I'm looking at buying a used 1990 mazda mx6, 5sp, 4 cyl, coupe with 166k on it. has aftermarket headers, he says its straight piped, may or may not have 02 sensors, I didn't check because I didn't think about it till later, its an hour away and no CEL.

Clutch feels good (probably not for long). Have to rev to 4k rpms and ease out on the clutch just to get it to move without stalling out. idles a little rough, not to bad, revs up in neutral fine, probably needs a tune up in addition to whatever is causing the issue. 5th gear it won't accellerate at all, and if you hit a hill it will slowly decellerate.

No idiot lights on dash, no codes, engine is dry and not showing signs of leaking, pedal to the floor and accellerates like it has 50hp.

i've no idea what is going on and it's the only car in my area that even drives for $1000

 

Check the rubber boot from the air filter box to the throttlebody for cracks. If it is cracked/split then the engine movement makes the crack larger and the ECU thinks it is at idle and denies it fuel.

 

sounds like a money pit.
read this US Law: I just bought a used car and discovered the catalytic converter is missing. Is that legal?

 

Caution! A cheap car can turn out to be costly mistake.
When you turn on the ignition without starting the engine all dash lights should come on. After you start the engine most of them should go out. If the lights don't come on the bulbs are burnt out or removed on purpose.
In 1990 it was probably equipped with OBDI which used a Morse code to indicate trouble.

Are you sure it is even firing on all 4 cylinders?