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bad fuel efficency

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My friend recently purchased a 2001 520 exc with a fresh top end rebuild on the engine. Why does his bike only get 20mpg? What should he expect to get. The bike runs great so I don't think the jetting is messed up. The bike has the fmf pipe and header. It seems to get 20mpg no matter the riding or terrain.

Thanks for any ideas.

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If it really is running good the plug should look good too.

If the jetting is correct it must be losing fuel out the overflow tube. Check the float height using the overflow tube. Assuming that he has a clear drain tube, bend the overflow tube around so it is vertical and open the bowl drain. The fuel level should be 8mm above the bowl gasket.

I get 35-45 mpg with my 540 depending on the type of riding I'm doing.

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Right on, Thanks. I was thinking the same thing because I always see gas come out of the drain tube when we load the bike in the truck.

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The carb has an o-ring in the carb surrounding the float needle seat.

They get hard from the alcohol in the gas.

Phillips head screw holds it in. Air pressure in the inlet removes it easier.

You will find a brittle broken o ring.

Sorry this forum really has no one posting the most basic fixes that are well known on the other site.

Almost amazed actually.

The gen 1 fcr needs a 48 pilot jet. A Ocemn kehein needle in the 5th clip from the top position and a 162 main.

Doc bob

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We checked the float height and it seemed set right, I will have to check that o-ring next. There was a vent hose that was kinked shut, would this cause the bad fuel efficeincy? We want to get this figured out so we can ride to the Grand Canyon before it gets to hot.

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We checked the float height and it seemed set right, I will have to check that o-ring next. There was a vent hose that was kinked shut, would this cause the bad fuel efficeincy? We want to get this figured out so we can ride to the Grand Canyon before it gets to hot.

All the old o rings die.

Bob

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