Hi guys, I have twin Dellorto DHLA 40's on my A15 and the rear carby is backfiring every now and then, but not at idle and not at WOT, mainly when cruising doing about 2500-3000rpm, my first thought was that it could be the rubber seal inbetween the manifold and carby letting air in and leaning the mixture? any thoughts?
Also does anyone know the normal position for the idle air mixture screw, as in how many turns out.

The normal position is tune-by-ear. In other words, do not set it "to a certain position" and leave it there. Tune it to best lean idle (as with any carburetor), and synchronize the two carburetors (as with any twin carbies).

Turning the idle mixture screw to a certain position is a probable cause of the backfiring.

hahaha sorry dd, think i didnt explain properly, what is the STARTING position for the mixture? for example 2 1/2 turns out then tune?

I had a similar problem with my 240z (triple 45 dcoe webers) it would cough and spit at light throttle openings.

I stripped and cleaned them out and also replaced the soft mount gaskets.
It still coughed and spitted.

In the end I re-set the linkages and it was all good.

Maybe this could be your problem, check them first before spending your hard earned.

Cheers,

G

I would like to hear some options on this also?!
I have a single side draft 45dcoe on my mild a12 and that tends to give off a little bang when I blip the throttle to downshift, gives me a bit of a fright sometimes. Though bit may have been too much fuel, ie. running just a little to rich... But not sure, haven't had the chance to have it dynoed yet so could be anything...

Synchronize your carburetors. I don't use a "starting setting" -- just adjust them for best lean idle. But just as importantly, synchronize them (adjust the linkages dynamically).

and use a manometer to adjust air bypass valve, im aware of ALL of that :) but with all carbs ive always started with the mixture screw about 2 turns out and adjusted back in to lean it out, until you can hear a change then bring it back out til it runs better, that was the position I was wondering about, where people 'start' from hahaha all good.

2 turns out is the rebuild setting. Not an adjustment per se.

Yes, it is "best lean idle", just as you say:
Quote:

You can use a special synchronizer hardware or manometer -- or just a piece of hose to your ear. When both have the same tone, they are in sync.

ok cool, that's not a bad idea, havn't heard that one before lol
cheers for the feedback guys