I thought I had old fuel, so I siphoned it out and replaced. Motor still hard to start, won't idle long, and accelaration has to be done by feathering the throttle lever. When it dies, it backfires. Sometimes top out speed is 30, sometimes its 60, not consistent. I'm thinking either gummed up carbs, or fuel filter clogged up. Where do you experts think I should start ?