coked up exh ports 2

[nobbie]

hi all and happy new year

i have recently acquired a cheap 855 cc,3 cyl diesel.no numbers on it but i think it may be a kubota or universal diesel.the bore and stroke are 72 x 70 mm
anyway upon trying to start it i found that the valves were leaking and upon inspection i have found the exh ports to be full of carbon.this in turn has stopped the valves closing fully and resulted in u/s valves although the seats seem fine.
there is not much play in the valve stems,stem seals seem ok...well definetly not hard as in cooked.the bores are rather shiny but with no lip and the pistons seem tight in the bores.
so what could of caused this i am wondering.
the enginge drove a refrig unit for a container or semi so i am guessing that this engine has run without load at high rpm[the pump was wound up] and the carbon has been soot from fuel burnt and possibly a design fault in the head..ie ports too shallow.

so i intend to renew valves,lap them in and see what happens,

so any theorys or experience would be appreciated.

 

[Deltona]

Does it have EGR (Exhaust Gas Recirculation)? This can cause bad coking problems as can worn valve guides and stem seals.

 

[nobbie]

no egr on engine
i wonder if they had the exhaust outlet pointing forward,when the truck was rolling it would create a large back pressure?
stems and seals seem fine,maybe they were using the wrong oil.
ive never seen it so bad and ive done a few engines.

 

[racear]

What I see many times is too rich a fuel circuit that washes the oil off the cylinder and then both the rich charge and oil gives the coking action in the port
racear

 

[dicer]

This engine must spend many hours at or near idle speed and or very low load?

 

[nobbie]

the engine drove an 8 amp 415v generator as well as cooling gas recirc,so you may be right ..minimal load..

 

[CH5OH]

to low load ,definetly