But at 15 mpg I doubt the engine would even run it would be so rich. I'm betting on a fuel leak to somewhere outside the engine, maybe a cracked fuel like that mainly leaks when the engine is running (vibrating) and evaporate quickly off the warm crankcase. At 15 mpg excess fuel through the carbs and cylinder rings when the bike was sitting would cause the rods to slap the excess fuel/oil mixture blowing more gas through the surge tank further enriching the fuel mixture.
Try filling up and either immediately ride an hour before refueling to check mileage or at least turn the petcock off when stopped for more than a minute to elimate excess fuel drain past the floats and petcock. Would be interested in the results. Since replacing the fuel lines is a simple 30 minute job I'd try that first.
You may still have a leaking petcock and float valve (I had both) but 15mpg is pretty bad. Even with a leaking petcock and leaking float valve (1 leaking float valve) when parked a few days before refueling I still got 40 mpg and a slightly raised oil level. Enough float valve/petcock leakage to give you 15 mpg would cause MAJOR rod/oil slap if the excess fuel got into the crankcase.
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Last edited by Spencer1; 05-31-2013 at 11:10 PM.