Rebuilt an '80 911 that was disassembled by someone else and not labelled. I thought I had everything just right but on first start up it back fired and flames erupted from the throttle body! Minor damage; not sure where to start though. Valve timing was set and rechecked several times, ignition timing at TDC1 to start, all new ignition components, gaskets etc...the only thing I had to guess on was vacuum hose placement. I've rebuilt lots of motors including boxers, but this is my first Porsche. Any help would be greatly appreciated! Thanks

 

Sounds like combustion is still taking place when the intake valves are opening. Could your ignition timing be 180º out?

 

Thanks for the suggestion! Gave it a try but unfortunately got the same result...was ready this time with the fire extinguisher so no damage at all! It seems to be coming from behind the throttle body/fuel distributor area. Perhaps I have the fuel lines or wiring wrong? I think I'm going to have to pull the motor again...