dellorto carb

[shaun everard]

Does any one what the flow rate is for a dellorto vhsb tube 268?? And also the measurements for a k44 needle??

[Richard Gray]

Shaun - What do you mean by "flow rate"?

You can find the dimensions you need for the K44 on Tom Barth's Green Flag Karting website:

http://www.greenflagkarting.com/dell/pdf/needle_k-u.pdf


or


_________________
Richard


"Running into the sun but I'm running behind"

[Riley Will]

_________________
Riley Will
BRC Engineering
rwill@brceng.com
(403) 216-0630 wk.
www.brceng.com

[shaun everard]

Basically I am trying to get as much information on the dellorto set up I have. So I can go to my local dealer for mikuni carbs as I have a 41.5mm tm pro series flat slide that im trying to get working. On my cr as it worked brilliantly on other kart engines I have had. But no one seems to have tried it before or they dont want to tell me what needle and tube I will require.
Im not sure if there is a formula for working out what I would need as there is for exhausts.

Cheers for the help so far if I get it working I will post my findings.

Thanks Shaun

[Ian Harrison]

Hi Shaun

I don't think anyone is hiding anything, I think that they just don't know!!

The "flow" through a tube is irrelevant as you always run it with a needle, so the question is what does a particular needle/tube combination flow at a particular throttle opening. If we look at WOT the flow is defined by the tube area less the needle tip area. Of course "in the real world", lots of oher things affect this flow and jetting tables/programmes need to be taken with a large pinch of salt! Most if not all ignore the fact that if you fit a main jet with a larger flow capacity than that of a particular needle/tube combo the flow capability of the "set" is reduced.

I've never quite understood the poblem with taking the top off, having a look and gaining the knowledge to make a jetting decision. Anyway I digress.

I don't know of anyone running this carb on a kart. I will post the standard jetting for a CR250 in a TMX but I presume that the Pro-Series also uses a different series of jets and needles.

However as that's what we ran in our other Mikuni Carb, it may well give your dealer a start.

The Sudco online catalogue has some good info. Give it a google!!

To be fair, if you ran the carb without problem on other 250 kart motors, you are probably better placed than most, if not all to make a start.

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Richard Gray]

Shaun - I have an Excell spreadsheet which I set up to calculate the "effective jet" sizes at various throttle openings for the dellorto where you choose the needle and atomizer. It uses the annulus calculation compared to the area of the main jet. Shoot me a PM with your email if you'd like a copy. I don't recall if I included the K44 needle, but it is easy to enter the dimensions available as indicated.
_________________
Richard


"Running into the sun but I'm running behind"

[Ian Harrison]

Hi Guys

Shaun if you zoom in on Riley's he just posted these figures for the K44/268 combo with a 41mm venturi.

This software is a little more sophisticated than Richards spreadsheet.

Might be nice for Richard to post his figures for the same combo. Then we can see.

My own "annulus" spreadsheet indicates a maximum useable jet size of: 228, which is 10 up on Riley's

Any better offers

For those with poor eyesight Rileys figures are:

%age throttle - Equivalent jet size
00 - 97
10 - 97
20 - 103
30 - 124
40 - 140
50 - 155
60 - 167
70 - 182
80 - 195
90 - 207
100 - 218

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Richard Gray]

I don't know if I would use the words "more sophisticated", but the graphics sure are better I am afraid that I cannot plug the numbers directly into my spread sheet because I would need the dimension of the needle into the atomizer at WOT (my spread sheet does calculate the available clip positions, btw) and I do not have a 41mm carb handy to make that measurement. And, no, we cannot assume that the 41mm is the WOT dimension for slide over end of atomizer.
_________________
Richard


"Running into the sun but I'm running behind"

[Ian Harrison]

Hi Richard

You actually posted the chart with all the needle dimensions on it above!!

