has anyone tried to bore out there chokes on dellortoes dhla40's as mine have 32mm chokes at the moment i will need 34-35mm i have a lathe and grinder i can use at work and the finish will be no problem but is it just a striaght bore or is there more to it
i know about eurocarb but i am to tight to splash out £60 ish if i dont need to
thanks dave

I've seen it done with Weber DCOEs seemed to work Ok

Theres more to it , the chokes are venturis so are tapered in from each end, I've bored them and made new ones for weber IDF's, the maximum size recommended for 40's is 34mm, not to hard to do, establish how far in from one end to the minor diameter then open it up to the new diam. then machine the new tapers, and blend in to give a slight radius at the narrow point, might pay to have a practice before starting on a real one.

Wimmera

How much is it for a new set of chokes then ?, I've got DHLA's and might need 32mm's, I'll have a look today to see what Chokes are in mine already

Jason

They sell up to 37mm for 40 DHLA carbs.
http://www.racecar.co.uk/dellorto/carcarburettors/dhla.htm
http://www.racecar.co.uk/dellorto/
If you have them in 32mm the taper is already there, you are just taking some out of the thicker part in the centre. It looks like a delicate operation but if it cocks up you can always resort to plan B and buy some.

[Edited on 5/12/04 by Peteff]

I knew 36 was available, but, the recommendation from the tuning manual is " A 34mm choke is definitely the maximum size to use in 40 DCOE or 40 DHLA body"

Wimmera

Bother,, i have a pair of 27's, a pair of 30's and a pair of 32's, oh well, looks like i gotta but another pair of 32's

Jason

it looks like i will have a go at work then
what it seems like is two tapers that converge in the center and the center diameter should be 34mm
i too have seen chokes up to 37mm for 40s
so i dont understand why 34mm should be the maximum unless the venturie (spelling?) is restricting flow above 34mm diameter
as pete say what have i got to loose
cheers dave

The restriction is in the body, 40's will handle chokes from 28-34, 45's 34-40 and 48's 36-42.

Wimmera

so why do they offer up to 37mm chokes for 40's

Beats me, perhaps the bigger is better mentality buy a few, interesting to caculate the area of a 37 mm choke and compare it to the area available around the butterflies.

Wimmera

after a long think this is my idea and probabley wrong
if the bodie diameter is 40mm and the length of the choke is fixed then every time you change the central diameter ( the choke size where the two tapers converge) the the angle of the taper must change
and so if the angle become to shallow the choke doesnt work as it now almost a striaght tube
does this make sense or complete crap?
dave

http://hometown.aol.com/dvandrews/dellorto.htm
It's interesting although not altogether relevant to your original question.

Yes that could be some of the problem, it would be interesting to compare 36mm chokes from 40s"& 45's to compare the angles, but as mentioned earlier the available area around the butterflies in the 40's is less than the choke area when the size gets up a bit which must create a restriction . Much better to alter the angle when opening them up.

Good luck with it

Wimmera

Angle will be the same, and as long as the edges are smoothed it should be OK.
A long while back I did some work in Fluidynanics on the flow through nozzles and diffussers -- A venturi is just a convergent nozzle connected by a short straight section to a difffuser. In a pefect venturi the pressure before and after is the same --- ie pressure recovery is 100% but it dosen't work that way you get losses due to skin friction and turbulence. Smoothing the surface of the venturi will reduce losses at entrance and in the centre section but the big losses are at exit -- if the outlet from the diffuser is angled at more than 15 degrees the air flow will break away from the surface and turbulence and back flow fills the void.
This back flow and turbulence robs the air flow of recoverable energy so the pressure after the venturi never recovers to the pressure at entrance.

As it is practically impossibe to keep the flow attached to the wall in the diffuser section many carb venturis are truncated and don't continue the 15 degree angle to the full body diameter. so as long as the exit from the venturi blends in smoothly it won't cause a problem.

Personally I wouldn't use any venturi bigger than 34mm in a 40mm carb body --- 40mm carbs are OK up to about 140bhp after that they start to become restrictive if bigger is really needed go for 45s or 48s.

[Edited on 8/12/04 by britishtrident]

[Edited on 8/12/04 by britishtrident]