I have an 91 jeep wrangler that I put a 89 305 TPI in and two months ago it was backfireing really bad so I put new spark plugs in and it seemed to fix the problem. And it is beginning to backfire again and I was wondering of anybody could help me out. I don't want to just buy new plugs again because the ones I get are very expensive. Any help or suggestions would br much appreciated.

 

Start with some basics first, set base timing with the EST disconnected to 6-8*BTDC
Verify that you're plug wire routings are correct for the firing order.

Is the engine setting any codes?

Is the back fire through the intake, or the exhaust?

 

It is through the exhaust and I am not getting any codes and the wires are in the correct order

 

Do you have a scan tool to monitor ECM inputs?

 

Update.... The valvs opend early so I made a few adjustments and it is runng good again! Thank you for all your help!