My Yamaha 150 2 stroke at idle every 5 seconds gives a back fire or repeated stutter. When accelerated it runs better. The one tell tale sign of something wrong is the port side middle cylinder's spark plug shows no carbon/oil/or use. I did put a timing light on the spark plug wires to check for sparks while idling and all showed spark. All the other spark plugs showed the usual black oil and were wet. This middle spark plug showed a bit of unused fuel/oil but nothing like the other 5 plugs. Any insight would be helpful. I was leaning towards the middle carb. I am waiting for my manual to come for better insight. Next move will be a diag. from a mechanic (trying to avoid). Thanks.
