My son recently purchased an 05 Silverado HD. It had an issue on starting so we replaced fuel filter and cap, lines going in and out and clamps because it kept loosing prime as soon as you shut the truck off. Then a fuel leak sprouted and we replaced the passenger side fuel injector return lines. It has had an oil change and a new air filter installed. He did the bottle test on the valve in the fuel rail and nothing leaked out. My son informed me that when he hits 2k rpm the truck slightly bogs down. It also blows black smoke on heavy acceleration. The truck has no codes stored on the computer and the check engine light is not on. The fellow he purchased the truck from replaced the pump gear, 2 injectors and fuel sending module. It still does not start right up, it takes about 15 seconds. I was wondering if anybody has any idea of what could be causing this. He thinks it might be tuned. Is there any way to find this out? Appreciate any help. Thanks.