“Shafty” Drive Train – Poor Fuel Mileage?

[bgk]

In my growing collection of GS bikes, I have two 1000cc models, a 1979 GS1000L chain-drive and a 1980 GS1000G shaft-drive. I ride each of these bikes regularly during the riding season. Same rider, same weight, same riding style, etc.

The 1979 chain drive bike gets a solid 40 mpg or better, and that?s with hopped-up carbs, exhaust, and ignition, (Dynojet kit in carbs with individual K&N pods, a MAC 4 into 1 pipe, and an electronic ignition from a later-model GS).

The 1980 shaft drive bike is bone-stock, and it only gets about 30 mpg.

Both bikes are well-tuned, with batteries and R/Rs and coils output checked, new NGK plug caps, etc. Carbs on both bikes have been cleaned and float levels have been checked, etc. and both sets of carbs have been dynamically synched with a Motion Pro set of mercury gauges. The insulator colors of the spark plugs from both bikes are generally medium tan to light tan.

Both bikes run nicely with no hesitation or ?flat spots?, plenty of power, etc. There are no weird noises or wobbles or vibration or anything else detectable while riding either bike. When I put the bikes on the center stands, with transmissions in neutral, I can spin the rear wheels by hand with no unusual resistance or ?tight spots?.

Both speedometer/odometers seem accurate, as I often ride past unattended police radar trailers parked on the side of the road, which display the radar-detected vehicle speed on a big LED display sign.

So, how come the shaft drive bike gets such poor gas mileage compared to the chain drive bike? (I?ve read posts on the GSR from other guys with big bore shaftys, and they are getting around 40 mpg or better.)

Could it be something in the transmission or clutch?

Is there a diagnostic test of the drive train that doesn?t require pulling it all apart?

Thanks in advance for your responses.

[duaneage]

Check the brakes, one might be dragging
Check tire pressure.
Check the wheel bearings.
Are the wheels aligned? Poor alignment will worsen gas mileage.

I get 48 around town and 55 on the highway on my 650 so I don't think the shaft drive is a fuel waster. Granted a chain might be more efficient but not that much more, certainly not 25% more.

[redliner1973]

I get 40MPG regularly on my '83 GK, Sounds like you jave another issue with it. 30MPG is way low...

[dpep]

I agree that something is wrong. Have you checked your compression? I range between 35 and 40 with my '80 850, and my riding style is fairly brisk. I am also pushing a Winjammer fairing and carrying a heavy top box. Riding steady, level, and sane will push it over 40.

Do check the brake thing. I have had my rear brake drag in the past. That will also eat up your disk in pretty short order. On level ground put the bike in neutral; apply and release the brakes; then roll the bike forward or back. It should do so easily. Try it after you have ridden for a while and everything is warm. You can also tell by touching the disk after a ride and see if it sears the flesh on the tip of your finger.

[LW_Icarus]

Check the brakes, one might be dragging
Check tire pressure.
Check the wheel bearings.
Are the wheels aligned? Poor alignment will worsen gas mileage.

Just curious how you would align a shaft drive bike?

[bgk]

Good morning. Merry Christmas.

Thanks for your responses.

I?ve checked compression and air pressure. I have not had the rear wheel off to visually inspect the wheel bearings, but I have had the bike up on the center stand and pushed and pulled on the rear wheel by hand. I have run the bike in gear while on the stand and have looked carefully for any rear wheel wobble.

Like LW_Icarus, I don?t know how to align a shaft-drive bike, or if it?s possible to do so. (It?s certainly possible to check alignment, but I don?t think it?s adjustable.)

Yesterday (Christmas Eve) was unusually warm in New England (around 45 to 50 degrees F), and I managed to wrangle ?flight clearance? from my wife to ride for a couple of hours in the afternoon.

I rode the G model on the more traveled, better-plowed suburban/rural roads at moderate speeds and used the brakes lightly. I was particularly light on the rear brake (most of my braking is typically on the front brake anyway). Every half hour or so, I stopped, put the bike up on the center stand (tranny in neutral, engine off), rotated the rear wheel by hand, and touched all three brake discs with my bare hand.

