2007 Service Man/Handbook says spark gap variable of .038-043.
as you can see below I use NGK Iridium IX (DCPR8EIX) pt #6546. NGK recommends the #7 hotter
plug which in some informational areas means 70-100 deg's hotter spark. Somehow I tend to think
this will bring on a hotter engine as well. By all accounts my plugs read exactly as they should, slightly
dusty with a light gray, little more gray on back plug. Chris Wadell Cycle Solutions
http://www.cyclesolutionsonline.com/ put the Jet Scream unit over the throttle body back in 2008
Thunder in The Valley, Johnstown, PA. and recommended that plug with the conversion. It has always
burned true to the charts. And had since installed the SE 10 mm plug wires.
My engine until recently since the Gap was reduced from .040 to .038 was when warm/hot backfire
1/3 of the time at start up, and progressed to cold start up.
I was headed to Pittsburg & for the real test of the fine tune yesterday. The Bike never shut down,
and this was in bumper to bumper 3 mile traffic near Squirrel Hill coming and leaving the area. The
ambient temp was @ 80 deg. There was not the uncomfortable heat coming off the engine when in
stalled traffic either, but then again I was all leathered up. I have not had a bad start in over a dozen
starts since the adjustment.
I believe the Coil has been overworked which is causing the problem of shut down. A closer look
after more miles may allow another adjustment to .039
I somehow feel the Bike is running a slightly higher RPM in normal operation now due to the Gap.
More like a shorter stroke connecting rod.http://www.custom-car.us/ignition/spark-plug/gap.aspxAs a rule, a properly gapped spark plug will burn hot without being too wide at high rpm to cause a misfire. Ironically, the car manufacturer's recommended spark plug gap is not optimal! The recommended spark plug gap is designed to be adequate for cold starting and smooth driving on a car that is in need of an engine tune up. If you drive your car normally and tune the engine regularly, you can increase the spark plug gap by about 0.010" for better performance and better fuel economy. However, if you drive at full throttle most of the time, you should reduce the gap by about 0.010" for better performance. The spark plug itself, and the residue that forms on it, would indicate whether the gap is too big or too small. A light brownish discoloration of the tip of to porcelain insulator indicates the proper operation of the spark plugs with the gap being ideal or close to ideal for the most recent engine speeds. Thus, to check the spark plug gap at high engine speeds, you'd need to run at full throttle and immediately turn the ignition off without allowing the engine to idle. But ultimately, you'd need to run your car on a dynamometer to find the best spark plug gap, and the right ignition timing for your engine.
Remember that when you increase the spark plug gap you need more voltage from the ignition coil to create a spark across the spark plug gap. We'll discuss ignition voltage at a later stage. When a greater voltage is required to create a spark, cold starting and firing fouled spark plugs become more difficult. Therefore you should ensure that your secondary, high-tension ignition wiring is at least 8 mm in diameter, and that it is always clean, dry and in peak condition. Also note that it is not advised to adjust the gap on a multi-electrode spark plug as this will affect the proper operation of the spark plug.