I am setting up a Viper for a trip out to West Yellowstone and I am really confused by the recommended jetting changes found in the Service Manual.
As you go higher, the air becomes less dense so you need to run smaller fuel jets so your engine isn't running too rich. The service manual has you go to smaller mains (which makes sense) but LARGER pilots (which doesn't make sense). They also have you turn the fuel screw out another turn as well (which doesn't make sense either). I am sure there is something that I am not thinking about correctly... can anyone explain the logic to me?
As you go higher, the air becomes less dense so you need to run smaller fuel jets so your engine isn't running too rich. The service manual has you go to smaller mains (which makes sense) but LARGER pilots (which doesn't make sense). They also have you turn the fuel screw out another turn as well (which doesn't make sense either). I am sure there is something that I am not thinking about correctly... can anyone explain the logic to me?
dirtjumper895
New member
Well, this is how it was explained to be. At elevation, there is less atmospheric pressure. So, there is less pressure on the volume of fuel within the float bowl. In order to maintain the proper "nozzle" pressure through the pilot jet, the orfice size must be increased.
turning the screw out allows more air, and leaning it some. Sounds about right especially if you increase pilot size.
dirtjumper895
New member
stingray719 said:
This is true on an airscrew (usually seen on a 2stroke dirtbike), but not on the fuel screws in the TMX33 Carbs used on the vipers.
By opening (turning screw out) allows more fuel, actually richening the mixture.