some of you might remember the air sucking issue i experienced last fall, i went thru 3 intake gasket sets and even with the thick ones from Best Gaskets (14046 E), it still sucked air. not as much as with the thin metal ones, but enough that the engine would stall when brake cleaner sprayed on between manifold and cyl.head.

now, i took the manifold down again, cleaned the surfaces and mounted it without gaskets but with marking paint on the head can now clearly see that the 2 surfaces are uneven and only seats correctly on the 8 spots where the bolts are. looks like the manifold was tightened too much.

i have to find a machine shop that can make these surfaces even.

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How are the surfaces in heads? That could be checked with metal ruler.

did not check that yet, but i guess the manifold side is more prone to be uneven.

I would double the gasket thickness ( two thin or two thick gaskets both sides ) and see if that would stop the vacuum leak — as you know you manifold or head matching surfaces  are somewhat irregular from your matting  paint test 

A good trouble shooting matting test—- for sure 

Should be the an easier fix than other options if it works — worth a  try ! 
A machine shop can plane the manifold and the head but much more involved and some expense as well 

Not sure if you need to be doing both heads and the intake surfaces 

Machine shop will advise on that 

Many have issues with our intake manifold seal  ( to many )  leaking air ( vacuum leak ) which cause engine performance issue ( cylinders running lean and hotter than normal ) 


after a winter of filing and fidling both surfaces of the intake manifold it still sucks air on one side, not as much as before, but still not a good idle when hot.

when you look at the new pics, the marking paint test before i mounted the manifold wasn't that bad, especially when you compare it withe the first pics.

guess i have to find a machine shop in my area :(


Taking look at your photos...have the forward ports been ground on?  They look off-set a smidge to me, but might be camera angle. 

These Caddy intakes are interesting to me. The opposite of a performance manifold. Short runners heading mostly uphill.   Works good for torque, not so good for revs.  

But, of course, who takes their 390 or 429 to redline? Even the transmission short-shifts to prevent that. 

The red line on both the 390 and 429 is ——4,600 rpm so who is looking for revs from these engines ?

These engines are about torque not revs at that redline 

480 lbs of torque is a lot for any gasoline engine to be making

Torque is addictive I have been told —making our year Cadillacs a  very unique driving experience

The 429 engine with the TH-400 trans is an amazing unique combo and driving experience  and why I own mine for 59 years 


Tony, I totally agree with you on the torque.  That's a main reason these cars feel so nimble.  

My former 1961 Series 62 coupe with only 25k original miles did not feel anywhere as snappy as does my '64.   Yes, it's a different engine and different transmission. That's my point. The 429 and TH400 is a great combination. 



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