alfa sprint - popping under deceleration

[ar15625]

Hi all,

I have recently got a 1981 Sprint Veloce (1.5 dual webers) and its seems to be out of 'tune'. the problems are:

- when you take your foot of the accelerator pedal, the car backfires (pops) through the exhaust as the revs drop off...(say from 3500 back down to 800)

- the drop off the revs is rather slow (takes 2-3 seconds to return from 3500 back down to 800)

- The spark plug colour looks fine (greyish brown and clean) so I guess it running with the correct mixture (possibly lean)

It runs a bosch distributor standard (with not electronics like later sprints/33s)

My question, it this likely to be timing or fuel (or both)....

The car accelerates and drives well. I checking the timing, I have 4 timing marks on the flywheel a "T" and three single lines "I"

My questions,
- it this likely to be timing or fuel (or both)....
- any idea which of these marks is the static advance ?

thanks

[jol]

The boxers have quite a overlap in the cams, ie. both valves are open for some time. Air/Fuel is drawn into the exhaust where it ignites.

As for ignition;

[ar15625]

thanks. Here are the timing marks I have....
As you can see I have a "T" and "I" and "I" followed by "II" ....

I plan to set the static timing, at idle speed with vacuum off using the first "I" ....

[Oelholm]

The popping noice is one og THE most well known characteristics of the Boxer Alfas - Enjoy it

In Denmark we usually say that the 'Suds "fart"...

[ar15625]

thanks. I think the static timing is actually set at the "T" mark and not the "I" .... will check it out again tomorrow....

[Greek]

Yes it is a feature of the boxer engine especially if there is the default excaust and not an ansa or form a 33 s2 .

If your carbs are not tuned , the pops are just louder or more often by the way...

[giulia_veloce]

Hey Guys.

The popping is normal and can sometimes sound good.
Its caused by a lean micture in the idle circuit.
Sometimes cured by adjusting the idle mixtures correctly,at engine operating temperature.
The right way is to go up 1 size in the idle jets and reset the idle mixture screws.
Car will start easier from cold,the progression from idle mixtures to main jets will be heaps better,and goodbye popping.
Idle mixture jets work from idle to about 2,500 rpm.
I have done many idle rejets for my customers cars,and they love it.
For the cars with points,make sure the point gap is right and the ignition timing is correct before rejetting.Same goes with electronic ignition.

Robert

[33gold]

I concur it's too weak if popping, a burble is right not pops IMO