I have a 97 civic with jdm h22a and m2b4 tranny. I changed my tranny fluid today and took car for a drive and was on the interstate when all the sudden my car started sounding like a lawnmower like it was missing and I pulled over to check nothing was out of the ordinary no cell but it backfires when you let off after accelerating and smells like it's running rich. Any ideas guys

 

Have good fuel good spark thinking maybe timing idk

 

The only thing i can say at this point and based on the information given is check for any and all stored codes.

 

If all you have touched recently is the tranny fluid, it is probably time to go over some stuff on the engine. I would start with checking your plugs and wires, and condition of the cap and rotor. If you don't find an issue there, My next thing would be to pull the valve cover and timing covers, and check the timing, it is possible that the auto tensioner failed and has caused a timing belt issue.