93 r100rs- gas mileage = dismal

[ricochetrider]

My 93 RS gets THE worst gas mileage- EVER.

I'm thinking maybe 37-39 MPG at best. This past summer, I took a weekend ride up to the Adirondaks with some buddies- two 84 R100RSs and another guy on a 73 R75/5.
I'd be on reserve, and everybody else would still have plenty of gas left- even (especially) the guys on 84 Last Edition RSs, with their 40mm carbs. The guy on the /5 with his dinky toaster tank was still rolling plenty of fuel when I hit reserve. Oh BTW, the guy leading the "charge" was riding
F-A-S-T! So, let's say we were totally flogging the daylights out of the bikes. And those guys all got WAY better mileage than me.

My bike gets crappy MPGs no matter if I flog it or dog it. wtf.

Anyone have any comments, input, suggestions, or
knowledge about a possible reason for this?

 

[jad01]

I'd be on reserve, and everybody else would still have plenty of gas left- even (especially) the guys on 84 Last Edition RSs, with their 40mm carbs. Anyone have any comments, input, suggestions, or
knowledge about a possible reason for this?

I'd get an '84RS... they're neato!

I thought that I read somewhere that the later model R bikes ('88 and forward?) had lower gearing and slightly less fuel tank capacity than the earlier ones ('84 and back). Seems there have been a few posts on the forum citing mediocre fuel mileage in the later models. But, I think you get better brakes & suspension on the later ones (trade-offs). Also, how is the state of tune of your bike? It might be due for carb adjustments (rebuild?), etc.

 

[20774]

Some past discussions about RS mileage:

http://forums.bmwmoa.org/showthread.php?46641-Whats-your-R100-getting-for-mileage

 

[20774]

I'm thinking maybe 37-39 MPG at best.

Do you know for sure? Some people (me included) actually keep numbers on miles traveled and gallons refilled. When I go on reserve, and then fill up, I back calculate how many gallons it took to get to reserve.

The reason I say this is that what controls the number of gallons to reserve is the height of the straws secured to the top of the petcocks. It's possible that those straws could be taller than normal in your tank. That would mean fewer gallons to reserve. But if your gas mileage is truly 37-39, then the height of the straws makes no difference...you will go reserve sooner but you will have a deeper reserve amount of fuel...in the end you will go the same amount of distance.

In my calculations, I have also been able to calculate (within reason) how much fuel is below the reserve level in each saddle of the tank. That knowledge can come in handy...it did one time traveling from Hoover Dam to Kingman...I wasn't sure I was going to make it but I did...came within 0.5 gallon of having to walk.

let's say we were totally flogging the daylights out of the bikes.

That can severely reduce gas mileage! But the fun meter must have been pegged!!

 

[88bmwjeff]

My 88 R100 RT gets in the mid 30's on average. I've kept a runing MPG chart for the last 5+ years. Also, I've run out of gas and was only able to get 4.5 gallons in it. The straws don't seem unusually long either. I think the flap in the filler significantly limits the amount the tank can be filled. Lastly, I understand your frustration regarding distance (or lack there of) one can go on the tank.

One thing I'm curious about is that Bing recommends rejetting the carbs because they're were "lean from the factory" to meet emissions requirements. I'd be curious as to the MPG numbers of the stock jetting compared to Bing's rejetting. Unfortunately, I haven't had the chance to do a test myself yet.

 

[lkchris]

I think your mileage sounds pretty normal.

The difference between your bike and the others is mainly the rear drive ratio and yours is lower, thereby spinning your engine faster than the others at the same speed. All the 5-speeds have the same transmission gearing.

Back in the day it was common for me on my '84 R100RS to get better fuel mileage than an '83-'84 R80RT, most of that due to the fact the latter engine was always spinning 1000 rpm faster.

It's not the bigger carbs on the '84 as they are sized to match the bigger valves in the heads. Your bike has smaller intake and exhaust valves to correspond with the smaller carbs and this is in some way a good thing, as the big-valve heads were found to be fragile. The small reduction in horsepower this creates is compensated for by the lower rear drive ratio.

Of course, thrashing and going fast always reduces mileage, but I bet you can keep up in 5th as well as you can in 4th most of the time. It's simply mythology that these engines need lots of revs.

All the bikes in this era were jetted pretty lean, and richening them is NOT going to improve fuel mileage. There is NO jetting change on any Airhead that will improve them from factory specification. Verify your needle jets are set at the proper notch. New ones might help, as they do wear.

And, yes you do have smaller fuel capacity than the '84s ... this from moving the electrics out of the headlight shell to under the tank.

 

[jad01]

Some past discussions about RS mileage:

http://forums.bmwmoa.org/showthread.php?46641-Whats-your-R100-getting-for-mileage

Yes, yes, I remember the trick to better gas mileage now... keep your fly zipped (post #6)!

 

[SCBuckeye]

On my '78 R100/7, I have consistently stayed around 40 MPG - usually a little lower - 36 to 38. I am not a "hotfoot" rider and always accelerate slowly and don't ever race it or hot rod - just to be nice to my bike - AND, I can't afford to have to replace something that would break sooner!

However, a weekend ago, I decided to ride up to see my son and his family in Lexington, KY. - about a 370 mile trip each way.

Got there OK, and stayed the night. When returning the next day, I decided to ride with the traffic flow on I75 south and I40 East. I stayed pretty much around 70 to 75 MPH, so I expected that my fuel usage would diminish.

To my surprise, I actually topped 40 MPG - right around 41 to 42. I was shocked!

To measure mine, I fill my up to the same exact spot in the filler throat (mine has two holes to allow air to escape, so I fill to those hole's very tops). Then I read just how many gallons I used to fill up that that point. Then compare to miles traveled since last filled to the very same level. The Reserve amount makes no difference when calculating this way.

It is good, as Kurt mentions, to know exactly just how much fuel you have left when you do have to use the reserves.

I typically only use fuel from the left petcock - which uses fuel from the left side of the tank left of where the frame tube goes down through the tank. When this side shows that I need to use the reserve, there is still quite a bit of fuel in the right side ABOVE the reserve level. At that point, I turn on the right side petcock.

When the right side needs to be put on reserve, I REALLY know that I better get fuel soon. I have never measured, but I figure I have around 10 or 20 miles left.

This above procedure gives me two warnings about fuel level. However, what I try to do is mentally calculate based upon my near consistent 40 mpg. Around 200 miles, and I have used up around 5 gallons. With a 6-gal. tank, only a little over 1 gallon left! I start looking for a pit stop!

 

[braddog]

37 to 39 would be about what I get with my '77 R100RS, and it's running fine. The only time I exceeded 40 MPG was when I was able to burn a couple tanks of 100% non-ethanol 93 octane gas.

 

[barryg]

bmwrider88, how many miles on your bike?

 

[lkchris]

37 to 39 would be about what I get with my '77 R100RS, and it's running fine. The only time I exceeded 40 MPG was when I was able to burn a couple tanks of 100% non-ethanol 93 octane gas.

The nikasil motors were carbureted a little leaner than the iron motors, as they dissipate heat better and of course needed to meet stricter emissions requirements. Even then, of course, they needed exhaust system oxygen to finish burning fuel.

In addition, the iron motors have higher compression, which improves fuel economy but could require a richer mixture.

I saw 50 mpg a couple times on my '84 RS, admittedly going a little slow or in mountains.

There is limited usefulness in comparing iron and nikasil motors, as they are almost two different things.