Race Engine Forum UK

Hello Mr Croft,

Following your advice just joined this nice forum. Please help me with my 40 IDF jetting problems regarding my Fiat 131 Racing, it's got:

8mm dome pistons
42in-38ex fully ported head
4-1 exhaust, 55mm endpipe
Abarth profilo 068 cams in & ex
40 IDFs on Fiat waffle manifold slightly opend from 32 to 33,

Current jetting is choke 34, F11, main 150, idle 55, air 210, and the car does not perform as it should...

Thanks in advance,

HI, welcome!

Thank you for posting here.

Assuming 2 liter and 110 degree cam timing, with Weber 40IDF I'd start with:

34 choke
F11 emulsion tube might be OK but the richer F9 may be better
mains 145
idle 55
air corrector 180

Make sure your ignition timing is 10 deg +/- 2 deg at idle (850rev/min) and max 34-36 deg total (static plus centrifugal) at 5500 rev/min.

It is critical that the carbs are balanced and the idle mixture screws are set. It is also important that the pump jets work well. If this jetting does not improve matters let me know here as soon as possible. Long-distance 'fault-finding' takes time but I am better at it than most.

Also (from 'search = IDF' facility) this is worth reading:
http://guy-croft.com/viewtopic.php?t=16 ... ht=jetting

GC

Mr Croft ,the abarth 068 cam lobe centers are not 110 deg, but 99.5 in and 100,5 ex. What do you think of 36 chokes?

Thanks for the fast reply,

Hi

I would not run that timing on 40IDF:

068 cam lobe centers are not 110 deg, but 99.5 in and 100,5 ex
Reset cams to 110 deg, they have too much overlap for those small carbs. If you want to maximise power go to 44IDF.

As for 36 chokes you can use these but I maybe would not do that in 40IDF, the secondary choke fuel signal will be poor, better to stay with 40/34 or go to 44IDF with 38mm chokes. These carbs are hard to get in paired setups, but I have some good used 44IDf for sale on behalf of GCRE.com member Leon Bester.

If there is a pickup/ acceleration problem it is not a choke size issue. Choke size (strictly related to carb size) controls the torque response - small chokes for bottom-end, big chokes for top-end power.

I assume, from your obvious technical knowledge, that you are running one or two straight-through big-bore silencers - if not you could have a back-pressure problem.

Forgive me if I do not spend long justifying my technical answers, but you can trust what I tell you.

GC