A CESSNA 340 CONTACTS A FENCE ON TKOF AT 12S.

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DEPARTED MONUMENT STATE ARPT A NEWLY PAVED 2200 FT ARPT ON RWY 32. RWY 32 HAS A 3-7% SLOPE DOWNWARD. CONDITIONS AT THE TIME HAD A QUARTERING TAILWIND OF 6-8 KTS OUT OF THE W. I CHKED PERFORMANCE DATA AGAINST THE CONDITIONS WITH AN OAT OF 12 DEGS C AND CALCULATED A TKOF COULD BE MADE IN AN ADJACENT GND ROLL. I DID NOT FACTOR THE DOWN SLOPE IN THE CALCULATION BUT MENTALLY THOUGHT IT WOULD ADD TO MY PERFORMANCE IN A POSITIVE FASHION. I ALSO FIGURED IF I WAS APPROX 500-800 FT IN THE GND ROLL BECAUSE OF THE STEEP RWY GRADIENT I WAS COMMITTED TO TAKE OFF. I PERFORMED A FULL PWR HOLD THE BRAKES TKOF AND AT AROUND A QUARTER OF THE WAY DOWN THE RWY GOT AN ERRATIC MANIFOLD PRESSURE INDICATION SURGING ABOVE REDLINE AND BACK DOWN. THE WASTE GATE HAD RECENTLY BEEN REPLACED ON THE L ENG THE ONE WITH ERRATIC INDICATION. I WAS BALANCING MY ATTN BTWN ENG INST AND AIRSPD INDICATION AND OUTSIDE. I TRIED TO STRETCH THE TKOF ROLL MAX AND AT APPROX 95 KIAS ROTATED. ON ROTATION I CLIPPED A FENCE AT THE END OF THE RWY. I PUT A HOLE APPROX 1 1/2 INCH X 7 INCH IN THE L SIDE BELLY OF THE PLANE AND ALSO CAUGHT THE FENCE ON THE R PROP AND R HORIZ STABILIZER. THE AIRPLANE HAD NO APPARENT MALFUNCTIONS EXCEPT A LACK OF PRESSURIZATION. I RETURN TO MY HOME BASE. MY ROTATION WAS WHAT I WOULD CONSIDER NORMAL FOR A RWY WITH NO GRADIENT. I DID NOT FACTOR THE STEEP GRADIENT FOR TKOF WHICH I BELIEVE MADE THE TRANSITION FROM RWY TO AIRBORNE DELAYED BECAUSE OF THE DOWNWARD MOMENTUM OF THE PLANE. THE PLANE HAS BEEN INSPECTED AND AN ESTIMATE OF DAMAGE BY AN A/I. I WAS OPERATING IN A NARROW MARGIN OF SAFETY AND FAILED TO CONSIDER THE NEGATIVE ASPECT OF THE DOWNWARD MOMENTUM ON THE LIFTOFF TRANSITION.