225 Slant needs a good idle. what rpms are you spinning?

If the throttle is closed, the engine cannot run unless it is getting air somewhere UNDERNEATH the throttles.
Just Cuz the Idle speed screw is all the way out, does Not mean the throttles are closed, nor that the engine isn't getting air somewhere else.
At idle, the more timing you give it, the higher will be the idle speed, up to about 25/30 degrees. After that the engine may begin to complain after it warms up.
If the valve lash is too tight, it will complain sooner.
How many parhs are there into the combustion chambers? Trick question; Four, the intake valve AND the exhaust valve, and the two valve stems.And that's if the head gasket is sealed
How many paths are there into the intake port?
Well besides the throttles ... there is the PCV, the Booster hose, possibly an internal air bypass system, a hot-idle compensator, a base gasket, the intake to head gasket, and possibly a cracked exhaust manifold under the intake plenum, Or if the engine has an EGR valve/passage, add that to the list. Soooo like said;
Just Cuz the Idle speed screw is all the way out, does Not mean the throttles are closed, nor that the engine isn't getting air somewhere else.