Motorcycle Repair: engine stalling under load, yamaha virago, unleaded petrol

Question
Hello Bill,
My question is not necessarily Honda (as you recently helped me with),but could affect any bike I imagine. A Yamaha Virago XV250 that runs like a dream, unless you open the throttle quickly in top gear, then it stalls, recovers quickly when you throttle back. Also, when traveling up a long hill, even if sitting on 90 - 100 km/hr comfortably, if I reach the end of the throttle it does the same thing. It will sit on 100km/hr all day and not miss a beat, only in the above circumstances. Changed the plugs and run it on 91 unleaded petrol as advised by dealer. Carby is a Mikuni? downdraft.
Thanks in advance if you can help. Regards, Ken

Answer
Ken, you didn't say if it was a stock bike with OEM air filter and mufflers or not. ANY changes to the intake/exhaust system will affect the original jetting calibrations. When you get that flat spot, try pulling on the choke a little at a time to see if it gets better or worse. If better, the you need a large main jet or you have some flow restrictions. Be sure that the fuel cap is venting properly, the petcock/fuel valve screen is clean and there is not sediment lying in the bottom of the float bowls.

In the US we have alcohol-added fuels, which require a larger main jet for proper running on the older bikes.

Bill Silver