Uneven running

[Brett C]

I'm wondering if the collective expertise can offer any suggestions on the following:

My spider has been running unevenly at constant rpm. It's like a slight miss every few seconds, but only happens at constant throttle settings and is most noticeable below 3000. However, apart from an initial hesitation, it runs smoothly under acceleration. The car is running electronic ignition and I have replaced the plugs and leads, but no improvement with this problem.

I am thinking it's a carburetion issue and have some kits on hand for them as soon as I can find the time. I have no experience with side draft webers, so it would be nice to pin point the problem before I tear into them. Incidentally the fuel pump is new as are both the fuel and air filters, and it does seem to be running rich.

Any thoughts?
Thanks, Brett

[Mick A]

Having a specialist adjust the fuel mixtures may help with this problem.

[giulia_veloce]

Agree with Chowderboy

Might be as easy as turning the idle mixture srews out half a turn,to leaky needles + seats,to even a vacuum leak at the brake boosters.
Need more info= Engine capacity,year model,,carbys= Weber or Dellorto,who touched it last time= an Alfa mechanic or a backyarder.who fitted the elect ign,was it set with timing light.Whats the air fuel mixture at idle?tappet clearances ?
Compression test?are the 2 carbys balanced ?,cracked carby mounts etc etc etc

Ok,,from what you said= constant miss at constant throttle is usually lean idle jets,or incorrectly set idle mixture screws,,but read above.
As usual,,it could be an easy fix,,but many choose to do it the hard way.

Ok,,im bored,and impatient.and wants to go to Grafton rally now,not in the morning
So to keep me sane,,I wrote the above.

Hope it helps

Robert

[Brett C]

To fill in the blanks:

It is a '73 Spider 2000 with Webers. The car has a total of 80000 miles and I don't know if the carbs have ever been overhauled. I believe the car was always maintained by Alfa specialists until I bought it 3 years ago. I assume the electronic ignition was fitted by them some time ago as it is an older unit.  I had Marenallo Pur-Sang sort out some issues it had initially but I have maintained it since then. I have not messed with the carbs except to remove them to fit new mounts and give them an external clean. I believe the vacuum system is tight as I had a lot of trouble with the brake system initially and went through that pretty thoroughly.

I haven't checked the timing although since the problem seems to vanish under acceleration I was not expecting it to be ignition related – but I'll happily stand corrected!

I will have a go at resetting the mixture screws first.

Thanks
Brett