I just rebuilt a 350 and I can't sort out the demon that is making it backfire and not start. I know the timing is correct. I even pulled off the front cover to see if the cam was lined up correctly, it was. I have checked teh rockers then adjusted them extremely loose and still it backfires. The distributer is a HEI and i replaced the module because i know those go. I did have the heads ported. Could a nicked valve seat or two cause the engine to only backfire and not start? Any advice or ideas would be appreciated.

Are you sure of the firing order? re-check the plug wires to see if they are not crossing. Also I know you said that your timing is correct...but maybe the distributor is out by 180 degrees. If it's not that it may be trouble with the carb.

I have to agree, Check the distributer for 180 off, I've been doing this for 20+ years and still get caught by that sometimes. You did good by loosening the rockers up, 'cause a valve hanging open will cause a pop..

Check that distributer over good too, i have had the heads where the rotor goes on (by the advance weights)...loosen up and spin on the distributer shaft, I have seen the gears spin on the other end too and did'nt notice that the roll pin had snapped, and the gear had turned so make sure everything with the distibuter is good, then bring her up on compression on number one cyl, get TDC on the crank and drop that HEI back in rotor to number 1 on cap..I am sure you know all of that as well as the 18436572 firing order...

Good luck

The only time I have had a small block backfire and or shoot flames out of the carb was when the timming was way off. Bring #1 to top dead center and then very the valves are close the rotor should point to the wire going to #1, Insure the firing order is correct 1.8.4.3.6.5.7.2 and that is a clockwise rotation.

I've run into different timing sets where one set had the dimples with the crank at 12:00 & the cam at 6:00 & the other had them both at 12:00. If you guessed wrong (I've never had a set come with paperwork that tells you which set you have) & you don't check your valves, you could turn your mufflers into a pair of pillows like I did on my Suburban. 454's back-fire like they mean it. I guess the moral of that little tid-bit is, assume nothing. Do it by the numbers every time. A nicked valve seat would cause back-firing but not prevent starting.