Have juice to the coil,but no fire.is there a sencor on the motor,if there isn't any oil in the motor ,it won't fire?

Hi Mate, You should have a '85 - '87 twin carb thumper like mine. On the L/H side of the engine close to the sprocket is some wiring comming out of the engine (from the exciter coil that generates the high voltage supply responsable for the spark plug coil.) It tend to break down and give a weak spark or no spark at the plug. Follow this cable up the frame to under the seat where you can unplug the red/black wire. The exiter coil should measure 300 ohm to earth +- 10%. If the value is way off, lay the bike on it's side and strip the L/H engine side cover off. If the prominent centre coil fitted to the assembly looks visually burnt - it had it! Your local electric motor rewinders should be able to rewind it for you or ex Japan - $$$ or buddy up with a trusted engineering shop en have the housing machined to acomudate a Yamaha YFM 350 Big Bear centre coil. Note the difference in hight at the mounting brackets. The other x4 coils on either side of the centre is the generators for the electricals such as lights. the other two wires in this harnass should messure in the reason of 30 ohm going off memory. The CDI do seldom corrupts. These bikes are also renown for cracked cylinder heads should you have a good spark but weakish compression causing majour starting problems. Best of luck... take your time with it. Great bike once issues are sorted!

There's no oil sensor on your XR600's engine. If you aren't getting spark at the spark plug, check all of the bike's electrical connections for corrosion. Physically unplug each one and look for discoloration, melted connectors, etc., and scrape away any corrosion. Spray each with some WD-40 upon reassembly. Eliminate the kill button as a culprit by leaving it unplugged and check for spark.
If this fails to cure the problem, then one of the components that make up the ignition system are bad: CDI unit, pulse generator, alternator, or ignition coil.