Race Engine Forum UK

Greetings from Australia Guy;

I have just discovered your new look forum/website and appreciate the help you offer.

I have a 2L motor in my 131, with extractors 4,2,1, mild head work and
a set of 40 DHLA Dellortos on a straight shot style manifold I welded up.

The car has been running like this for a while but I get the occasional back fire thru the carbs, with splutter when cruising at 2-3K rpm (idle to progression?)
I think they may be running a bit lean. They were straight from an Alfa GTV.
Can you suggest jetting to try for this setup? I'm sure the chokes are 32mm.

Thanks

First fit new pump jet diaphrams if not already new and check that all 4 pump jets are squirting properly with engine off and when throttle opened

Jetting 40 Dellorto DHLA on 32mm chokes:

145 mains (to be honest it may run on weaker 135/140, not sure)
No5 emulsion tube (No 6 also OK but a richer tube more suitable for higer-tuned motors)
180 or leaner 185 air correctors
45 or richer 50 or 55.9 idle jet
40 or richer 45 pump jet.

My jetting will not damage the engine but to get it perfect dyno test is advised. It should be 2.8-3.2% CO at idle, easily less than 400ppm HC if idle mixtures and balance correct, and under load from 3500rpm up 3.8-4.5%CO.
There will always be a slight bit of hesitation on the changeover from idle to main jet as the butterfly throttle passes over the progression drillings but it should not be so pronounced that the car is undriveable around 2500-3000 rpm. Weber IDFs are the worst.

That is just a start, there are so many things that can cause this characteristic, report back after rejetting please.

GC