How To Improve Gas Mileage

[Jerry]

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.

[sixtysix]

Fuel Mileage Addendum
By Gary Granger

Now that Jerry has covered the easiest and most common ways to improve fuel mileage, I will cover some other things (less likely) that may be causing bad mileage.

Exhaust Baffles - If your baffles rattle around or have become loose or are rotting out, this may cause mileage to drop suddenly. I think it has something to do with the loose stuff blocking the exhaust. I replaced my old drilled out Jardine slip-ons with new ones, (the same exact Jardine slip-ons) and picked up about 3 MPG. If you hear the baffles rattling around it may be part of your problem.

Wheel bearings - Anything that causes rolling resistance can cause increased fuel usage and that includes 20+ year old bearings packed with 20+ year old grease. Check them when you have the wheel off and replace them if they seem stiff or notchy.

Petcocks - On my 80 Standard I found the vacuum line to one of the petcocks to be wet with gas when I pulled the tank off. The petcock had failed so that fuel was being pulled directly into the vacuum port in the carb boot and into the cylinder. A rebuild kit fixed this up. A special may not be able to fail this way (not sure), but if any vacuum lines are wet with fuel then some repairs are in order.

Carbs - Besides the already mentioned cleaning and tuning, there are a couple of things that can cause bad mileage with carbs. One is leaking diaphragms, I inspect these with a small intense light in a darkened room to insure there are no pinholes. Another is worn main needles or needle seats (emulsion tubes). Either can cause too much fuel to get by at any given RPM, please see the Factory Pro site for more info on how this works. On early model carbs the needles could also be set to high, which does the same thing.

Weight - Increasing weight and wind resistance by adding a fairing, windshield and/or bags can easily add 75lbs or more. More weight is less mileage.

Engine Condition: If you have poor compression, stretched out timing chain, worn cam lobes, etc. You will not be able to get optimum mileage out of your bike.

Bottom Line on Mileage: A myriad of factors including those mentioned above as well as, wind, towing, ambient temp, driving style, and many other things, can affect mileage. Even different fuel blends can cause mileage to suffer. An XS/XJ in good mechanical condition should get between 33-40 MPG, don’t expect miracles, start with Jerry’s recommendations if you feel the need to improve mileage.