Small Engines (Lawn Mowers, etc.): B&S horz. twin 18hp wont run, secondary coils, spark tester

Question
Hi,

I have an 18hp B&S engine in an Agway lawn mower.
Model: 422707
Type: 152-01
Code: 9403055B

For no apparent reason one day it would not start, I checked the fuel lines found a crack (no leak) and replaced all the line & filter, and put new plugs in it.  Sometimes it fires but won't run.  Pulled the ground wire on the magneto and the plugs still have spark, adjusted the gap to what I beleve is the correct setting (do you have a spec?).  Now the weird part, I pulled the right side plug to check for spark, grounded it on a piece of tin mounted to the engine, got spark, and the motor started?  If I removed the plug from the engine it will stall, put it back on quick and it picks back up.  Put the plug back in and it won't start?  Side note, from time to time the carb will spit gas out of the top (air filter has been removed.)  Is there anything else to the ignition?  Can the coil work but not be strong enough (spark is blown out by compression)?  Can one wire be bad and the other good (the reason I could get it to run on one cyl)?  Any info or ideas would be appreciated before I start throwing parts at it.

Thanks,
Ryan

Answer
I have one coil, on a single cylinder engine, with weak spark.  Weak spark was comfirmed with an inline spark tester.

The engine is pretty old, when was the last valve service?  Have you ever checked the flywheel key?

It could be a bad coil but not likely since the coil uses the same primary and secondary coils to produce spark.  Are there any holes in the spark plug you can remove and have the engine run.  

Coil gap should be .010-.015.  Did you swap the spark plugs from side to side to see if you have a bad spark plug?  I have had brand new spark plugs that were bad.

Let me know.
Eric