As I'm working on a 260z engine, I found somebody decided to plug some holes in the balance tube by using two 10mm straight thread cap screws (bolts). The original holes were tapered but they forced metric cap screws into the holes, ruining the threads. Notice they were only able to get the screws in part way. I've attach a photo, I circled the two locations. Without drilling the holes out I was able to thread the holes with a 1/8-27 tapered pipe tap, thus allowing me to use 1/8" plugs. All other plugs I've found seem to be the proper threads.

Jim

Well let's hope fixing those two plugs helps your running issue. If the two 10mm head bolts were screwed in somewhat tight, they probably sealed "mostly". If those bolts are the only vacuum leak you found, they probably weren't large leaks.

Oh, and I don't see anything else glaringly wrong in the pic, but was wondering... When the flat top to round top conversion was done, did they (you?) use a new intake/exhaust manifold gasket, or reuse the old one?

I can't remember on the gasket, swapped carbs years ago. Going down another rabbit hole now with brake problems. Can we still get the gasket?

I was considering a master vac leak, pumped on brakes, pedal went to floor, resivor for rear brakes was dry, filled up, fluid going somewhere. Rebuilt cylinder, now other issues. Should put that on hold and get back to vac leak problem.