Motorcycle Repair: 1978 ATC 90 wont start, air fuel ratio, motorcycle shop

Question
I recently was given a 1978 ATC 90 that has been sitting for a few years. We flushed the fuel tank,and sent the carb. to the motorcycle shop to be rebuilt. With fresh fuel and a new plug we tried to start. I know it's getting fuel because the plug gets wet with fuel. I have an inline spark tester that lights up, and when I pull the plug and ground it against the head I have a blue spark. It seems to have compression because I can feel it with my finger, but I haven't put a gauge on it. I've pulled the rope about a thousand times with no luck. Any thoughts?

Answer
Jason, as you know you need to have compression, spark and the right air fuel.  All you have to do is figure out which one of the three is out of wack.  

You have to make sure the timing is at the right time with a timing light.  It's also possible the plug is fouled with fuel or oil and the spark is jumping some other location than at the tip.  If the spark jumps down inside the plug, the engine won't run.  

Check the compression.  If it's below 125 psi, you may not have enough compression for the engine to start and run well.

The last thing is the air fuel ratio.  If the ratio is too rich, less than 9:1 the engine won't run ir too lean, greater that 17:1 the plug can't ignite the mixture.  If you put a little fuel in the air filter and the engine runs, the mixture is too lean.  If it doesn't fire it may still be too rich or the ignition.


Good luck
Rich