Gas mileage on rebuilt 1974 R75/6

[davidcboyer]

Greetings. I’m a long time BMW rider and MOA member but new airhead owner. I rebuilt a 1974 R75/6 with 61,000 miles - notably for this question - new rings, honed cylinders, rebuilt carbs and electronic ignition (EnDuraLast). Compression and leak down test are favorable after the top end rebuild. The bike operates great - starts easily, good power, smooth throttle through the range. My question - I have about 500 miles on the rebuild and I am getting about 33 mpg with 2/3d’s highway at 60 mph and 1/3 city. Seems like I should be closer to 50 mpg? On the carb rebuild I kept the original needle jet and jet needle and put the clip in the 3rd position from the top of the needle. The plugs (see pictures) show some tan on the top of the insulator and dry black lower on the insulator and on the threads. I balanced the carbs manually.

What would you do for next steps on gas mileage? Should I lean out the midrange by moving the clip on the needle jet one step up? Should I have replaced both the needle jet and jet needle at this mileage? Is this possibly an issue of balancing carbs with a balancing tool? The bike is a joy to ride but I wonder about long term issues if I am burning more gas than needed.

 

[20774]

Welcome to the forum! Nice looking machine! What are the carb numbers? I might have considered changing the jet needle and needle jet...also maybe some new floats. Have you set the float level in the carbs properly? One way to check the float settings is to leave the taps on for a bit, then shut them off. Carefully, drop each bowl. The height of the fuel in the center of the bowl should be about 24mm or one inch. If the level is too high, the fuel will be more easily sucked up into the carb. Also be sure that the choke (or enrichener) is completely off when you move the handle to the off position.

 

[ebeeby]

I like that plug color...

 

[ccolwell]

Some odd years ago, ok, decades, I had an R100, Ducati 900SS and a Honda 650 Hawk. They all got 33 mpg. I think it had more to do with my right wrist than the bikes.

 

[melville]

My R75/6 struggled to break 40 mpg until I lowered the needle one step, so more of it was sticking out. It also had a 'fat burble' at cruise throttle until I did that--an occasional but steady soft miss.

I think we started out in the same place, with the needle in the third from the top groove. I went to the second from the top and it ran much more cleanly and mpg went up substantially.

You may also have a worn needle jet. The brass will get abraded by the steel needle. It really doesn't take much to turn your 2.66 needle jet into a 2.67 needle jet. The effects can be significant.

 

[davidcboyer]

7

Welcome to the forum! Nice looking machine! What are the carb numbers? I might have considered changing the jet needle and needle jet...also maybe some new floats. Have you set the float level in the carbs properly? One way to check the float settings is to leave the taps on for a bit, then shut them off. Carefully, drop each bowl. The height of the fuel in the center of the bowl should be about 24mm or one inch. If the level is too high, the fuel will be more easily sucked up into the carb. Also be sure that the choke (or enrichener) is completely off when you move the handle to the off position.

Thanks for the ideas. The carb numbers are 64/32/10 and 64/32/9. I did put in new floats on the rebuild and set them per the specs but I will check the fuel level per your method.

Looking a little further at the Bing carburetor book I see that my carb numbers are listed for an R75/5. The R75/6 carbs should be 64/32/13&14 according to them. The needle jet is different for those two carbs - a 2.70 for the 9&10 and a 2.66 for the 13&14. So if I replace them I should go with the carb numbers and put in a 2.70?

 

[stevenrankin]

forties

I would think you should be getting about forties for mpg. At 61K, there is a very good chance the needle and seat need changed. I can only give this advice from experience in having my mpg drop from the forties to the thirties due to worn needle and seat about about the same mileage intervals.

As Kurt suggested check your float setting. Double check the needle setting, you may improve mpg by changing it a notch. Otherwise the plugs look good which means all your other work was tip top.

Don't forget to write to tell us how you make out. St.

 

[20774]

The carb bodies are more or less the same during this time period. I would certainly rejet the carbs to match the /6 numbers given in the book.

 

[manitou_commando]

Itn has been stated that the cheapest thing on a BMW sits astride the seat
Dud, gas toady was $3,51. What is your objective?

 

[melville]

Itn has been stated that the cheapest thing on a BMW sits astride the seat
Dud, gas toady was $3,51. What is your objective?

I paid $6.379 per gallon just two days ago.

 

[stevenrankin]

Objective

I think the objective in this case is to get the fuel milage the bike is supposed to deliver. UNLESS the owner is running wide open throttle all the time, a R75/6 or any BMW airhead bike should get at least 40mpg to maybe 48. 33mpg is a sign something is wrong somewhere.

It isn't a matter of the owner being cheap or the cost of gas. LOL, of course, I don't ride with the owner so who knows, maybe 33mpg will be all he gets despite a properly set up bike? St.

 

[nickrides]

I think a good benchmark is the plug color, and those plugs look spot on to me.

If you try and lean it out some more to gain some MPG, you may run into problems like preignition and burned exhaust valves. Not good to say the least.

I would leave it alone and enjoy the bike as is.

Side Story

When I was in my early 20's I had a Dodge Tradesman maxi van with a hemi engine.
I put Edelbrock headers, 4 barrel Holley carb and a cam in it. It was fast!
Then I morphed into a broke college student. the 7-9 mpg was killing me. I got a Holley jetting kit and started leaning it out to gain a few MPG. I ended up burning valves in both heads, ouch. That cost me a lot more than the fuel cost. I put it all back to original settings and starved for several years, but it ran great, and if you wanted to be a hot rodder just drop a gear and smash the gas pedal. AND I could bang my GF in the back.
High speed cruising on the interstate you could almost watch the gas gauge sink.
Nick
1978 R/80

 

[stevenrankin]

Plugs

I can't argue with the plug color, it is the right shade.

