1978 kz 650 it starts but blows blue gray smoke through the exhaust pipes. There is also
Oil build up in the exhaust pipes and has low power what might this be?

If that is a car, or a bike, you are talking about, it sounds like worn piston rings and worn valve seals, and it is a 78 so that is probably right, if it hasn't been maintained properly. What is the mileage ?

Tick

I also have a 1978 KZ-650 (Blue) and if the smoke is blue-grey and goes away after the machine is well warmed up, then the valves or valve seals are leaky. If it blows smoke more or less all the time, the rings are shot or the cylinders are scored from running with low oil, no oil, cheap or wrong grade oil, or combination. If that's all that's wrong with it, you could get it bored out and buy oversize pistons and rings. But that's a four cylinder bike, and a bore job isn't cheap and it's even less cheap when you have to do it 4x.

If the smoke is black to blue-black, maybe you have a carb or 3 that are so messed up it's running horribly rich.

There's lots of stuff to check. I'd join KZ-Rider and ask around there: http://kzrider.com/

Motor's tired--rings are worn out