+1 for a brake servo (booster) problem.
I had a similar issue about 18 months ago with number 4 cylinder. I'd discounted the booster as there was no burning of brake fluid through the exhaust.
Found out that the leak in the booster wasn't very big, and there was no noticeable smoke at cold start-up, or any at all when the engine was warm
However, after everything else was checked, there WAS enough leakage to affect the performance of number 4 sparkplug, in that I could never get a good colour on that plug, and adjustment of the airscrew made almost zero difference.
When I disconnected the servo hose, there was just a hint of dampness on the inside, where vacuum from the inlet manifold had sucked it through.
Changed booster, issue disappeared, air mixture screw worked perfectly again.
1963 Giulia Ti Super Rep
Last edited by GTA R; 12-09-2018 at 02:49 PM.