weber idf 40 jetting

[Adrian3]

i know this has been discussed many times but i can't seem to find recommendations for UK petrol and for the following set-up:
Engine is 1720, standard cam and i want to keep the 28 venturi.
i will be getting a rolling road session but just don't want to damage the engine getting there.
So, any successful combinations of mains/emulsion/airs/idle/pump numbers would be good.

[Timo]

Hi Adrian,

40 IDF's come out of the box with 28 venturi, 115 main, 200 air correctors, F11 emulsion tube, 55 accel. pump jet, 50 idle. They will run ok on a 912 but will be kinda restricted.

Ideally you want to step upto a 32mm Venturi and then change the jets to suit. But might be best to wait until rolling road time to do that, costs can soon mount up if you end up buying everything twice!

From memory I think mine is as follows:
32mm venturis
125 mains
52 idle
180 air correctors
F7 emulsion tubes
55 pumps (I think?)

Best of luck

[neilbardsley]

I'm pretty similar to Tim but 190 air correction.

My mains said 115 but the RR found they had been drilled and weren't the same.


However, my 912 engine had 165 main jets. Worthwhile doing a RR

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[Adrian3]

thanks guys.....i'll be at sub 3Krpm most of the time so28 venturi's should be ok for me.....thanks Neil, hadn't thought of PO's drilling out jets, i guess there must be some sort of jet gauges available.....do the sizes denote hole size? e.g. 200 is 2mm?

[neilbardsley]

Sorry I was talking about the main jet size.

Have a look at Bruce's article

http://www.sparkingplugs.com/3.html

I strongly suggest you aren't below 3k mostly. Peak torque and happiness in these engines is 3-4.5k. The switch between the idle and main circuit isn't discrete but generally complete by about 3k. The main circuit maybe be dominating long before that.

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[Timo]

Adrian, if you are staying with the 28mm venturis for now please don't use mine or Neils setup as reference, the correct main jet size is determined by the venturis you are running to a degree. There is a ratio, I think main jet should be 4 - 4.25 x the size of your venturis, so somewhere between 112 and 119 would be your ideal size.

According to this ratio 32mm venturi main jets should be somewhere between 128 and 136, however RR suggested mine was happier on 125 mains.

[neilbardsley]

Tim I have 28mm chokes. Sorry I should of mentioned that before.

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[robs912]

I've found that the only way to get the jetting really spot on is lots of expensive time on a rolling road or install an afr gauge and you can do many hours of driving under different loads to get it just right. All part of the tuning fun!
32mm vents
125 mains
F7 emulsion
47 idles
220 air correction

Cheers Rob.

[Timo]

Neil, I should have realised, hence the 115 mains.