REDNUT, I would suggest you do air flow mods first to increase mileage.
A K&N air filter and a complete 4" exhaust will support all future mods and in the mean time improve your efficiency, power, response, and milage.
Adding fuel at this point will decrease milage.
There are some things you can check if you have the tools.
First, the two rubber fuel hoses on the back of the engine, they dry rot and reek havic on driveability, you probably should just go ahead and replace them.
Second, fuel pressure, Remove the feed banjo bolt and drill and tap it 1/8 pipe thread and reinstall it with a guage attached, 0 to 30 psi will be needed.
You should have 21-23 psi at idle and as you raise rpm the pressure should raise smoothly to 30+ psi, if not, you need a new overflow valve. $79. 00, it bolts in at the top of the pump just under the rad hose.
Third, Locate your lift pump between the starter and motor mount on the left lower of your engine, mounted with it is a fuel heater, there is a screen in the bottom of it, you can unscrew it like an oil filter, remove it and clean it out, I have found some that where almost completely clogged.
Pump timing is not something you can check your self, you will have to take it to a cummins shop that does spill port timing or come see us at JRE.
There are many other factors that affect milage, the list is huge!
Start with what I have suggested and you will be on your way to better milage for sure.
Ted Jannetty
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Ted Jannetty, Owner President Jannetty Racing Enterprises Inc.
97 3500 4x4 club cab 5spd. RED, JRE stage 3 power kit, JRE 4" exhaust
JRE 1/2 inch fuel system, Bosch 300 hp injectors, Prime loc,
Cummins chrome kit, US Gear 20% over drive, 3. 54 posi, Autometer
Ultralite 50lb. boost, and Pyrometer, BD exhaust brake.
99 Ski-Doo 800 Formula III
99 Honda Forman 450 ES
2001 Ski-Doo MXZ 800 X
2000 Transam Ram Air WS6