Hi
I am trying to get a really nice 47 DeSoto back on the road. I suspect the last owner bailed because of a problem I have discovered. The engine, which is a fairly recent rebuild, runs great until the choke begins to open. Then the vacuum sags to 15" and it runs LOUSY. I shot some WD-40 along the manifold gaskets, and it picked right up when the spray was near the center intake runner. I pulled off the manifolds, put a machinist's square on the flanges, they looked good. The gaskets looked pretty cooked; it was hard to see a definite leak path, although I am convinced there is one.
Rebuilt the heat damper, replaced the bolts mating intake to exhaust, and reinstalled assy. using all new studs and NOS nuts.
Used Fel-Pro gaskets, with a little Hylomar on the intakes and a little hi-temp RTV on the exhausts.
Same **** situation.Exactly.
Now I'm ready to do it right (and spend enough to ENSURE it's right).
Suggestions?
I have had many Mopar flatheads over the last 35 years, and I've never seen this bad behavior before.
H E L P

i'm not exactly sure, but i would recommend calling the good guys at www.vintagepowerwagons.com. they sure seem to know their stuff!!

Ward, Coat the manifold surfaces with machinist bluing or some other dye indicator(even a light coat of spray paint) and then using a flat sanding block sand very lightly in parallel strokes along the surfaces. Metal showing thru is high and indicator left is low. Maybe this will help determine where the leak is exactly. Another question to think about is: Do the manifolds get slightly uneven against each other when they are tightened down?

Ward;
When I installed the manifolds on my Desoto engine I hand first tightened the 4 bolts that connect the intake and exhaust. I then mounted the manifold assembly to the engine and tightened the 13 nuts and bolts to the specified torque. Once this was done I then tightened the 4 bolts that connect the intake and exhaust to the recommended torque. So far (2000 or so miles) I have had no problems.
Don Coatney

Is it possible that the manifold is cracked, and the crack open up once it gets hot? no amount of flatness or gaskets will fix that. Maybe try stacking two gaskets everywhere with some high temp silicon. that'll give you some extra "crush" to compensate for a warped manifold.