Fiat Twin-Cam 1800 Weber 40 IDF carb jetting

[Marc]

Hello Guy,

I have a 1756 TC with 40/80 Alquati cams, Plex ignition, Abarth Valves, Springs Keepers, Dual 40 Idfs, waffle intake, 9 to 1 pistons.
Headers free flow exhaust.
I am using a set of IDF 70's, VW stuff.

She seems to be running a bit lean coming off idle progression circuit, sneezing a bit. I started with the stock twin carb jets for a 1608. I did not change the pump jet, might be a little short on squirt time, its a 50, do you think I should rejet everything to a higher volume of properly mixed fuel or try a longer pump jet like a 40?

[Guy Croft]

Marc, hi

She'll need enrichment, bigger motor, quite highly tuned, so check this out.

std 40 IDF 1608 jetting is:
choke 32mm
emulsion tube F11
main jet 125
air corrector 210
pump jet 40
idle jet 55

Suggested jetting for your 1800:
choke 32 or for more power 34mm
emulsion tube as 1608
main jet 135 or 140 on 34mm chokes (1608 too small-lean)
air corrector 175 or 180 (1608 are too big-lean)
pump jet 45
idle 55

Now, apart from jetting:

1. Check that the pump jets really work well and if not replace the diaphrams. If they still don't work well take out the pump jet bypass in the float chamber and make sure it's not seized.
2. Make sure the float level is right - it should be 10mm from float to cover held in the vertical posn without gasket with the float needle in light contact with its seat.
3. Make sure that the 4 off 2-piece idle jets located in the upper, outer body of the carbs are not blocked, and that the 'O' rings are in place.
4. Idle mixture and balance (barrel to barrel and carb to carb) are critical, searcb by IDF on this site and you should find something already written by me on this.
5. Do a compression test - a bent valve can cause spitting.

That should get you started I hope, come back to me on this.

GC

[Marc]

Thanks Guy!

I had no reference for an 1800 with dual carbs,I will rebuild them,and re-jet to your no#s...I have the right idle/ progression and emulsion tubes,and 32mm chokes, will add the pump jets and the mains and air correctors to start,she is lean as it stands now. I will check the floats and pump jet group. How much low speed flexibility will I have with 34mm chokes? Thanks again Marc

[Guy Croft]

Hello marc

flexibilty, not precisely sure, bigger chokes add top-end torque and reduce low-end, but not in this case I think to the extent that you would find it unpleasant to drive.

G

[Marc]

Just what the doctor ordered! Pulled the head to check everything, put in a competition head gasket, changed the jets, power is like it should be, no sneezing!
Power is now intoxicating!
Can't stay off the gas pedal! Think I"ll go MGB hunting. Good thing I just rebuilt the gearbox and driveshaft, all this power would have caused a driveline failure in no time. Can you tell I'm having fun!? Thanks again...

Regards Marc

[Guy Croft]

Thanks for the feedback Marc,

yup, once in a while I get something right!

GC