MG Car Repair: MGB GT Power Loss & interior steam, manifold vacuum, gt power

Question
I have a 1969 B and am very new to the British car world.  Car ran flawlessly for 6 months then I garaged it for about 3 weeks--temperatures got down to 11-15 degrees for 5 days.  It started fine this morning but when I got it on the road there was nearly no acceleration.  RPMs went up but it took about 5 miles to get up to speed--even slight hills slowed me down to 15mph.
Also, this car has steam which smells like antifreeze coming in through the defog vents--always has since I have had the car--begining to fog the lower part of windows permanently.
Thanks much for your help.

Answer
Hi Travis,
The steam out of the defroster vents is coming from a leaking heater core. The heater core is in the black can in front of the firewall in the center.
As far as the engine not pulling you need to determine if the RPM goes up and the car don't go or is the engine just not have the power to pull the car? If the RPM goes up and the car don't go it is a clutch problem but if the engine just don't seem to have the power to pull the car you need to first run a compression test. Throttle open and at least 4 or 5 revolutions on each test of a cylinder. Then if that is ok check ignition timing and carburetors for adjustment. Also test the intake manifold vacuum at idle and at high RPM. Let me know,
Howard