Boy, does my son's slant run crappy!

some good points have been made from vacume problems to carb problems...could be ignition problems or something....but I would start with a couple easy things.

to me it sounds like it could be a couple things.

since when first started it runs good and then gets worse.....that makes me think choke!....sounds like the choke is on at start and runs good and once warmed up,the choke should be off to let it keep running good....it does not sound like the choke is all the way closed because it would probably be stalling.

But it does sound like the choke is not opening all the way ,as said before...verify that your choke is working properly (that is where I would start).

I also would not rule out a fuel pump,there cheap and easy enough to replace if it came down to it.
I have had them give me similar problems before.

And I would check your fuel filter.....I have had this happen multiple times!

Start the car up and it runs great.....the longer it runs and the more fuel that the pump draws through the filter.....if you have a worn out paper filter or dirt inside or something it will eventually pull it to the fuel feed hole from fuel pressure and it will limit fuel flow and cause the problems you have.

if thats the case,there cheap and easy to replace and I would check this by disconnecting the fuel line right before the filter to relieve and pressure in the filter and then reconnect it....start the car back up and see if it runs good again,if so than you have a bad filter.

or if you don't mind the extra work,pop the filter off and wipe off the inlet and try blowing through it....if air does not flow out the other side freely and easily,your filter is plugged.

pretty basic stuff but worth mentioning none the less!