Race Engine Forum UK

Hi Guy, I finally got the 13/15 Weber 40IDF's I purchased from the GC website on my 1976 1800 124 Fiat Spider with electronic ignition.
The front carbs both show a reading of around 7 on the synchrometer.
The back carbs both show a reading of around 3 on the sychrometer.
The fuel pressure is 2.5lb's
Adjusting the Idle mixture screws doesn't make much difference.
Could this be a fuel pressure problem?
Thanks.....pete

no else both carbs would be affected.

You need to adjust the balance betw carbs via the balance quadrant.

G

Hi Guy, I thought that the carbs had to have the linkages between the carbs disconnected before balancing can begin.
Is it your belief that these carbs can still be balanced when the starting point (carb linkages disconnected and air bypass screws closed and idle mixture screws turned out 1 full turn) and initial readings showing 10 on the front pair of carbs and 3 on the back pair?...pete

No you don't need to disconnect one carb from another but you certainly need to hook them together to get the carb-carb balance right.

Have a look at this photo Pete.

If each carb is flowing equal on each barrel then the problem is likely just carb to carb. But you don't have to have the carbs balanced relative to each other to do the barrel settings.

Tuning these is a bit of an iteration. As you optimise the balance from barrel to barrel and carb to carb and get the mixture settings perfected the idle speed climbs and you have to relax the idle speed screw to keep the engine at 850 rpm.

You have to have the engine at FULL operating temperature to perfect the settings, so you have to get them the best you can, drive the car a few miles and then do it all again.

You'll notice - betw 750-850 rpm - as you close the idle mixture screw the engine starts to miss on that cylinder as it's too lean. Or should! As you approach the optimum idle mixture (and it MUST be at that kind of speed) the engine speed will climb and then as you wind the idle mixture screw out the speed will stay the same and then fall off again as you massively over-rich. So - to a point - best idle mixture will correspond to highest idle speed. If the engine pops and bangs when you lift off the throttle after hard accleration in 2nd or 3rd gear - one of more cyls is TOO LEAN.

Colortune is by far the best tool for setting these - see links.

I hope this helps a bit to begin with,

G

Thanks Guy, your instructions and picture are great. The engine is running smoothly, just waiting for the colortune to get here to see how close I got....thanks again....pete

Good-oh!

Thanks Pete,

G