Heat related vacuum leak?

[Godwin]

Engine in the 60 is a 2F that was rebuilt about 30k miles ago, and I think this problem has always been in existence since the rebuild. Engine is desmogged and has the stock intake and exhaust manifold. When cold and at an idle of around 900 rpm the vacuum is 17". After reaching operating temperature of 185F the vacuum drops to around 13" and will drift up and down a bit.

So if this is a heat related vacuum leak what are the likely suspects? Crack in the intake manifold below the carb? Pulling the carb and taking a look would be easy enough but would a crack be evident and if not how to check for one? What else could cause this drop in vacuum?

 

[tetonguy]

EGR Valve?

Y0u said its desmogged? Is the EGR valve still installed but vacuum lines shut? If so the plunger in the valve itslef may not be seating closed. if it is open you will loose vacuum there. Mine was stuck open about 1/8" inch and on initial visual it looked like it worked, took a good soaking in carb cleaner to fiind and fix it. Any RPM change when you spray carb cleaner around the manifold? Might try listening for audible leaks with a stethescope or a tube to your ear too. Any real variation on vacuum at higher RPM's or is it only a heat symptom?