IDF Tuning.

[sptcoupe]

A sealed up bleed back is the same as a 00, which means all of the gas that is pumped into the venturi by the acelerator pump when you acelerate actually gets into the fuel air mixture. In some cases, it may be just right with all the gas getting into the fuel/air stream, but it may also be too rich. Various sized bleed backs let you properly adjust this circuit. On most performance modified motors a 00 or 35 is the right answer. On my 1800 motor, I was running 25s, and have had anywhere from 00s to 55s on various modified engines. Think of the jet size as representing what percent of the gas that the acelerator pumps pushes toward the pump jet is bled off and returned to the float bowl. So an 80 will "bleed back" about 80 percent of the charge to the float bowl. A lot of guys will seal off the usually leaner jets that come with the IDFs to get a 00-sized effect, but it just takes experimenting to get the car to run absolutely smoothly. If a sealed jet works, then great. If you have a stumble on acceleratio it is likely to lean. A puff of black smoke from the tail pipe when you acelerate and it is too rich.

Do not change the bleed backs and the idles or any other jet at the same time. I always get the idle and full throttle circuits dialed in first, then do the acelerator circuit. Do one one circuit, one jet at a time.

I think you are very close. I would focus on the idles and get a 58 in there, as I think you are lean in that circuit with the 55s. They can be hard to find, so try a 60 and see if that smooths it out from idle to about 3500 rpms. If you can't get it to idle smoothly with the mixture screws at least 1/3 a turn out, then you are probably too rich with a 60. If you have to turn the screws out more than about 3/4s, you are likely too lean.

I also would urge you to get some sort of fuel pressure regulator in line between the fuel pump and the carb inlet. The IDFs are sensitive to fule pressure, and only need 2.5 - 3 psi. If you are just a bt out of that range, you won't know what you are chasing when you have stumbling or uneven idle, etc.

[lanciahf]

Success! I swapped the 55 idle jets with 60s and 175 needle valves for 200s and all is well. The car is nice and smooth with no hesitation. I do have a idle jets in the size of 57 that I might experiment with. Thanks for your help!

[sptcoupe]

Glad it worked. I would definitely try the 57 idles.

[lanciahf]

I tried the 57 and the car still went pretty good so I might keep them. but but but

The idle remains uneven, with pooping on deceleration which leads me to think the throttle plates are still open on one of the carbs. This evening I took everything apart again (SORRY GEORGE!!!)and noticed this on Carb # two. I saw it before but I assumed it was okay since I bough them from a fellow Fiat owner. Looks like the shaft was cut down and hammered over to secure the central linkage piece. Unfortunately the piece is not locked down and has enough play to effect idle. Since there are no threads left I was thinking of having a welder tack the piece to the shaft????

Maybe I should back up,,,, the carbs are Series 70 VW type mounted on a OEM Fiat 124 Waffle IDF manifold. Looking at the lack of room between the two carbs maybe the series 70 can not be mounted on a waffle manifold but only on the PBS manifold? Anyone know for sure?

Also CYL 1 air bleed screw has to be unscrewed out a lot to balance with cyl #2. I noticed when I got the carbs that the # 1 air bleed screw was out a lot more than the other three. Like three times as much. As a consequence CYL#1 needs more turns of the mixture screw to smooth the idle. BTW I verify the air balance with a nice tool by SK. Any idea why this is?

Should I just give up up and look for a set of series 13/15 IDFs? Anyone have a set for sale?

[mdrburchette]

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