Problems with New PW Unit

After cold water pressure washing (with average results) for about a year I decided to purchase my first diesel fired, 13 hp Honda dd driven system to improve out results as our client list grew. I have been very happy with results obtained however , Just 2 months and 25 hours into the new unit I experienced burner not firing. Upon taking to shop (under the 2 year warranty) I was informed that there was a diesel carbon ball the size of a tennis ball blocking the burner causing it not to fire. This carbon ball also created a secondary fire inside the burner etc etc etc. although this particular service will be covered under warranty the service department said that the manf probably would not cover burner replacement due to sulfur issues from the diesel. I have always purchased the low sulfur diesel. What do you think might be causing this? We do not abuse the unit and are very careful and quick to change out tools and use at least a 5 minute cool down before shutting off the equipment. I did install a 90 degree exhaust stack to redirect exhaust gases outside if my box truck,

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If the elbow is JUST an elbow, and the ID is the same size as the stack, it won’t affect the burn. Do you run your engine at full speed to keep the charge rate up? Could be the burner just needs to be leaned out a little.

The elbow is the same size, however I did extend the stack so that exhaust clears the truck. And yes it is run at near full throttle

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The extension is about 3 feet long

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Well it is more than just an elbow. I run about a 3 ft length from elbow to exit the truck. I do run it at full speed. Repair shop said that have seen exhausts set up like mind with no problems.