And they can also be read of Rileys spec (in setup 1)

So the K44 is:

A = Needle top diameter = 2.50mm
B = Tip Diameter = 1.40mm
C = Overall Taper length = 30mm
D = Diameter at the top of the taper = 2.06mm
E = Secondary Taper Length = 20mm

Just interested to see how your spreadsheet varies from Riley's software. Both will be different, but both will provide useful information when taken with the Viper pich of salt

Shaun

Standard jet needle (needle) and needle jet (tube) for a Honda CR250 using a Mikuni TMX carb is:

Jet Needle: 633 S-9
Needle Jet: 6B HY 38-73

I hope that helps

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[shaun everard]

Thanksevery one for the help given most people on other forums have not even bothered to reply at all I am now going to go and try to get it sorted out. If any one is interested in seeing the carb I can post pictures if wanted. Also Ian if your interested in seeing it I can bring it for you at cadwell go meeting if your interested. Thanks all again Shaun

[Richard Gray]

Ian - I need to determine the length of the needle remaining in the atomizer at WOT in order to calculate that annulus, from there, using the needle info published, we can calculate the annulus at any throttle opening and thus give a theoretical equivalent jet size. The measurements I have are for the VSHB34 (and 30, 36, 37, 39) but I don't have a 41mm in my data base. I do have a carb, but it is packed away from my recent move and I have no idea in which of the 60-75 boxes it 'lives". We did a last minute "panic pack" and all boxes didn't get properly labelled


BTW, when I do my flow bench analysis, I monitor the pulse on the atomizer (and idle jet circuit) as well as the venturi. Even though my bench is a manual data collection device and charting used for comparison is manual also, I can detect the differences in atomizers and needles wrt throttle opening.
_________________
Richard


"Running into the sun but I'm running behind"

[Richard Gray]

Guys - Just to show how varying our calculations can be, I looked up, in John Savage's JetTech Pro software, the equivalent jet size using a 41mm venturi, K44 needle and a 268 atomizer (main jet 195, idle jet 65) and get the following result:

%Throttle position / Main Jet Equivalent
0 / 116
10 / 137
20 / 155
30 / 169
40 / 182
50 / 194
60 / 205
70 / 206
80 / 206
90 / 206
100 / 206



I trust John's calc's and will compare them to mine when I get a chance.....
_________________
Richard


"Running into the sun but I'm running behind"

[Riley Will]

How many "mm" of verticle slide movement does a VHSB39mm carb have?
_________________
Riley Will
BRC Engineering
rwill@brceng.com
(403) 216-0630 wk.
www.brceng.com

[Ian Harrison]

F+++k Riley, I got paying work to do here.

Firstly remember that the 39 Dellorto venturi isn't round, it's formed (roughly) by the intersection of 2 circles, sort of fat number 8

The 39mm VHSB Dellorto has a height of 40.6mm

The slide total travel is 41.5mm from seated (it dosen't quite sit at the base of the venturi by about 0.1mm), even with the stop wound fully out.

At full throttle it clears the top of the veturi by 1mm.

Hence 41.5mm total travel.

The top of the tube within the atomiser sits level with the bottom of the slide when its all closed up.

With a K44 in the top slot and the 1/2 slot washer on top (lowest position), the needle tip is 47.29mm below the bottom of the slide.

From my calcs this means that in the lowest position the needle tip is:

(47.29 - 41.5) = 5.79mm into the tube at WOT.

Measurements are subject to the vaguries of the Harrison measuring stick and eyesight of course Hope this helps.

Richard, apologies, I misread/misunderstood the request in your post. As I said everyones calcs/software will be slightly different unless they are based purely on mathematical areas. One thing absolutely, completely, cast iron, certain . . . . . . . they are all in reality wrong

I'll give you an example:

To rely purely and blindly on theoretical calculations is to simultaneously seize both cylinders of your BRC on the dyno . . big style!! Can you feel the pain. Suffering is the best teacher

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Richard Gray]

Thanks Rily and Ian - That is perfect information for calculations for Shaun...if he is using the 39mm Dellorto...which one is he trying to compare? If it is the 41mm carb as indicated earlier, then does someone have the same infor for that carb?


I am not sure I understand why all of our calculations do not agree, unless we are dealing with different dimensions, assumptions etc. It would seem to me that if we all have the same starting point (needle into atomizer dimension) and then use the published data for the needle, then we should come to the same conclusions, correct? That said, the reason I don't use % throttle is precisely the problem already demonstrated. Instead I use mm of slide travel and calc the annulus value at 5mm intervals.
_________________
Richard


"Running into the sun but I'm running behind"