As I am not an expert on Bing carburetor operation, I can only say, my gut feeling based on my experience with my two current R bikes as well as once owning a R75, 33mpg points towards needles and seats being worn.

There is no need to do major mechanical changes. Easy thing first is to move the needle a notch. That is simple and easy with no cost, if moving the needle a notch makes things worse (too lean and pinging) or makes no change, then I would change the needles and seats. They do wear out and are not that expensive nor hard to change.

Unless there is some major change to this bike such as a different final drive ratio or LOL, perhaps an incorrect odometer, a constant wide open throttle or perhaps riding in a lower gear than needed, The bike should get in the 40's for mpg. I mean for crying out loud my Euro spec R100RS turns in 44mpg average in mixed riding and while I am not a speed demon, I am a quick and smooth rider who likes to use the throttle a lot. St.

 

[davidcboyer]

Updates

Thanks everyone for your experience and ideas on my gas mileage. A few updates given the various comments:
* I checked the fuel levels in both bowls - they were both at about 24 mm. The floats appear to be correctly adjusted.
* I know that my odometer is about 10% off based on riding with a GPS for many of my miles. I made that adjustment in calculating gas mileage to get the 33 mpg in my post
* I am a modest throttle rider - never full out on the throttle. Most of the mileage is at 1/4 to 3/4 of the throttle which is mostly using the needle jet as you know.
* The carbs are 9&10’s as I have noted. I called Bing and having the 9&10’s means my bike was early in the 1974 run. Most of the R75/6’s were 13&14 carbs. The 9&10’s had different size main jets, needle jets and idle jets.
* It turns out, in talking to Bing, that I had changed out the needle jet and jet needle using a rebuild kit supplied by the seller. Their kits do not come with main jets and idle jets because they don’t wear. I rebuilt the carbs in January so forgot that I had done the needle jet and jet needle.
* Because all of the jets are interdependent, Bing says that I should use jets that are for the 9&10 carbs to be consistent with the main and idle jets
* The big question for me, which I have asked the seller, is whether the rebuilt kit was for a 9&10 per the carb numbers or the 13&14 carbs of most of 1974. The former is a 2.70 needle jet and the latter a 2.66 needle jet. The other way to find out is to remove the jet to see the size.
* If I have a 2.70 per spec, my best option would be to move the needle position from 3 to 2 and monitor for signs of too lean.
Thanks for your help. Or, as you suggested, just ride it like it is!

 

[20774]

Thanks for the update. Sounds like a plan!

 

[stevenrankin]

Needles

Ah more information! With so many different variations on the Bing needles, it could very well be the previous owner got the wrong set for the carbs. So despite being new, they just are not right.

You have an excellent plan in operation and I am looking forward to reading your results. Good luck. St.

 

[lkchris]

Well, my 2012 R1200RT (Camhead) seemingly can never return less than 50 mpg. It's a bigger and heavier bike than any Airhead ever.

Just always remember the Bob Dylan theory of engine fueling: A carburetor gives your engine what it wants, fuel injection gives it what it needs.

Your carbs are fueling your engine on overrun, slowing down. Not so much with FI.

Bing recommends new diaphragms yearly.

The only time I ever got 50 mpg with my R100RS (the Airhead most likely to return best mileage) was in the mountains coasting a lot. Lots of downhill, thinner air.

Friend of mine on R100RS and I once did a ride with a nice lady on R80RT ... she needed fuel way before we did. Why? Her motor was turning 1000 rpm more than ours at same speed, same transmission gear.

The fact it's old and small doesn't mean your 750 will get better gas mileage. Any R100 will do better. A newish F750 will likely do 60 mpg.

 

[stevenrankin]

again

My first BMW was a R75/7. A one year wonder that mostly was a R75/6 with a different frame and cosmetics. That bike got between 42 and 45 mpg and I rode the heck out of it. So I keep saying, your R75/6 should be getting the same numbers.

Now IF you first try changing the needle notch and nothing happens, that most likely means the previous owner purchased or was sold the wrong needles.

Is there anyone else out there that has a R75/6 who is getting 40's mpg?

As for fuel injection gettin better mileage and mpg in the new bikes and OM bikes, it is real nice you get 50's and 60's, you should. If you own/owned an R75/6 what mpg did you get?

I used my 78RS as an example that even the most powerful and European tuned bike can get 40mpg. My current R80RT gets 45mpg average, and yes, it turns 1000rpm higher at 60mph than does the RS. I get better mileage on the R80RT. Oh boy one or two mpg better but STILL mid 40's. The guys I know who ride R90/6 get 40's.

Even if the new needles installed on the correct notch don't make a difference, it will take a lot of hard evidence to convince me 33mpg is normal for that bike. St.

 

[melville]

Here's real world info from my R75/6:

https://www.fuelly.com/motorcycle/bmw/r75_6/1975/melville/136001

I think you can probably tell when I lowered the needle one step. Looks like later in 2015 if you go to Chart All Fuel-Ups:

https://www.fuelly.com/motorcycle/bmw/r75_6/1975/melville/136001/fuelchart

The end of the chart was a trip to Montana where the throttle was pretty much pinned the whole time. That, and the odometer was malfunctioning and I was guessing mileage per tank from Google Maps info.

 

[stevenrankin]

I see a increase

I see a increase with improved mileage around August 5th or 7th. Before is what I would consider low for the bike, after is more in line with what I would expect, of course, there are extreme highs and lows. LOL, yeah the last part of the chart would jibe with a WOT.

I put a request in another forum for riders to post their mpg. One hit so far at 45 average over a thirteen year period on a R90/6 with the owner's second bike a R75/5 similar.

Thanks, St.