Your car did have an ECU and the injection system was both mechanical and electronic but my guess is none of this is at play now because you have a mechanical LPG system providing the fuel now.

Maybe what 280 is referring to is the fact that a manifold that was designed to be a dry flow only manifold is now being forced to be a wet flow manifold. An intake charge with fuel in it is heavier and not able to make sharp turns as easily as air alone is. I'm assuming that the fuel and air are entering at the stock throttle body location. If that is the case, you have a horrible, very sharp, very sudden turn from that location into the center two cylinders. Probably not much of an issue in a dry flow scenario but a bad deal for a wet flow scenario. This might explain why those center two cylinders got in trouble. The rest of the cylinders have a little more room and time for the intake charge to make the turn into their runners. I'm not saying this problem can't be overcome, I'm just saying you may need to be aware of it so you can do something about it.

If you can afford it, the LPG injection would do a better job, but I'm sure the current set up could be made to work decently as well.
Regards, Eric

__________________
89 300E "Benzer1" 15.924 Uncorrected
93 400E "Benzer3" 14.200 U.C.
95 E420 "Benzer4"
92 300E "Benzer5" 16.299 U.C. Future turbo CNG
87 300D "Benzer7"
87 300D "Benzer8"
87 300D "Benzer9"
87 300D/70 AMC Javelin "Sidewinder-Benzer"
87 300TD "Benzer11"
06 E320 CDI "Benzer12"
05 E320 CDI "Benzer12A"
71 AMC Javelin AMX 401 "Sidewinder"
74 AMC Hornet 401 "C.K.10" 13.63 U.C.
74 Bricklin SV1 "Presto" AMC 360 pwrd.