Hey people, I wanted to know what i was doing wrong on my 85 250sx.I put a new piston and rings in it because it was smoking and i set the timing with the 'T' mark on the fly wheel lined up through the plug hole in the side of the left case.but i am not sure how the cam marks should go.As far as i can see there are only two marks exactly 180 deg. from each other.I have put these horrizontal across the top of the valve casing in either 180 deg location which puts the lobes either at 8:00 & 4:00 or 10:00 & 2:00 oclock.I was told by a friend that one of these positions would work,not so.i have fuel and fire to the pistin but will only backfire or try to run rough...Please help.....

 

The marks are supposed to align with the head casing just like you said.

The lobes are always to be pointed downward...which in your case is the 8:00/4:00 positions.

The only other thing I can think of regarding the timing that you may be overlooking is that ,maybe the chain is jumping one tooth when you are asembling it.

Remove the valve covers and set the crank to TDC. Both rocker arms should be loose and no tension on the valves whatsoever. This will eliminate the timing issue. If both of the rockers are taught against the valves, you have the camshaft put in upside down.