Shims and Things

[Fostang]

I was wondering if one engine that is equal to another would also require shims in the same places when putting the same 1.7 rr's onto a different one.

How is it that the stock 1.6 stamped rocker arms don't need shims to make them work properly?

How do you make sure you've put the right amount of shims?

Would incorrect shims make an engine run rough or have 3 spark plugs that have a brown black finish on it while the others are white with a light brown coat. Which would in turn point to that these 3 cylinders have incorrectly set shims on them, right or wrong?

Also at idle the car smells rich has a somewhat shaky idle for a fuel injected car and also an audible miss. While at high rpms it has backfired twice.

I think I'm pretty close to working out the bugs on this fueli conversion. Just need some input on this subject. Any suggestions or info will be greatly appreciated.