere around 13.5 a/f ratio. Covers on it needs 55 or maybe smaller.
Mains are much worse, covers off and a 140 main 160 air with.8 emulsion tube couldn’t better 17.5 a/f ratio.
Now with covers back on a 122 main is too rich!
I have the air box on and I know the float chambers and air jets are sealed to the air box with the covers on but my thinking was that the same “wrong” pressure signal is supplied to the fuel in the bowl as in well as the jets?
I was expecting a little change but with the covers off it seems like the mains are impossible to set up?

[nmauduit]

is the backplate of your airbox hole lining up with the carb's hole ?

[vxah]

Yes, 100% and sealed to it!

[nmauduit]

I'm not sure what you mean by "100% sealed to it" , I'm referring to the cuve atmospheric venting hole, one per carb about half an inch in diameter (this plays a role in fuel level and flow during operation, if plugged somehow there would be a depression inside the carb at locations intended to have atmospheric pressure ... unless the top cover is craked open)

[vxah]

Sorry, I meant I’m 100% sure the vent holes are lined up with the backplate. I straightened up the backplate and used sealer in the low spots to make sure.
That’s the thing though, how many are actually properly sealed to inside the air box?
I now have it reasonably drivable with the covers on but it’s still a bit rich here and there. Take off the covers and it will be un-drivable due to leanness!

[Craven]

This effect is also apparent with the Weber carb; take the twin setup on the Hillman Hunter from way back, where the cover was raised up by a spacer, not sure if this was solid or partly ventilated. Perhaps some form of depression created in this area.
FWIW.

[mbell]

My THEORY would be that the air box is at below atmospheric pressure so with caps on the air correction side of the jets receives BELOW atmospheric pressure air. With the caps removed the air correction side of the jets receives atmospheric pressure air.

This change in air pressure effects the operation of the air correction side of the jets, where higher pressure causes more air/bubbles. More air/bubbles generally leans the carb out.

[vxah]

Yes that’s a good theory and one that I would go with although, with covers on the fuel in the float bowl also receives less than atmospheric pressure so would this not reduce fuel flow through the jet?

The Holbay dcoe’s were a bit of a special with long auxiliary venturi tubes, odd progression holes and no air box vent, to be avoided I’m told?

[mbell]

with covers on the fuel in the float bowl also receives less than atmospheric pressure so would this not reduce fuel flow through the jet?

The air pressure in the float bowl and jet holder area are the same. So with no difference in pressure between the two side of the main jet, I think there will be no difference to fuel flow rate or fuel level in the jet holder whether both are subject to higher or lower air pressures.

[rgh0]

Can anyone explain to me what the “fluid dynamics “ if that’s the correct wording, of the DHLA jet cover fitted or removed is please?
Bit of an oddball question I know but I’m trying jet up the 45E’s on my Elan and found that it makes a massive difference but, not the way round I was expecting!
With covers off I needed a 58 idle jet with a .9 air jet to get somewhere around 13.5 a/f ratio. Covers on it needs 55 or maybe smaller.
Mains are much worse, covers off and a 140 main 160 air with.8 emulsion tube couldn’t better 17.5 a/f ratio.
Now with covers back on a 122 main is too rich!
I have the air box on and I know the float chambers and air jets are sealed to the air box with the covers on but my thinking was that the same “wrong” pressure signal is supplied to the fuel in the bowl as in well as the jets?
I was expecting a little change but with the covers off it seems like the mains are impossible to set up?

Unlike Webers the DHLA jet square black plastic covers are directly vented on the side of the cover to atmosphere, make sure this vent is not blocked. If the vents are not blocked having the covers on or off should make no difference

The Weber round metal covers are sealed and the jet chamber vents to the airbox, so if you have a significant pressure drop to the air box removing the covers may make a difference. In reality you should have no significant pressure drop to the airbox.

cheers
Rohan

[Craven]

It’s worth taking a closer look at the Internal air supply to Weber DCOE carb, from the attached pic’s it can be seen the air intake does not directly enter the main body but flows up into the top cover where due to the design pattern of the gasket into a sealed space above the jet block. The air intake eventually reaches the float chamber etc via small gap in the top at the side of the round jet cover.

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