Hi everyone,

Hoping someone can help as I'm pretty stuck. Working on a 1985 GMC c2500 with a sbc 350 engine that I rebuilt, everything brand new. I've been trying to turn it on the past few weeks and it seems like it will almost start but dies, and backfires/pops out of the carburetor and sometimes shoots flames, so I think something is wrong with the valves. Last week I verified that the engine timing is correct, I set it to tdc of the compression stroke (verified by watching the number one valves and once the exhaust valve began to open I turned the crank opposite [counterclockwise] and then set it to 8 degrees before tdc with the mark I have on the harmonic balancer, both valves seem closed so Im pretty sure it's tdc compression) then installed the distributor and set the cap on so the rotor points directly to the spark wire that I'm calling number 1 for cylinder 1, and then I follow the normal gm firing order. Brand new distributor cap/rotor/ignition coil/spark wires/spark plugs. Still getting backfire, and I have a piece of tape on where the pcv valve plugs into the valve cover, and I notice when I turn the engine over the tape pushes outward, like there is air pushing on it (I'm assuming one of the valves isn't sitting/closing properly or in correct timing so this is why). I had the valves brought to the machine shop and cleaned up and I believe reassembled correctly. All I did to put together the valve train was tighten the nuts on the rocker arms until the pushrods had no up and down movement only a little side to side, and went through all exhaust/intake valves doing this method. Rotated the engine over 4 times (8 crankshaft rotations) and checked each one. It is a hydraulic flat tappet cam with hydraulic lifters, I set the lash with no oil in the lifters. All new rockers/lifters/pushrods. Does anyone have any advice? Did I overtighten the valves, and can I just untighten then to fix this? Was the machine work not done properly?

Also, to set tdc, what I did was use a piston stop tool and with a piece of tape on the harmonic damper mark where the piston hit the stop tool (I used the 0 on the timing tab as the spot to mark on the damper). Then I rotated the crank counterclockwise and when the piston hit the stop tool again I marked on the tape at the same location (the timing tab 0 mark). To find my tdc I simply just found the midway point between these two marks, and figured setting that new line to the 0 mark on the timing tab would put me at tdc. This is correct thinking right? What exactly is the small indent/groove on the harmonic damper for?