I recently purchased a 91 NA. After changing the plugs, I got two trouble codes -- 11 (Intake air thermosensor) and 33 (Port air solenoid).
Before I changed the plugs, the car would not pull much past 6500 and seemed to be out of breath. Now the car will sometimes idle poorly and still feels breathless. Also, The engine is smooth until 6500, and then I feel a vibration from the engine (?). I will check the sensors and see if I unplugged something -- BUT WHY CAN'T I GO TO 8000 RPM?

Thanks
TODD

 

check VDI operation and engine trouble codes

 

check cats for clogged

 

my 90 NA ran like **** until I replaced the air flow meter.
I tried a used one, it worked better but still there was something wrong.
Then I broke down a bought a new one from mazda =
big bucks.
Installed in mazda's parking lot and from the second I started the car, I thought I bought a brand new car.
No Joke

 

My car had those same symptoms, it turned out that the computer wasn't sending a signal to the 6 port solenoid, so the ports weren't opening. I got a summit racing RPM switch to activate the solenoid, and the car performs much better now that it can breathe properly (it feels like I've got an extra 15-20 hp at high RPM). Check the 6 ports and VDI to see if they're working. An easy way to do that is to get a pressure guage (it'll only need to read max 8psi) and plumb it into the solenoid and get the guage in the car. Take it for a drive and see if the pressure changes. If it doesn't then you know why your car has no power. Good luck.