11 replies to this topic

[rafael]

Hello,

I have been browsing around on this fantastic forum for some time. My compliments, Dave.

I want to jet my carbs (dhla40F). Alfa GTV 1971 with 1962cc engine

According to some info on jettings for alfas, 2 setups become clear:

ch m.j em.tube air i.j
32 145 7772.10 240 58
32 150 7772.11 210 57

I try to understand the diffs:
the 7772.10 with start the main jet earlier, and that can be counteracted by going to larger air corrector. Does that make sense? Which one is better?

I am using now the 32 150 7772.11 210 58 setup, Well to be honest I am using 3x 150 and 1x 148 which is of course not correct
I notice that:
- the idle adjustment screws are out about 6 turns which is a lot
- there is a gap between 2000 and 2500 rpm

According to me the gap can be caused by:
- too small idle jets
- mismatch in main jets

Questions:
- should I buy 4 new 150 jets to make sure that they are all equal (alternative is to mix 3 used ones with a new one).
- which idle jet should I be using? On this forum I read 60 to 65 as the best one. I wonder why alfa did not install larger idle jets in the first place. Larger idle jets can also be explained from the amount of turns of the idle adjustment screws.

[davesideways]

Hi.

Put the 148 main jet in the rear most cylinder, that will run the hottest usually, so can run leanest. Ignore main jets. Unless changing to 155.

Try 60idle jet.

Alfa used small idle jets as the engines were new, not over-bored, ported, worn, missing airboxes etc. These factors mean you will find an older engine needing more jet size?

If you use plain airfilters the engine will need more idlejet size than the same engine using an airbox. Factory cars had airboxes?!

32 145 7772.10 240 58
32 150 7772.11 210 57

Which is better? The better setup is the one that works the best. Hard to say, the 10tube is for smaller CC engine, so 10 tube will operate earlier in a large engine, yes, the air corrector is bigger to slow down the 10 tube, yes, the main jet is smaller as the tube is richer/earlier in operation. You get the right idea, but there is no best answer. Proof is in the eating.

I advise fitting 60 idle jets, if that doesn't work try a 200 air corrector or 155 main jet.

Ideally you should try :

32 155 7772.11 210 58
32 150 7772.11 210 58/60
32 150 7772.11 200 58/60
32 145 7772.10 240 58/60
32 150 7772.10 240 58/60

Expensive though, but that's all the factory did, try jets till they got it bang on.

6 turns on idle screw is ok. Often when changing the idle jet there is little need to change the idle screw, as the system was never marginalised or overloaded in the first place, so there is no effective mixture change at the idle screw 58 or 60, the low limits of the idle screw's metering capabilities have not challenged the idle jet. During progression the idle jet feeds the idle screw and progression holes, so don't expect to be down to 4 turns with a jet change.

[rafael]

Dave,

Thanks for the quick reaction!!

Yes I use the factory airbox. Engine is stock with high compression pistons (10:1), whereas stock is 9:1

On the pump jets. Somewhere on this forum you mention squirts of 1 meter for 1.5 seconds.

That is with 33 jets about 8cc for 20 pump actions for a single carb (2 throats). Is that correct, or should I have 8cc per throat (20 actions) => 16cc per carb (20 actions)?

[rafael]

short update:

this is the new setup:
32 150 7772.11 210 58
main jet cylinder 4 is 148, as suggested by Dave

pumpjet 40 (downwards) with a volume of 8cc for 20 pump actions per carb (2 throats); was 6cc in previous setup.

Car behaves much better. Stable idle. picks up immediately on throttle. Gap between 2000-2500 disappeared

idle adjustment screws adjusted with carb tune.

[davesideways]

Often bad running/misfire in low-end/light throttle is down to the carbs airflow balance...

8cc good.

[rafael]

Update from my side.
I have been trying different set-ups over the last days. I test each set-up by driving the car to detect smoothness and flat spots. Besides that I measure acceleration from 70 to 120 kmph in 5th gear.

Some results:
Idle jets 60 work better than 58. More smooth operation, and better acceleration.

Tried the following:
32 150 7772.11 210 60 best
32 155 7772.11 210 60 second
32 145 7772.11 210 60 worst

clearly 145 main jets are too small for 2000 engine. 155 too large (rich)

Still to do is 32 150 7772.11 200 60 which should be equal to best one so far except that the main will enter earlier due to smaller air corrector. Is this correct?

After that I will try the 7772.10 emulsion tube options

[davesideways]

With 200 versus 210 it'll just be a touch richer all over the higher rpm + the overlap from the idle jet to main jet will happen marginally earlier; a result of the main jet operating more aggressively; it'll fire earlier with less vacuum/depression. It'll be "richer".

10 tube will be alot richer in mid rpm.

[rafael]

Another update:

I have tried 4 additional combinations. The results are from good to bad:

32 150 7772.11 210 60 best
32 150 7772.10 240 60
32 145 7772.10 240 60
32 150 7772.11 200 60 worst

Results based on average acceleration times 70 to 120 kmph in 5th speed in two directions (up and down the highway).

This concludes the experiments, which confirm the factory settings except for the idle jet (58).

[davesideways]

No suprise there then

Still it's nice to know. The progression/pickup from low rpm is probably the most important thing, so check it still works well in town/slow driving.

[Magnus K]

Rafael and Dave:
Interesting thread! I'll think a bit more about this when trying to jet my own Alfetta GTV with DHLA 40Fs in the spring/summer using a VEMS wideband lambda sensor.

My current jetting:
32 150 7772.10 200 60

Was thinking of switching to:
34 145 7772.8 200 60

One question though... My car is a track-day car and I'm thinking of switching from 32mm venturis/chokes to 34-35mm venturis/chokes. Good or bad idea? I would like to cure a lack of power above 4000 rpm and wouldn't mind sacrificing some of low/mid-range power.

EDIT:
I should add that there seem to be some problem getting 34mm chokes from dellorto.co.uk. Should I go for 33 or 35mm if asked?

Best wishes
Magnus

Edited by Magnus K, 15 January 2012 - 08:25 PM.

[Magnus K]

I got the 34mm venturis/chokes and 7772.8 emulsion tubes so later in the spring I will give this setting a try:
34 145 7772.8 200 60

Best wishes
Magnus

[davesideways]

Didn't see this.

I think you will find the 34mm chokes ok, 35mm too big unless tuned engine.

You had best try to see the results.

The jetting you provide should work ok.