H140 Excessive black smoke from exhaust

[Tanner712]

I have 2 of the choctaw H140 heaters. The one works perfect, The other smokes a lot of black smoke while running but does produce heat, I have verififed all fans are working, changed the fuel filter, and cleaned the burner assembly and fuel nozzel. Any ideas what might be causing this?

 

[155mm]

run some kerosene thru it, like 50.50 with good clean diesel, and add some diesel 911 to clean the tank. or run external fuel with good diesel to eliminate any fuel tank issues.
Verify the boot is clamped on both ends on combustion fan. also check no mud dobbers built a nest in fan or fan outlet.
exhaust outlet is clear, didnt forget to take the cover off, (i have done that, caused alot of smoke)

does the thermostat cycle off and on correctly?
is the box sealed fairly well with no big air leaks?
what fuel pressure you running? more isnt always better. i am running 90psi at 3100'