Carburation quiek?

[StescoG66]

Wonder if anyone can advise. First of all engine is Alfa Boxer 1.7 8V with twin 40 Weber IDF
Car runs great, pulls cleanly from low rpm and revs generally cleanly with no splutters or stutters other than being a bit of a pig when cold.
However, on motorway on a steady cruise I get a strange pulsing sensation. This seems to be when cruising at about 3000rpm on very light throttle - apply even a slight bit of throttle and it disappears. Carbs were running a bit rich (have since adjusted) but were recently cleaned plus I balanced them too. Tickover when warm is pretty good, the odd little miss, but I recall this being normal on the boxers.
Any thoughts on where I should be looking?

Thanks in advance.

Addendum. A bit of research indicates this is about the point it goes from idle to main jets. What should the jet size be?

Edited by StescoG66 on Tuesday 30th May 05:51

[StescoG66]

Mods my sausage fingers - title should read quirk

[richhead]

if its at steady throttle at speed, it may be the beginings of some carb icing happening, can you try getting the intake somewhere warm?
If it happens at lower speed aswell, it sounds like a jetting issue, maybe get it on a rolling road with someone who knows about carbs( not so easy now)

[gazza285]

I chased a fault on a poor running Alfa for months, eventually traced it to a porous inlet manifold on one bank.

It ran great on WOT though, so that’s how it was used until it fell apart.

[StescoG66]

Thanks folks.

It does seem to be on a steady light throttle at speed........

However tonight I checked the mixture screws and they were all over the shop. One was under one turn out from seated, while another was nearly 5 turns out.

I have taken them all to 2 1/8th out from seated will see how that fares. Will double check ignition timing tomorrow.

[Sardonicus]

Sounds lean to me so your mixture/pilot screw suspicions may be right over advanced ign timing can give a similar sensation