There's such a wealth of info. I thought I'd give you all a shot.
Here's what I've got. 94 zuk rf900r. I did a WHOLE lot of work on this bike not long ago, rebuilt carbs, plugs, chain, sprockets, etc. The bike ran like a raped ape when I was done with it. He brought it back the other day and it won't start. No trash in the fuel filter I installed, so I haven't pulled the carbs (should be no need to), plus out of despiration I shot some starting fluid through, and it didn't even stumble or try to start(although it backfired like a canon). I pulled a plug wire at a time, and tested spark with my spark tester and each one shot a solid and steady half inch spark. I also had installed a 2' ignition advancer back then. It seems as if the ignition timing is hosed. Anyone have any ideas on testing this, or any way of testing the black box? I don't know anyone who has one to try and swap. Ideas??

Sorry to ask the obvious, is it out of gas???

Nope not out of gas, got it hooked up to aux. fuel cell....

wrong spark plug wires connected to wrong plugs, or wrong wires conneced to wrong coils? That would make it backfire like crazy but not start. Ohterwise take out the advancer and see if it works.
MikeG

Ok Ok, I got it. Thanks guys, now I feel like a putz. :oops: Turned out to be fouled plugs. I'd have never thought it because when I pulled them and checked them for spark they were sparking. They were a little black, (bike runs rich, I didn't put the jet kit in, and I'm not going to spend the time tuning it either, seeing how I'm doing this for free.) But a friend of mine from down the street came down took one look at the plugs and said "it ain't gonna run on those" so we cleaned them with a silica blaster, cleaned them up, and sure as shit bike fired right up and runs like a top. I will tweak the mixture a little to lean it out but I'm not gonna do plug chops and mess with needles and jets. All that work i did that I mentioned earlier I only charged him $150+parts. But thanks for the help.....appreciate it. :)