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I've been noticing some wierd symptoms for the past four months, where within a few minutes after a really hard run my bike will "cough" or "stumble" when rolling the throttle back on from the 3k to 6k RPM range. I tried everything I could think of: flush fuel system, change plugs, clean air filter, check coils & wires, check carbs, etc. The problem has never occurred after any kind of "normal" riding, only after riding like a total idiot for at least 10 or 15 minutes, slowing down, and then getting on it again. To makle a long story less long, I discovered quite by accident that I was getting vapor lock! I went to take off the fuel tank yesterday and the line had so much pressure in it that it hissed for a couple seconds. I replaced the fuel line with one that has a fiberglass braid around it, making it as short as possible so that it has no spots where it runs horizontal, put in racing fuel and voila! No more vapor lock. I tried the best I could to recreate the symptoms today to no avail, but it was a great ride! :cool:

Racing fuel typically has a higher boiling point and lower vapor pressure than pump gas, which makes it less prone to vapor lock. I take back what I said in an earlier post about higher octane fuel having less energy. Holy $#!% does this bike run good on Sunoco GT racing fuel!

Has anyone else noticed these type of symptoms? Probably not this time of year, but in hot weather or high altitude it could happen to you too. I'm not talking about passing a few cars or going "only" 80 or 90 MPH here, more like climbing a big mountain at 9000 RPM or making a 10-mile long top speed run. Perhaps the problem is that we're getting higher-volatility "winter" fuel down here due to it being winter up North, since the problem has existed only sporadically since November. I might try to get some kind of insulating material to put over the fuel line 'cuz running $5 a gallon race fuel isn't very economical. But it sure was fun! :smilewink
 
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