PLT OF A BEECH BARON; BE58; DSNDED FROM ASSIGNED ALT DUE TO BOTH ENGS 'SURGING' AND LOSING PWR. RPTR FAILED TO ADVISE ATC AT ONCE DUE TO FREQ CONGESTION.

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DEPARTED OFF M39 AT XA00Z. I PICKED UP MY IFR CLRNC ONCE AIRBORNE AND WAS CLRED TO OSH AS FILED TO 7000 FT. WHILE CLBING I ASKED FOR 11000 FT TO PICK UP A LITTLE BETTER GND SPD. I WAS THEN CLRED TO 11000 FT. I REACHED 11000 FT AND STARTED THE NORMAL PWR REDUCTION. I STARTED WITH REDUCING THE RPM'S FROM 2500 (CLB PWR) TO 2300 (CRUISE PWR). THE MANIFOLD PRESSURE WAS FULL FORWARD AND L FULL FORWARD (ABOUT 21 INCHES). WHEN I STARTED TO LEAN OUT THE ENGS (14 GAL/SIDE); THE ENGS STARTED TO SURGE IN AND OUT WITH PWR. I IMMEDIATELY PUSHED THE MIXTURES BACK TO FULL RICH. THE ENGS CONTINUED TO SURGE SO I SWITCHED THE AUX FUEL BOOST PUMPS TO HIGH. THE SURGES WERE FREQUENT ENOUGH AND VIOLENT ENOUGH THAT I COULD NOT MAINTAIN A SAFE AIRSPD. AS THE AIRSPD BLED OFF; I IMMEDIATELY STARTED A DSCNT TO ESTABLISH A SAFE AIRSPD. THE CTL FREQ WAS CONGESTED; HOWEVER; AS SOON AS I COULD BREAK IN I INFORMED ZME ON 119.25 THAT I WAS DSNDING. MEMPHIS ASKED WHY I WAS LEAVING MY ASSIGNED ALT OF 11000 FT AND WHAT ALT I WAS PRESENTLY AT. I STATED THAT MY ENGS WERE SURGING AND THAT I NEEDED TO DSND TO MAINTAIN A SAFE AIRSPD; AND THAT I WAS PRESENTLY PASSING THROUGH 9500 FT. HOWEVER; I WAS REALLY AT 10500 FT (I WAS PAYING TOO MUCH ATTN TO THE RPM'S AND THE MANIFOLD PRESSURE AND DID NOT LOOK AT THE ALTIMETER CLOSE ENOUGH). CTR IMMEDIATELY CLRED ME TO 9000 FT AND ASKED IF I NEEDED AN ARPT TO LAND AT. I STATED THAT I WOULD LIKE TO CONTINUE MY DSCNT AND THAT THE ENGS WERE STARTING TO RUN SMOOTHLY AGAIN. ONCE LEVELED OFF AT 9000 FT; I ASKED CTR FOR 7000 FT. CTR AGAIN CLRED ME TO 7000 FT. WHILE DSNDING THE ENGS SOUNDED AND FELT GOOD. I AGAIN MANIPULATED THE CTLS TO THE CRUISE CONFIGN AND CONTINUED TO OSHKOSH; WI; WITHOUT FURTHER INCIDENT. UPON REVIEWING THIS SIT WITH OUR DIRECTOR OF OPS AND OUR DIRECTOR OF MAINT; I LEARNED THAT I HAD THE MIXTURES TOO RICH AT 11000 FT. I ALSO LEARNED THAT IN THE FUTURE I NEED TO LEAN OUT THE ENGS MORE AT HIGHER ALTS AND THAT TURNING ON THE BOOST PUMPS PROBABLY MADE THE ENGS SURGE MORE. I ALSO REVIEWED THE PLT OPS HANDBOOK PUBLISHED FOR THE BEECHCRAFT BARRON 58 AND LEARNED THAT I WAS LEANING THE FUEL TO MIXTURE IN ACCORDANCE WITH THE AIRPLANE MANUAL.