Backfire into intake
 

My 914 had not run for 3 years. I soaked the cylinders in WD40 for a couple days before starting. I discovered heavy rust in the fuel tank so I replaced:
Tank, hoses and pump.
Injectors and engine bay fuel lines
And Flushed the long fuel lines until they ran clean

The engine started but there was no power, poor idle
I Replaced:
Points/plugs/condenser/coil.
Reset timing and idle (The dwell is no high side- 58)

Power is back and idle is good.

The Problem:
When I give it gas, it will back fire into the throttle body. It seems to happen a 2k - 3k rpm range. It's doesn't happen every time.

Ideas?

The motor is 2l with FI.

The culprit is certainly attributable to the vehicle having sat for soo long. Sea foam is some amazing stuff. Add a pint to the engine oil as well as a pint into the fuel tank. That should clean up some sticking injectors, clear out any gunk on the valve stems and lifters. It might take a little time after running the car but it will clear up.