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Old 10-18-2016, 05:20 AM
  #12  
1QwkSport2.5r
 
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Join Date: Oct 2006
Location: Cottage Grove, MN
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I run converted glow engines as diesels and regularly get dark exhaust oil on all of them. .25 up to .45 size engines. The last engines I ran were K&B Sportster .20 and .28s on a 10x6 APC prop. Both engines ran dark (very dark brown - almost black) when set to peak mixture and compression settings. RPM for the engines was 10,800 and 11,300rpm respectively. Higher prop loads gave a lighter color residue, but I am not sure those bigger props would fly the model as well. I haven't tried it, so I'm not sure. My ST .45 conversion turns a 13x6 APC at 10,800rpm and runs rather dark residue as well. The fuel I use is 31% ether, 22% castor, 47% Jet A, and 2-3% Amsoil Cetane boost for ignition improver.

Perhaps too low of an ether content requires more compression which makes the residue run darker?