Why does my 225 run so rough cold?

You've probably got choke problems. The choke thermostat may be faulty or missing, or the choke pushrod bent or otherwise brutalized. The choke pull-off may be faulty or its link improperly adjusted. See choke adjustment info here, though your '75's choke thermostat is not adjustable (if you want an adjustable one—which, really, you definitely do want—get a #1234 Electric choke kit). Check and make sure the thermostatic air cleaner system is intact (including the flex duct from the bottom of the air cleaner snorkel down to the sheetmetal heat stove surrounding the central collector portion of the exhaust manifold) and working correctly (flapper in the snorkel blocks the front/cold-air entrance of the snorkel, so all the air has to come through the sttove and flex duct). Another thing to check on this: cracks or leaking gaskets at the exhaust manifold central area. If there are, this will feed exhaust into the intake and make the engine run very poorly when cold. The heat riser valve is a good thing to check, too; with the engine cold, find the 3" diameter round disc facing front, on the central portion of the exhaust manifold. Grab and rotate it counterclockwise (viewed from front). It should turn easily and spring back clockwise when you let go.

Make sure the distributor vacuum advance is working (diaphragm not blown, hose not cracked or holed). Route the hose from the distributor directly to the carburetor's spark advance/OSAC port, without stopping at the widget on the side of the air cleaner housing.

The carburetor is likely due for some attention. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download. Tune-up parts and technique suggestions in this thread. And don't forget these engines need periodic valve adjustment.

You will want to get the three books described in this thread as quickly as you can.