Topic: sudden high speed miss- any ideas?

[Jay B]

Sorry I haven't done much in the way of diagnosis yet, my bike's in the garage still too hot to touch. I've put about 1000 miles on my 350f since I redid it last winter, and it's been running great. The other evening I was running it hard up through the gears, and all of a sudden at about 9k or so it began missing badly. I backed off the throttle and it ran fine below about 5500rpm or so. This evening was the first chance I've had to look at it. I'm running the stock fuel filter screen and also an inline filter. I removed the line downstream of the filter and got way more gas than this little engine will ever burn. Next thing I checked were the points. Sure enough, I found a set had slipped open to about .024". I thought great, that's gotta be it. So I adjusted them back down to about .016", hoping this would cure things. Nope. Went out for a ride. It starts and idles great, and runs great till about 5000-5500 rmp, then misses pretty bad. If I struggle up to 70 mph, about 7k rpm and back off the throttle, it will quit missing and run fairly well. It just won't take any throttle. My plugs were a perfect light tan before this happened. Air filter is new. Complete tune-up about 300 miles ago.The points were too hot to mess with, but they both looked OK opening them with a screwdriver and looking at them. No noises, ticking, cam chain noises,etc. Probably the biggest clue here is the fact that this happened in the blink of an eye to a very good running bike.  I was just wondering if anyone has had a similar experiance, or could suggest a good place to start. If not, I guess I'll start tearing into it one of these first nights.

[Gordon]

First thing I'd do is check the condition of the plugs since the high-speed miss began.  The same thing used to happen to my 400 when I ran it fast, before I got the carbs completely sorted out.  It would foul out the plugs at high rpm and lots of throttle. 

[Green550F]

my 550 had a sim issue. bad cap arcing to head, new plug cap fixed it for $4

[Steve F]

I was just gonna suggest checking the ignition caps and wires too, but Green550F beat me to it.  Had something like that happen on my 750F and it turned out to be a bad wire.  It's kinda strange that it only shows up at the higher RPM's. Fixed the wire and it smoothed out.

[Jay B]

Thanks guys. My thoughts were along those lines also. I'll take a look at it soon and let you know what I come up with.

[TwoTired]

As the combustion environment gets more severe under increasing power conditions, it takes a higher voltage to jump the spark electrodes inside the cylinder.  When the voltage increases, defects in insulation regarding the ignition lead become more critical as it is taxed closer to its limit of containment.
 When the voltage leaks through insulation defects, it makes a spark at the defect and can follow dirt trails on leads and the coil body emanating from the leak to an electrical ground.  These can be visually found by observing the running engine in a darkend garage.  Its a mini lightning show.

It will use your finger as an electrical path, if you want to find it by feel.  But, most find that rather uncomfortable, as well.  If you use this method, put one hand in your pocket.  So, there can be no electrical path through your heart.  The spark won't physically damage your hand or fingers, but the sudden stop of hitting stationary objects due to muscle recoil can easily draw blood and lingering discomfort.

Cheers,

[Jay B]

Carbon fouled plug, #2 cylinder. Kind of an odd deal. Looks to me like a chunk of carbon, hopefully from oil consumption during break in, let loose during my Italian Tune-up and got jamed into the end of the plug. Explains the sudden-ness of the problem.  Like TT said the spark must have been arcing someplace during light loads and when the going got tough it couldn't handle it. I'll be keeping close watch on that cylinder and maybe have to do some exploritory surgery next winter if it keeps up.