The '51 GMC Maintenance Manual says to crank the idle speed down to 350 RPM so no mechanical advance was taking place while checking ignition timing. The book says that 2 degrees is in by 400 RPM. I don't know if the '51 GMC and Chevy trucks used the same distributor, but I do know that the 55-59 years did.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
