Cure for that flat spot!

[Biker_Bob]

I've streetfightered my FZR600 Foxeye, it ran fine before but after it felt like it was holding back and surging. I went through all the electrical connectors and re-checked everything I'd had off and back on again. I removed the carbs and cleaned them out thinking some sh*t had got dislodged from the tank. No joy.
On a long weekend in Wales I noticed that the bike hesitated when the wind blew!
Riding along if I lifted my legs up and clamped them either side of the engine it tried to cut out.
So, I bought a roll of Gaffa tape and taped up the gap between the tank and frame - problem solved.
The long term fix when I got back was to extend the carb static air pressure reference pipes by about 2 feet each and shove the end of the hose inside the frame spar.
Basically, the diaphragm of the carbs lifts when the engine sucks on the top side, to produce a pressure difference the other side of the diaphragm is referenced to normal atmospheric pressure. There's a pipe connected to a t-piece between carbs 1&2 and another between carbs 3&4. On the faired bike there's a bubble of air above the engine and everything is fine, but with the fairing removed the air rushes around the sides of the airbox and across the bottom of the tubes. Bernouli's effect causes the pressure in the pipes to drop when fast moving air blows across the ends of them, the result is that the carb diaphragm lifts too far and floods out the burn.
With the pipes stuffed into the frame the hollow frame acts as a static body of air (at atmospheric pressure) and everything is good. In fact, the long pipes generate a damping effect on the diaphragms and the bike runs better and smoother than it did with the fairing on.
I had exactly the same problem on a Honda SLR650 years ago when the pipe popped off the side of the carb whilst riding.
I wonder how many streetfightered FZRs are going round with a flat spot, where owner has messed around trying everything to fix it and not cured the problem - If your bike is exhibiting symptoms it's worth a look!

[willandrip]

That's a good heads up,Bob.

The yzf 750 and the TDM850 have a little valve added to those equaliser pipes to dampen their effect.