I think you have some sort of malfunction, with the mods you have you shouldn't be in the 20's. In fact... as poor as your mileage is, I would almost think that you would see lots of black smoke from your exhaust indicating a very rich mixture. The tips of your exhaust should be heavily coated in carbon. I know for a fact Triumph dealers have an exhaust gas analizer that should confirm a very rich mixture.
You know what you should try first though??? I would check the accuracy of your odometer against mile markers. If you go an actual 10 miles but your odometer only shows 7 thats is going to mess up your calculation for MPG big time. This happens with cars more than you might think... the two main culprits being the speedo head (the odometer itself is malfunctioning) and the vehicle speed sensor and/or its wiring harness. I wouldn't measure it against a GPS. Or is that how you are doing it...... Oh Oh.... The GPS may be spot on and everyone knows the speedo on Triumphs are about 10% off, and so is the odometer probably. Maybe we are all getting 28 MPG in reality but our out of wack speedos are just putting a rosy tint on things...
