Does it sputter out? Like its starving for fuel? Check to see if you have the gray fuel lines. They will need to be replaced. Also clean the fuel selector, carbs, and fuel filter.
Here's a thread you might want to read, I think it explains your problem in detail. You'll need to replace the grey tempo fuel lines with 1/4" rubber fuel line hose. If you can give us your model we can help you more.
