A vacuum gauge is good.
Check your vacuum at idle then watch it as you slowly increase RPM in park, no load. Vacuum should be very close to what it was at idle (assuming no EGR, variable valve timing,etc) as RPM goes up. Your not doing throttle snaps to high RPMs just slowly increase engine speed and let it stablize. IF vacuum is dropping off as RPM increases may have restricted exhaust.
Soung like something is keeping this engine from pulling enough vacuum on the modulator at road speed to shift into 3rd perhaps.

Last edited by BC59; 03/23/2012 7:07 AM.
BC
1960 Chevy C10 driver 261 T5 4.10 dana 44 power loc
1949 GMC 250 project in waiting
1960 C60 pasture art
Retired GM dealer tech. 1980 - 2022