R75/6 carbs vs /7

[srankin]

Well, never say never. In the case of the final drive, unless some moron did the major task of changing gears God help you if he did and did a half assed job, the number stamp is what you get. I don't have a list of specifications in front of me so I cannot confirm what you say about your drive being from an R60 instead of the bike you have.

I know in existing BMW literature online there is a specification sheet telling what drives go with what bikes. Kurt the moderator of this forum has far greater knowledge of this information.

One source I do use and is regarded as one of the best sources of airhead information is Snowbum's pages.

As for the speedometer, yep, the number on the speedometer dial corresponds with the matching final drive. Again, I don't know if a green dial speedometer is not the correct one for your bike but it is possible. A mismatched final drive and speedometer would in theory and in some cases cause a large discrepancy between indicated speed versus actual speed while riding. The person who installed the final drive may have matched the speedometer on the bike to that final drive, so you won't notice a discrepancy in speed. Of course, my thoughts leave out the possibility of a gear box change. Without a spec sheet in front of me, I can't say for certain all Post 70 to 74 gear box gearing is the same for the range of bikes produced.

I can read your reasons for doing what you are doing. Believe me, a well sorted airhead is a true joy to own. I myself have been tempted but have never found another bike to take my two bikes places. They just do what I want and do it well.

One thing take it or leave it as you wish. Don't fall into the money pit situation while working on this bike. Keep a clear mind that just because it is a BMW and old does not mean it is worth a lot of money.

I have said, I sorted out the problems with my RS the soiled dove. At the end of all the repairs I have far more into the bike than what I could realistically in todays market sell it for. I did not put the time and money in for resale and was very well aware of this fact. NO, I loved the bike when I bought it, loved it when I got knocked off it, loved it when I restored it to what it is now. Sentimental non logical things.

On my hard ridden 84 R80RT, I just put it on the lift to install engine guard bars with the thought of increasing my daytime lighting presence. The bike has 214K on it, looks fair runs great. I don't mind adding little things or spending money to keep it running and looking good. I know, I will never sell it in my lifetime.

Anyway, take a look around for specs for the bike you have, See if maybe you can find like minded people to help out in your area. Cheery bye. St.

[schrader7032]

Snowbum breaks down the gearing in this page:

https://bmwmotorcycletech.info/ringgears.htm

You can count the number of times the output at the transmission turns versus how many times the wheel turns. Put the transmission in 5th gear and then turn the rear wheel once while counting the transmission output.

[Andey]

I'm assuming the final drive is what it has been stamped with (37/11), I doubt the previous owners have had the know-how to change the inside components. If they did, i doubt the rest of the bike would be in the shape it was.

So I managed to track down a speedo with the correct ratio to match the 37/11 (long story short, i actually brought it when i first got the bike to replace the existing gauge assuming it was broken and i used 37/11 to buy without the knowledge of all the ratios and variations with different models and manufacturing years)

I've just installed it and it seems to be much more accurate (comparing it to a GPS speedo app on a phone)

Now I'm stuck wondering whether I should go back to stock 75/6 ratio or keep to this one, with higher torque but lower top speed?

[srankin]

Well, again, I am going to blab, tell me to shut up if I get annoying, LOL

I have two very different bikes, a 78 R100RS and a 84 R80RT, I say this because while the transmission gearing is pretty much the same for both bikes, the final drive ratios are vastly different. The RT has a low geared final drive hence, RPM at a given speed is higher than the RS which has a higher geared final drive. At 4000RPM on the RT, I am doing about 60mph. On the RS, I am doing 60mph at about 3500RPM.

The reason for the difference is to keep the engine in the proper range for most efficient torque, HP and fuel economy.

The RT with its smaller motor doesn't have the torque and HP of the RS. So by fudging with the final drive, it makes for a peppy bike in normal riding situations. In other words, the performance from zero to sixty is about the same as the RS, even though the RS is bigger and more powerful. (I don't have stopwatch or official data for this claim, just seat of the pants observation, so don't beat me up about it please, LOL)

Oh heck, I am going off into some kind of long thing here.
I will shorten it out, When do we ever use top end on the street? The best way to set a street bike up is for the type of riding we do most of the time. If it is city riding where there is a lot of stop and go, a low final drive could be a good thing. due to the extra quickness off the line, the extra pep at low speeds.
A higher final drive is great if most of the time we are riding at a constant speed in high gear.

