Motorcycle Repair: XV700 Virgo (699cc ) 1984, auto machanic, coil resistance

Question
I have an 84 700 yamaha virgo. front cyclinder,exhaust blows cold air.

Carbs have been checked and cleaned, coils have been checked by shop. If I pull the boot off the wire and hold the wire by the plug I can see the spark,however if I raise the rpms up the spark goes away. And I at least know fuel is getting to the bowl.

I work in the auto machanic field, but bike are not my thing. however on a car the is a compter to control spark is there  one on the bike?

Or is this problem internal as in vauvle stuck shut? any info would be greatly appreciated,.

Answer
Mike,
 I'm not sure if that year used an ignition module.  Follow the low tension lead from the coil and see if they lead to one.  You may be dealing with a capaciter discharge system a.k.a. points.  That aside, check that all your connections to and from the coil are clean tight and corrosion free.  You should really diagnose spark by removing the actual plug used, place it in the cap and hold ground/threads to grounded metal/cylinder head.  By removing the high tension lead from the cap you eliminate finding any error that occurs there and weaken the connection.  If you have a tight connection there and else where and the plug still fails to produce a consistant strong blue spark at higher rpms you may have a bad plug.  Replace w/ the proper one gapped correctly and retest w/ the plug out again.  If you still have a problem yo may be dealing with a weak coil (resistance checks are not always accurate and resistance readings can change with tempature changes) or a failing igniter unit or point set.

Hope this helps,
Mike