strong fuel smell

[mega3xl]

Hi, I have a 99 yukon SLT. When I crank the engine, it takes a while to turn over (and not always on the first try). There is sometimes a puff of smoke from under the hood and it back fires. There is a strong gas smell and the 2 & 4 spark plugs are fouled up. My MPG has decreased as well. What do you think the problem is?

 

[SunlitComet]

Do compression test on the 2 cylinders to check for a blown gasket. If all is good you injection assembly may be toasted.