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Old 07-29-2002, 05:12 PM
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How to improve gas mileage

How to improve gas mileage
by Jerry Fields
edited by Mike Hart


This is a recurring topic. I have an XJ with Vetter Windjammer IV (tall windshield) and Vetter saddlebags. Here in AZ (home elevation at around 5,700 ft.) I get 33-34 mpg. At lower elevations I get a couple MPG more, 34 - 36 on average. On a non-faired XS/XJ at sea level, your range should be 30-40mpg depending on riding style/speed. Mileage falls off after 60-65mph. High 30's to low 40's is realistic for cruising at 45-65mph.

What to do about it.....
  1. Put in new spark plugs of the right type: BP6ES. Check them for color; you should get a light tan on all 4 if the carbs are adjusted correctly. If not, adjust and synchronize the carbs.
  2. Clean/replace air filter element. On an XS you can replace with K&N, Uni or pods; for an XJ, OEM only, no after-market unit available (but there is a tip under construction now to make your own K&N XJ filter).
  3. Check tire pressures, (Dunlop 32 PSI front, 40 rear)
  4. Check the carb boots for leakage. Easiest way is to squirt starter fluid or WD40 at the carb boots with the engine running. If the RPM increases, the boots are leaking and will need to be patched or replaced. (Black RTV sealant, or see the Carb Diaphragm Repair tip for more info) (Make sure the clamps are tight!)
More advanced options:
  1. Clean carbs. Pay special attention to setting float levels.
  2. Check/replace coils and/or spark plug wires. (See New Wires in Old Coils and the tip all about coils.)
  3. Check for dragging brakes. Clean both master cylinders, paying special attention to the 'spooge hole' that allows pressure to release from the lines. (See the Brakes not releasing? tip)
  4. Along the same lines, you may have brake cylinder piston seals that are sticking and not allowing the pistons to retract, causing bake drag. Rebuild the calipers. (Sometimes a good polishing of the piston and cleaning of the seal/o-ring is sufficient)
(Both 3 and 4 should be easy to diagnose. Check the temps of your rotors after a while riding without having to brake. If they to hot to touch and hold on to with a bare hand, problem is in the brakes. You may also consider replacing the 20 year old stock lines with Chris Rawson's stainless-steel variety.)

Final culprit could be a slipping clutch. If the engine revs up a bit (under load) but doesn't increase in speed, clutch is slipping. Adjust clutch cable, replace friction plates, or install new clutch springs. A common set-up is using stock Yamaha friction plates and Barnett springs. Consensus is to install friction plates and springs at the same time, although some have had good results from just installing new springs or shimming the old springs.
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