Howdy - I have a 54 GMC 2-ton dump flatbed w/ 270. Overhauled it last year with rings & bearings, replaced the old cracked head with a used one that was milled and had new exhaust valves fabricated, milled the mated manifolds and installed a Holley/Weber 2 barrel carb. Valves are adjusted; good compression 150-180 in all cylinders. At idle, the engine misses on cylinder #4, but under a load (less than 10 inches vacuum) it pulls with all six as smooth as can be. It DOES have a vacuum leak (I can hear the whistle) that I've not yet been able to localize. My plan is to shoot some starting fluid against the block at idle to see if the vac leak is at the middle intake port, what else should I be looking for? Where else should I look for the vacuum leak? (A note: this truck has a two speed rear axle that shifts by vacuum.)
