Formerly dead reliable FP 40.  Starts but after a minute of rich running goes dead lean an surges all over the place.

Wont stay running,

It has about 5 gallons though it then put in a box for a few years. 

This inst a runaway, I know what that is about or just a lean needle.  Not overly hot.

It seems like fuel flow problem. 

Have flushed out needle, replace lines, changed fuel, added a filter , 2 different new tanks  leak checked tanks. 

Changed plugs,  does it with the glow lead on or off. 

Is this an air leak? 

One of the renowned here had a NV clogged with castor as ' He Hadnt flown it recently ' .

Said bits broke off / lean/rich/lean/ etc etc as the debri in the spray bar itself disolved in flight .

Sounds Similar ! .

  I had an O S 46  A that was doing the same when running half/half oil 5-10 % Nitro, tried other tanks, all the other stuff, then I ran it a couple times on 22% all synthetic fuel and it straightened up.
  Needless to say I didn't exceed 9500-9800 R P M ..
  This is a suggestion only, your mileage may vary, so beware...

If you know there's a problem in the fuel system, why take the engine out?  Or do you just wish to verify that it works before going on?

I want to reiterate what Kennith said about the O-ring -- these get hard over time, and seem to shrink.  The worst ones will just crumble off of the venturi.  They're a standard metric size, so if things still fit correctly use that -- otherwise use an inch size (metric O-rings about that size have a 1mm diameter ring, while inch-sized use 1/16).  I would check and replace as a matter of course.  I keep a bunch of O-rings on hand for just that purpose.

If it doesn't work on the stand after replacing the O-ring, check the crankcase seal.  There's no reason it should go bad if you haven't been messing with the bottom end of the engine -- but there's a first time for everything.