A CPR FLC IN AN LTT CLB ABOVE THEIR OPTIMUM ALT AND STALL THE ACFT. DURING THE RECOVERY THEY LEAVE THEIR ASSIGNED ALT WITHOUT A CLRNC. THEY DECLARED AN EMER AFTER THE CTLR'S PROMPTING.

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THE CAPT WANTED TO CLB TO FL510. I DIDN'T WANT TO; BUT COULD COME UP WITH NO REASON THAT WOULD CONVINCE THE CAPT NOT TO CLB TO FL510. WE WERE GIVEN CLRNC TO CLB TO FL510. AROUND FL450 THE CLB RATE IS ALMOST ZERO. I SUGGEST WE ARE TOO HVY AND PROBABLY SHOULD STAY AT FL450. THE CAPT SAID WE WOULD MAKE IT. UPON REACHING FL510; THE CAPT SAYS HE IS GOING TO PULL THE PWR BACK TO CRUISE. UNDER THE CURRENT CONDITIONS; CRUISE PWR IS NOT ENOUGH TO MAINTAIN ALT AND AIRSPD. AIRSPD DECREASED TO THE POINT OF STICK SHAKER AND WE DEPART FL510 WITHOUT CLRNC. PRIOR TO THE STALL; I REQUESTED A LOWER ALT AS PER THE CAPT'S REQUEST AND ATC SAID IT WOULD BE A FEW MINS. BY THIS TIME WE HAD LEFT FL510 AND I INFORMED ATC THAT WE WERE OUT OF FL510. HE ASKED IF WE HAD AN EMER THAT KEPT US FROM REMAINING AT FL510. I REPLIED 'YES SIR.' THE FLT PROCEEDED WITHOUT FURTHER INCIDENT. I SHOULD HAVE BEEN MORE ASSERTIVE AND EXPRESSED MY DESIRE NOT TO CLB TO FL510. ALSO; THERE WAS A CURIOSITY AND NOVELTY INVOLVED WITH CLBING TO FL510 ON MY PART. I SHOULD HAVE SUPPRESSED THIS AND DONE WHAT I KNEW WAS SAFER AND SMARTER. NO PAX WERE ON BOARD.