One time I was having a hell of a time figuring out a problem with my generator. It would start fine, and run great for a while, then load up and die. It would crank but not fire after thet until at least an hour had passed. After checking everything twice, I pulled an exhaust hose to get at something on the engine and happened to look inside. While the hose looked brand new on the outside, the inside had been badly burned and started to slough off. When it would heat up, it would expand and mostly block the exhaust. It would still flow enough to crank fine, just not enough to run.
The point is that while the hoses may look fine, you don't know until you look inside.