I'm having this SAME problem with my Twin Zenith Finny auto.

Idles fine and low in Park or neutral....just DIES in gear.......SOMETIMES.

My spidey senses were telling me it was a vacuum leak......related to auto vacuum demand.....???

When I checked the 'rear of intake manifold' to auto vacuum supply tube 'banjo fitting, it seemed a tad loose.

When i put the 14mm spanner on it...the head just broke straight off (and NOT coz I had eaten too much of Popeye's spinach). Seriously NOT COOL for my delicately balanced and extremely sensitive Zeniths!

Took the same bolt off the parts car and it was fixed.....for about 30 mins driving...she ran like a DREAM.

Now my fuel pump is sucking OIL out of the sump (through a stuffed and "no longer available" diaphragm???? I noticed a tiny little bubble of insoluble liquid in my glass pre-fuel pump filter, The pump seems to "lose prime" when let sit, and today I saw the glass filter fill with air bubbles immeadeately as I pulled the dipstick out .....Ah HUH !!!!!) and feeding it to my DELICATE carbs....so I have cleaned the fuel pump with the little 'hand-prime' lever off the parts car and will install after I get the little cork gasket that lines the mesh filter screen!

I don't know if this little tale helps YOU at all....but I feel much better!