I got a gs-r in my white DA and my rear bumper keeps getting caked with carbon.
I checked the valve clearance and everything checked out O.K.
How do I know if its running too lean or too rich?

 

you prolly have a blown headgasket or your piston ring is giving up. i know if your running rich youll waste alot of gas and you can smell the fumes. check your compression on your engine.

 

Does your car smoke? Carbon build up means your running rich.

Blue smoke = oil
Black smoke = fuel (too much)
White smoke = coolant

 

Try reseting the ecu. When eva the weather changes my car does the same and once I reset it, its fine.

 

Resetting the ECU evry now and then is not necessary. The ECU constantly updates based on sensor inputs every time you drive.

If the emissions sytem is working correctly, you shouldn't have to worry about too lean or rich. Your deposits may be from oil burning or there is a problem with the fuel ratio going towards the rich side. Or, you make a lot of short trips in cold weather.