The two front discs were never warm, but the rear disc was variably in the range from just slightly warm to fairly warm/almost hot, but certainly never too hot to touch. The rear wheel rotated very smoothly by hand, with little or no resistance, but would stop rotating as soon as I stopped rotating it by hand (there was no ?coast-down? after I let my hand off).

I guess that this means that the rear brake is dragging, but very slightly.

Could this slight dragging condition be costing me 10 miles per gallon?

I have never done any brake work myself, but I have a Clymer manual and I am becoming increasingly competent as an amateur motorcycle mechanic.

Any tips or suggestions for me, before I start my first brake adjustment/repair project?

Any other ideas?

[bgk]

Today is a holiday in Massachusetts and I have some spare time to play mechanic with my bikes in the basement.

I got up early this morning and pulled the 1000G rear brake caliper assembly off the disc.

Everything looked OK with the caliper and pad assembly, at least as much as I could tell from looking at the pictures and reading the text in the Clymer manual. (I think the Clymer manuals are not particularly well written and the black and white photos are too small, but the manuals are far better than nothing, for rookie mechanics like me!) Maybe I?ll need new rear pads sometime during the 2006 riding season, but there?s nothing from my inspection that would lead me to conclude that the rear brake is costing me 10 miles per gallon.

I rotated the rear wheel by hand (engine off, tranny in neutral, bike on center stand) and there was no perceptible difference in rotational friction whether the caliper was on or off.

I also pulled all four spark plugs and confirmed light tan color on the insulators, after my two-hour low/medium speed suburban ride on Saturday.

So, I still don?t get it.

Why is my fuel mileage so low?

[duaneage]

You can align a shaft bike by loosening the two bearings and moving the swingarm to the left or right until it aligns. The front adn rear treads should track each other. When I dropped my swingarm for painting I measured the distance to the frame on one side to make sure it went together right. The bike is straight at speed and does not travel to one side when I get on it.

I would check the brake pedal pivot (it goes through the frame) to see that it moves freely. I just parted out a 650 that had a brake pedal pivot that was tight, the rear brake drug on it.

[DaveDanger]

I think we all need to have dpep (Don in Jax) come over and check our brake disks by searing his fingertips! (Wicked grin).

[highmileage]

Just a thought...

Some years ago I had an '82 CBX that had a similar but more drastic problem. Rear brake got hot and would bind a bit after riding it hard.

Turns out that it had developed a very slight leak in brake hydraulics. No visual sign of leakage, mind you (no sign of fluid loss ever). But it would leak a tiny bit of air into the system, and unlike brake fluid, when air gets a little hot, it expands and makes the pads drag on the disc.

Diagnosed the problem by noticing that the bike got more difficult to push immediately after a brisk ride than after it sat and cooled for a few minutes. And yes, that disc did sear my finger.

Ended up over fixing it - rebuilt the rear master and slave cylinders and replaced rubber hose w/ steel braided (and replaced the toasted pads and rotor). I was not going to deal with this problem ever again!

[JMHJ]

I had a stock '83 GS750T (chain, TSCC) for about three months, and got low 40s. I now have a stock '83 GS850L (shaft, 2-valves per), and get mid-to-upper 30s with the same kind of riding. I just figure the weight's more (536lbs dry vs 498lbs dry - not too much really), there's 100 more ccs, and maybe the TSCC engine makes the power a little more efficiently than the standard one (just guessing). I do hope though, that a thorough tuneup might improve things, though the bike seems to run pretty well as it is.

[Einar J Skog]

Hi

I got between 40--45 miles on my bike.depending on the driving type.
That numbers was from this season.
This winter I also degreeing cams and put on a Lazer competition 4-1 exhaust.


GS1000GL
(1100 wiseco, KN filter, dyno stage 1,Yoshimura exhaust,....)