[Joe Sr.]

If I were to carbonate at 30 psi for 2 days I assume once the co2 has dissolved the co2 tank continues to push a pressure of 30 psi onto the beer unless reduced.

Yes.

If you do reduce the pressure prior to purging the valve or pouring a beer does the head space pressure drop to the new setting or does it remain at 30 psi?  

If you reduce pressure at the regulator, the head space pressure remains the same until you release pressure inside the keg (pouring or purging).  EDIT: It is also possible that the initial pressure (30) could dissolve into the liquid enough that the pressure in the keg reduces to your new setting.  This would mean that you were not fully carbed at that pressure or that you lowered the temp of the beer.

Will the co2 tank only actually put more gas into the keg once the pressure drops at or below the new co 2 pressure setting?  

Correct.

Does the excess foam come more from too much carbonation suspended in the beer, too high a head space pressure, or some combination- or are these in the end the exact same thing?

Not exactly the same thing, but they are directly related.  The amount of carbonation will be a factor of both the pressure in the keg and the temperature (colder liquid will absorb more CO2).  If the beer is at the right pressure for the carbonation you want when serving at 40 degrees, it will be overcarbed/foamy if you serve at 50 degrees. 

Excess foam will also come from too short of a serving line.  A standard recommendation is to start with a long line and cut it back until you get the pour you want.

Because I'm lazy, the way I typically carb and serve from my kegs is to pressurize them to 30psi and shake for a while.  I'll do this a couple times before I put them aside.  Once they go in the fridge, I chill them and tap them.  The first pour is usually crazy foamy, so I'll release head pressure to get a good pour.  I close the fridge and come back when I want more beer.  The keg has reached a new equilibrium and either pours well or needs to be bled again.  When the pour gets too slow, I add gas at approx 10 psi to get a good pour and then disconnect.  I do not have my kegs on gas all the time.  It's usually pretty easy for me to get the kegs to the right pressure for a good pour with a nice head and good carbonation, but I'm used to my system after using it for years.  YMMV.

[roguejim]

I guess my question is: once the beer is carbonated at the force carbonation pressure does reducing the pressure to say 7-10 psi result in any co2 coming out of solution?  Does it make any sense to say the beer tastes under carbonated but there is copious amounts of head?  Beer line is about 4-5 ft.

I've had this same thing occur when force carbing at 20+ PSI for a few days.  Very foamy head, and flat mouthfeel, at least for several days until the carbonation settled out.  Even with bleeding the head space pressure, and reducing the PSI to a serving level (10-12PSI), the foaminess would last for a few days.  Now, I carbonate at serving levels for several days, and might crank the PSI up to 20PSI for the last day or two, if I'm impatient.  Of course, I always bleed, and drop the serving pressure back to 10-12PSI before filling my pints (my beer line is 5ft).  Much more even carbonation.  Given your beer line length, your PSI should be ~10.

[oscarvan]

I believe my beers are properly carbed, between 2.0 and 2.5...

Fridge temp is 45º
30 psi for two days, then down to 10 psi
12 foot lines......

All is well.

[euge]

Because I'm lazy, the way I typically carb and serve from my kegs is to pressurize them to 30psi and shake for a while.  I'll do this a couple times before I put them aside.  Once they go in the fridge, I chill them and tap them.  The first pour is usually crazy foamy, so I'll release head pressure to get a good pour.  I close the fridge and come back when I want more beer.  The keg has reached a new equilibrium and either pours well or needs to be bled again.  When the pour gets too slow, I add gas at approx 10 psi to get a good pour and then disconnect.  I do not have my kegs on gas all the time.  It's usually pretty easy for me to get the kegs to the right pressure for a good pour with a nice head and good carbonation, but I'm used to my system after using it for years.  YMMV.

I do this as well. Thusly I haven't had to modify my serving fridge. Only I don't like to shake kegs that much. Mainly due to the weight. Rocking it back and forth works well but my laziness prohibits even that. Also, I have my regulator set on 42-ish, whatever the max psi is. Just connect and fill that baby up about five or six times during the first week and the beer will be nice and carbed. One could vent first but I like to blast the first 8oz to clean out the pipes so to speak... Then vent it to 3 psi or so, pour one and see how you like the carbonation.

I've followed these threads and tend tend to think people initially have their systems set too high. The beer gets over-carbonated and also gets pushed too hard. They compensate by increasing their line length but still get foam because the beer is gonna foam no matter what because of over-carbonation. They back off on the psi and everything quiets down.

[gordonstrong]

If you suspect the length of the line has something to do with it, you can test that by putting more resistance on the line.  Either raise the tap or lower the beer, or put a bend in the line.  If it starts pouring better, then you need a longer line.  Is your line chilled to the same temp as the beer?  That can also cause foam to come out.

[oscarvan]

If you suspect the length of the line has something to do with it, you can test that by putting more resistance on the line.  Either raise the tap or lower the beer, or put a bend in the line.  If it starts pouring better, then you need a longer line.  Is your line chilled to the same temp as the beer?  That can also cause foam to come out.

Forgive me oh great one for doubting.....

Whereas I agree with raising the tap or lowering the keg, isn't it true though that putting a bend in the hose will create a venturi in the hose (Bernoulli at work so to speak) and foaming will occur in the hose right at the kink.....?

[Slowbrew]

There are so many things that can cause foam.  One minor issue for me is the very first pour of the day, even on a balanced system.  The faucet itself is warm and causes the first beer to be more foamy.  Any pours after that, assuming they happen reasonable quickly, come out perfect.

Keep playing with it, you'll get there.

Paul