Oh I have ridden both of my bikes wide open throttle to see just how fast I could get them to go and still do from time to time but, on the street, city or country, it is VERY rare I can get a chance to do it. High speed driving awards from law enforcement have slowed me down considerably as well as age.

So, bottom line; Set the bike up for the type of riding you will be doing, unless you are going to be at a race track or riding the Autoban, I wouldn't worry too much about top end. Getting caught on a street doing top end will not have pleasant consequences. Enjoy the ride, St.

[Andey]

I’ll be doing a lot of city riding (probably doing the bike an injustice... but can’t be helped), not too much highway riding. Some, but not a lot, 15mins max before hitting a traffic light or roundabout...

Seems wise to stick with the lower ratio at the moment... I might shop around for a stock final drive and when the time comes around and I have time, I would swap to see how it is...

I’m definitely not after sports bike performance (I think there’s another bike for that purpose...) but would be great to achieve some balance, especially from the way it was designed and intended to run.

[srankin]

Yep, see how things go for now. As long as the bike is running and running well, ride it. later when you get established with its performance and such, you can upgrade if needed or desired.
Until you are sure this is the bike for you, don't put a lot of money into it, you won't get it back if you find you must sell or trade in.

Riding in the city, beware the cage drivers. I am led to believe the AU drivers are better skilled and more attentive than here in the US. You wouldn't believe the stuff I see cage drivers, and even motorcycle riders doing here on a regular basis.

That is another forum topic LOL. St.

[Andey]

Yes, the test riding continues every night after work. Breaking the new rings in and just testing the general health of the bike.

I’m pretty sure now the gear box and clutch need servicing... I’m getting inconsistent shifting... sometimes it’s great, sometimes just not engaging into the gears so convincingly...

Transmission actuation arm is sweating oil and I’m sure the clutch cable is stretching... Will be looking at that soon...

I think this bike will follow me to the grave, being the first and I’m pretty sure I won’t get my money back on it, might as well keep it... it’ll be a good base for the journey to other airheads...

[schrader7032]

Break-in should include upshifts which puts combustion pressure on the rings in one direction, then followed by downshifts using engine compression which tends to put different pressure on the rings.

As for shifting, be sure to pre-load the foot lever with a little up pressure while you make the shift. Additionally, continue to hold the up shift as you let out the hand lever. This helps to give a more positive engagement of the shift dogs. I was having issues with finding a false neutral between 4th and 5th. The recommendation from Oak Okleshen at the time was to continue the up pressure while doing the shift. Works well. If it get too fast with the shifting, I can find the neutral, so I am deliberate with my shifting.

[srankin]

I agree with Kurt on the break in and add what my first dealer told me when I was breaking in my new R80RT. He told me to keep the rpm up, don't lug the engine. I try on both my bikes to keep the rpm over 3K in towns or cities.

I also like his advice on shifting. My 78RS has a different flywheel and clutch system and, I am not certain but, I also think there is a difference between the shifting mechanism on my 84 RT. the two different designs make riding each bike different.

The RS has to be thoughtfully shifted, It does not like being hurried. The RT, is lighter shifting, can be shifted up and down without using the clutch and is much more forgiving of not doing things right.

I do know if someone who mated a 78 engine to a 85 gearbox and got a nice set up, I am just too cheap to do it, and given what the RS is, I live with it. Mind you, it is no slouch of a bike and can be ridden very aggressively and still give a great ride. St.

[Andey]

Thanks for the advice on the breaking in and shifting...

It took a little getting use to, but holding the shift and rolling back on the throttle seems to have fairly positive results in terms of convincing shifts... great tip, the bike seems to like it that way... not sure how anyone would figure out that method of shifting, but seems to work wonders.

I did get what sounded like a sticky clutch going downhill... a slow grinding sound when the clutch lever was pulled in... but didn’t last long and didn’t come back.

However, I did spring a leak in the bottom of the right cylinder... I haven’t re-torqued it since installing the new rings, and I remember seeing somewhere that it needs to be retorqued after 100-500miles or something, but for the life of me, I can’t find it... Clymers? Or was it somewhere else...??