Hi a mate has just changed his head gasket on his 2wd sapphire and now it won't run on all 4.
Good spark on cylinders 1 and 4 but very weak on 2 and 3. It has fuel on all 4 cylinders and has had rotor cap and leads replaced but still the same.
Ran on 4 cylinders before changing gasket.
Any ideas what this could be ? Totally standard engine with no upgraded parts.

 

Check the gap on the sensor in the distributor
Check your engine earth as well, make sure you have good contact

 

Also check the hall sensor is plugged in tightly in the distributor if that has been messed with (mine had worked loose) I have done away with it all now and gone coil on plug with cam position sensor. change out the plugs and if the fuel is older than 3 months I would add new.

 

put it to TDC and recheck rotor arm position
also recheck the cap to leads connections particularly on the 2 affected cylinders,

Steve