what could it be?
 

my 86 fc runs great intill it hits 4,000rpms when driving... then it will choke for about 2 seconds an continue to climb the rpms?....ive already changed plugs,airfilter an fuel filter but its still doing it ........help any one??????

I'm not up to speed on the 13B yet, but I believe the secondary throttle plates open around then. If something isnt quite right (TPS maybe??) then you'll be drawing more air with not enough fuel until the ECU catches up.

thats a good brobability never thought of that ....thanks!!!!!!

Yea hopefully someone who knows better than I will come along to confirm and give a specific fix.

ran into the same problem with my TII at 3800rpm, re grounded the engine, battery, and ecu. After that the problem never came back... ever. Its worth a shot, good luck!

Excellent point, and a commonly overlooked problem area. You may have just found the cure for the notorious dead spot in many F.I. Rx7's. I'm a first gen owner,and they have a problem with wiring oxidation in wetter climates. There may be other causes for that flat spot, but this is a free fix, and well worth trying. Heavier gauge , and additional ground straps, are a lesson I learned in my audiophile days. Good luck, and happy rotoring.