Stock 1957 Merc 312, BT manifold, Summit 600, 1” open spacer. 

Runs awesome cold and warm, but once it heats up, I get a very audible vacuum leak while driving once the load comes off the engine. Any speed, but it’s more noticeable around town with less road noise. 

The audible leak goes away as RPM gets close to or reaches idle.  Once hot, idle quality goes way downhill and becomes rough.
All bolts are tight to spec.  A propane search finds nothing.   A carb cleaner search finds nothing.  Changed out the rubber line to the distributor, no change. Capped it off at the carb, no difference. Swapped carbs, still there but seemed different, less or more depending on the carb.  Tried a rubber style insulator above the aluminum spacer, seemed a bit better, torqued down the base nuts a few inch pounds more and broke a mounting ear.  FVCK!
Checked intake bolt torque, all good.
Checked intake carb mounting base. Straight, no nicks.  No change. 

Today I pulled the intake. The gasket surface seems good with no sign of a blown gasket or change in seal pattern.
Scraped head surfaces, all look good there as well.  I looked VERY carefully at the rear of the intake where the power brake port was plugged. No cracks, no obvious issues.

I’m out of ideas. I’m thinking maybe an angle issue between the intake and the heads?  I pulled an ECZ-B off the shelf and I think I’m gonna try this next.  I’ll need to extract a couple carb studs broken off below the surface, (fun), but it’s better than this crap.  I’ll attack the ports with my Drexel just to say I did, but I’m sure for street putzing around it’ll make no difference at all other than making me feel better.
Save the BT for the aluminum heads I suppose. 

For the first time in my life, I’m actually out of ideas.  

Car is a 56 with Fordo.  This irritates the crap outta me because other than the dog whistle, this thing runs like a scalded ass cat. 

So are you telling me to use TWO stacked gaskets between the spacer and the manifold?  

I can do that of course.  I can put two under the carb itself as well. 

I’m running a 1” wood open spacer. I really like wood, the world’s best insulator, and in Texas that means everything. 

But assuming the metal centers on the crossover gaskets have ridges, I’ll remove them and reinstall the BT. I really hate to lose that manifold.  I’ll use the RTV, and hopefully, if lucky, I’ll have enough left to glob all over the internal gaskets in the carb and inside my FM as well.  Maybe seal the VB and servos up REAL GOOD.  🤣

Do you recall the part number for the truck intake gaskets with the restricted crossover?
Im going to see if the local Summit warehouse has some before I order from a mail order house 

Irritating

Forgive me but I don’t understand the vacuum leak in that area. I use an aluminum dual quad manifold with tubes thru it for the chokes and it’s never been a problem. I looked at my BT manifold and assume the clamping washer would hold the same. If it didn’t I think you would hear exhaust ticking or noise. Thanks. 

Intake gaskets. Not sealed well at the back.