I can't seem to start any of my posts without a bit of a back story so here goes. Last September at an ARD I developed an issue on the second day where the bike wouldn't rev out fully in 4th and 5th gear, I couldn't get to the top of sixth even with a long stretch of runway! Power was completely fine everywhere else and when I grabbed the next gear was perfect but there was a loss of power over 10,200rpm that felt like the bike was just being held back. Two sessions later and the rear/top cylinder exhaust header cracked so I assumed that was the issue, the crack was developing, stretching at top rpm and throwing out the fueling due to the exhaust gasses leaking. Header all-welded and repaired and I assumed good as gold but it's obviously hard to test the top of 4th gear when not on track. Had the annual service at Moto Rapido (that's another thread for at some point) and I mentioned the issue to them. All I had back was everything seems fine and they suggested to turn down TC on track. I found this a bit hard to believe that TC would be causing the issue when dead straight, on a runway at 150+mph. Fast forward to now and after getting carried away a few weeks ago I could have sworn the same thing happened again. Being back on track this weekend I didn't want to turn up and then confirm the issue is still there and then worry about riding for the rest of the day so decided the best bet was to get the bike on a dyno to test each gear so I booked in at DynoTech Performance as they're nearest to me with a dyno and Steve Jordan was typically booked up until July. I explained what I'd experienced and they loaded the bike on the dyno and started doing some runs. When they started doing power runs I could instantly hear and see on the graph the dip I had been feeling. Power was literally dropping off like a cliff on the graph and you could hear it as well. After the first set of runs he thought it might be the clutch plates slipping slightly at the top end which obviously put the fear in me but the other guy wasn't convinced so said to do runs again in each gear. This time round it was a bit inconsistent and didn't do the same in 4th as it has before but issues was still there but not in lower gears. They then had a brain wave that it might be the exhaust valve closing slightly under max load and with the vibrations from the bike just start to make it move. So a third set of runs and they watched the valve, did a run and they saw it start to move a little, held it in place and it did perfect runs right up, you could hear the difference instantly! The bike has the Healtech servo eliminator fitted which I found tucked in the fairing a little while back but what they guys said is that they always lockwire the ones that they disable to prevent something like this happening but they've never actually seen it happen. So I'll be doing this tonight and then hopefully having a smooth running day tomorrow. On a side note, bike made 172hp at the wheel which considering the heat in the cell, ambient temps at the time, I don't think they pulled in 6th as power figures wasn't the aim here and the fact it had sat doing an hour of runs I'm pretty happy with. Their dyno is VERY accurate as they put it often reading 10hp less than others. It's also very clear from the AFR how much this would benefit from a map, very lean at low rpm, rich up the top. Dyno below shows early run with the drop in red, later run where the valve moved slightly in green and then held open in blue:
