A B737-800 CREW; IN AN ATTEMPT TO TOP WX; CLBED TO AN ALT WHERE ENCOUNTERING MODERATE TURB CAUSED A DECAY IN AIRSPD.

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FLT WAS DISPATCHED SUCH THAT THE RTE OF FLT WOULD ENCOUNTER WX IN CENTRAL IOWA ALONG J60. AN AREA/BROKEN LINE OF WX WAS OBSERVED USING RADAR 160+ MI PRIOR TO IOW VOR. 80 MI PRIOR TO THE WX; ASKED FOR AND RECEIVED CLRNC TO FLY HDG 280 DEGS. THIS HDG WAS USED INITIALLY TO FLY THROUGH A HOLE OBSERVED IN THE NORTHERN END OF THE WX. OUR ALT WAS FL350. DUE TO THE DETERIORATING RIDE IN CONTINUOUS LIGHT TURB WITH OCCASIONAL MODERATE CHOP; THE ACFT WAS SLOWED FROM .79 MACH TO .76 MACH. THE WX RADAR INDICATED SOME PRECIP AT OUR ALT AHEAD OF US. ALTHOUGH CLOUD TOPS WERE PARTIALLY OBSCURED; IT LOOKED LIKE WE MIGHT BE ON TOP AT FL370. AT THE TIME THE ACFT WT WAS 151500 LBS. BUFFET SPDS WERE CHKED (150000 LBS; 237-262 KTS) PER THE RECENT BULLETIN SINCE THE CLB WOULD BE MADE ABOVE OPTIMUM ALT. THE FMC INDICATED AN OPTIMUM ALT OF FL354 WITH A MAX ALT OF FL379. THE CAPT AND I AGREED THAT 25 KTS WOULD BE AMPLE MARGIN TO CLB TO FL370. THE CLB WAS MADE AT .76 MACH. AT FL370; WE WERE CLR OF ANY WX; THOUGH CLOUD TOPS WERE STILL ILL-DEFINED. THE TURB INCREASED TO CONTINUOUS MODERATE. AIRSPD EXCURSIONS WERE +/-15 KTS. THE OVERALL TREND WAS TO THE SLOW SIDE. THE AUTOTHROTTLES DID NOT HUNT BUT WERE SLOW TO RESPOND TO AIRSPD CHANGES. DURING THE EVENT THERE WERE NO ALT EXCURSIONS. THE AUTOPLT DID NOT SEEM TO HAVE ANY DIFFICULTY MAINTAINING THE ALT. THE TURB ENCOUNTERED WAS MORE LATERAL RATHER THAN VERT. NOTICING THE SLOWING TREND; I MANUALLY PUSHED THE THROTTLES TO THE CHEVRONS TO GET THE SPD BACK. FOR A FEW SECONDS; THE PITCH LIMIT INDICATOR APPEARED. AT NO TIME DID THE STICK SHAKER ACTIVATE; ALTHOUGH THERE WAS WHAT SEEMED LIKE THE THINNEST OF MARGINS BTWN THE PITCH LIMIT INDICATOR AND PITCH INDICATED ON THE ADI. NEITHER THE CAPT NOR I REALIZED THE PITCH LIMIT INDICATOR WOULD APPEAR WITH THE FLAPS UP. MAYBE THAT WAS A FACT I NEVER KNEW OR HAD FORGOTTEN. AIRSPD WAS SLOW TO RETURN. WE DECIDED TO DSND TO FL350. AS WE LEVELED AT FL350; WE BROKE OUT INTO CLR AIR BEHIND THE WX. THE CAPT SENT A MESSAGE TO DISPATCH SUGGESTING THAT THE RTE SHOULD NOT BE USED. INSTEAD; USE A RTE TO THE S AVOIDING THE WX ALTOGETHER. NO REPLY WAS EVER RECEIVED FROM DISPATCH. I WONDER IF .76 MACH FOR TURB IS TOO SLOW FOR THIS AIRPLANE. IS THERE A HIGHER SPD FOR USE AT HIGHER ALTS? THE BUFFET SPDS -- HOW ARE THEY DERIVED? WHEN IS IT APPROPRIATE TO SLOW TO THIS SPD? THESE ARE SOME QUESTIONS I HAVE THAT ADDITIONAL LITERATURE AND EDUCATION COULD PROVIDE.