How is jetting worked out for carbs?

[jamezz666]

Hello Everyone,

so i have a 2.5 opel CIH and i'm planning on fitting twin weber 38 dgas carbs, but i've always wondered how jetting is worked out? is there a guide?
For my twin 45's on my other car, i used the guide in the link here www.s262612653.websitehome.co.uk/DVAndrews/webers.htm But that's for twin Dcoes ect and no other model of webber carbs.
I've struggle to find any info on how to calculate the jets you need, or a rough guide..

The standard carbs which are hard to get hold of, was twin Zenith 35/40 so i believe the 38's would be ok if jetted correctly.

using the guide in the link for tuning Weber twin dcoes ect, it gives me this for my dgas 38

Main 152
Air corrector 202
Emulsion tube F9-F16
Idle Jet 65
Idle air F8
Doesn't give you any other calculations..

Any help would be much appreciated.

Thank You

James

[tonybraz]

Hi I have used this book, well worth a read, used it to set a carb on 750 Formula race car

The Weber and Dellorto Carburetor Expert - Des Hammill

Tony

[gib]

As mentioned you need the graphs from the books. Once you have these you can work out the relationships and drop this info into a spreadsheet as the picture below. i did this for DCNF carbs luckily they are mostly linear. You will also need the descriptions for what each change does.

Weber suggestions by creativeq, on Flickr

The final part is all the sizes are for old school 100% fuel not the current stuff so for all calculations its worth upping the petrol side by 10% and go from there.

[melle]

Good advice above. Also remember that bigger is not necessarily better, which seems a common misconception. Read John Passini's book(s), this will give you a very thorough understanding of what you're working with. I can send you (and anyone else) a PDF of the Weber Tuning Manual if required, just PM me your email address.

gib, I like your spreadsheet, much better than the recycled envelopes and notepad pages I keep losing!

[gib]

@ Melle Thanks us Saab owners need to stick together !

Saab at Brooklands NMP by creativeq, on Flickr

Also if any one wants the .xls file it is for DCNF's as mentioned but you could use it for other weber set ups you would just need to change a few calculations.

[melle]

I'll PM you gib .