Gas and Oil Home Heating Furnaces - furnace back pressure

could anyone give me alist of reasons a furnace would have back pressure. im aware of the obvious plugged flue or combustion chamber that needs cleaning. my roof jack is 4 yrs old and is clear,and so is the cap. the combustion air tube is clear also and is taking in fresh air from outside. i have a new heat exchanger because the original cracked after only 4 yrs. i also purchased a new burner assembly ,so i basically have a new furnace. the only problem is fumes that are backing up right where the transformer hinge is. i can feel it blowing on my hand and also smell a kerosene/dirty smell. had about 4 different techs out and the problem is that they are immuned to fumes and say they dont smell anything. the unit starts fine with no noises or hesitations. also can someone tell what color the tips of the flame in the combustion chamber should be if youre burning kerosene. i have a cmf 80 po conv downflow miller furnace with a beckett burner and its a closet installation in a mobile home. thank you for any replies.

Is the burner the same size? (GPH)

Same burner tip as old one? (cone shot? 90degre?)

A new vent installed, and not undersized?

First you said 4 techs have been on this. What kind of draft did they read over the fire?????Is the barametic damper ok. Why are you burning Kerosene and not #2 or #3 fuel oil. Have they put the same gph and same spray angle nozzle in . Did they move the nozzle set up in or out of the air tube any. As for as the flame goes ,it should be just some wisp's of black now and then. It should be set with a combustion test for CO2 temp and draft.
Why the new burner? Was the cone head gone? 4 years a new heat exchanger are you over firing this unit?Air filter clean, no other fireplace or fan to the outside?;) ED

The air you feel at the transformer hinge is not back pressure. It is the static pressure that builds in the burner chassis. Your techs (all 4 of them) need to measure draft over the fire like Ed said. You don't have much of a chimney but that is how your furnace is designed to work. It may have been out of adjustment in the first place to burn out a combustion chamber and a heat exchanger. Someone needs to make sure you are running 100 psi at the fuel pump and using preferrably a Delavan .579 mh nozzle. Be sure the 'z' dimension on the retention adjustment plate is correct. And do not set up the flame by color. Get it adjusted with instruments. Set it up for 11% CO2 with 0 smoke. If you do that, you won't have to touch it again for at least another year.