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Old 11-24-2011, 08:41 AM   #1
bhightower OP
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R100RT Transmission - 4th Gear Failing to Engage

I am a new owner of this 1982 R100RT with 19k miles. I noticed this problem when I test drove it, but dismissed it as a typical clunky BMW transmission, and that it had been in a barn for 10 years undriven. With all the fluids replaced I have driven it about 40 miles. All runs smooth with one exception. When going to the 4th gear, it slips to neutral. 95% of the time. Less so when downshifting. I have tried different styles of shifting, with no change. To make it work, when I go to this gear, I must shift twice (2x the hand and foot movement). It then engages. The transmission is not making any noise and all else is perfect. I have read plenty about transmission issues on the web, but not this one. Question: Does anyone have some helpful insight into this problem? If it is drivable, but requires a rebuild, shall I just continue on until it is not drivable, before a rebuild? Is there a smaller solution, an adjustment, than a rebuild? Many thanks, Bill
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Old 11-24-2011, 08:53 AM   #2
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Quote:
Originally Posted by bhightower View Post
I am a new owner of this 1982 R100RT with 19k miles. I noticed this problem when I test drove it, but dismissed it as a typical clunky BMW transmission, and that it had been in a barn for 10 years undriven. With all the fluids replaced I have driven it about 40 miles. All runs smooth with one exception. When going to the 4th gear, it slips to neutral. 95% of the time. Less so when downshifting. I have tried different styles of shifting, with no change. To make it work, when I go to this gear, I must shift twice (2x the hand and foot movement). It then engages. The transmission is not making any noise and all else is perfect. I have read plenty about transmission issues on the web, but not this one. Question: Does anyone have some helpful insight into this problem? If it is drivable, but requires a rebuild, shall I just continue on until it is not drivable, before a rebuild? Is there a smaller solution, an adjustment, than a rebuild? Many thanks, Bill
Are you sure about the 19K miles? Sure it's not 119K? If you are, I would ride the piss out of it ... get it REAL hot ... Change the oil. Put about 2 cups of marvel mystery oil in it, then top it off with gear oil to 800mL. Do it again ... ride it hard .. and see if that shakes anything loose. Just a guess ... but it sounds like your fourth gear is just sticking to the output shaft.
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Old 11-24-2011, 09:36 AM   #3
Wirespokes
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No, there's nothing wrong with the transmission. But there is something you don't know about it - let me explain:

Like most motorcycle transmissions, dogs are used to lock up the gears.

In case you're unfamiliar with 'dogs', they are pins (pillar-like projections) extending from the gear face that mate to corresponding holes in the neighboring gear. There can be four or five of them around a gear and they lock two adjacent gears together.

When a shift is made, the dogs need to engage, which means they have to slide home.

There are two gears that have stepped dogs (fatter at the base than out towards the end, with a definite step in diameter), and I forget exactly which ones, but am pretty certain fourth is one of them. Perhaps at this point you've realized what the problem is - the dogs need to slide all the way home or the gears can separate, causing the popping out of gear you're experiencing.

You don't need to shift twice to get it all the way engaged, just get a feel for that extra little bit of effort, along with knowing how far the shift lever moves before being all the way in gear.
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Old 11-24-2011, 10:41 AM   #4
Tosh Togo
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And here's something to help the OP visualize what's going on...



Dogs and slots.
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Old 11-24-2011, 10:49 AM   #5
batoutoflahonda
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You could try a spline lube. I was surprised what a difference it made on mine, as I was having to double clutch it to get it to shift well under a load.

disclaimer: Some will disagree with the spline lube.
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Old 11-24-2011, 03:24 PM   #6
bhightower OP
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Thanks for your reply. The visual really helped and this sounds like the problem. Perhaps I'll also replace the transmission fluid once again and consider a higher quality lube. After 10 years of sitting in a barn, there could be residues built up in the tranny case.

I really do think the mileage is correct. The cleanliness of the bike, the fact that I know the previous owner, and the absence of any smoke, etc all lead me to conclude the mileage is correct.

Thanks again. I was really down last night, thinking I just bought a bike that needs a new transmission. I hate to be had. You made my thanksgiving with your helpful response. One more thing to be thankful for. Cheers! Bill
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Old 11-24-2011, 03:27 PM   #7
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How old is the clutch cable? Is it perfectly adjusted?

Have you tried the gearchange technique where you preload the gear lever before making the change?

Is the gear lever tight?

Do an oil change and try a synthetic in there. I've found it helps for smoother changes. Might not in this circumstance if it's a dog issue.

One thing at a time, to see if any of it works.
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Old 11-24-2011, 03:37 PM   #8
bhightower OP
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I think you are on to something that should help. The gear shifter is loose and needs a new grommet; not enough to cause it, but enough to add some play. Someone else had suggested the cable, but I don't think that is the cause as it works fine on all the other gears. It's just this one situation. I am also hoping that with use it will get smoother. One time when the engine was cold the gears were extremely unresponsive. I thought this might work its way out.

Thanks again for all the great advice.
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Old 11-24-2011, 04:00 PM   #9
Bill Harris
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And remember that BMW gearboxes are not at all like Japanese gearboxes-- they don't snik-snik-snik between gears but generally clunk and have to be shifted positively.
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Old 11-24-2011, 04:15 PM   #10
Wirespokes
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It sounds like your transmission is shimmed pretty well. Reason I say that is when the oil is really cold, and trans too, tolerances are tight, and it'll be stiff shifting. Synthetic will help that problem. Synthetics have been shown to help trans life considerably.

Since it's only fourth gear, I wouldn't suspect the cable or splines or anything else.

Another tidbit of data is that these gears weren't ever well machined - just kind of rough except for the gears themselves. What happened was that the dogs might not be totally concentric with the shaft, and they didn't all match up perfectly with the mating holes. It's not uncommon for a dog to shear off due to it being the only one carrying the full load. On an older well-used transmission it's possible to look at the gears and see which dogs made contact and which didn't. A company in Austria (HPN) will detail them for you so they all mate up properly, as an example.
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Old 11-24-2011, 05:41 PM   #11
boxerboy81
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Quote:
Originally Posted by bhightower View Post
The gear shifter is loose and needs a new grommet; not enough to cause it, but enough to add some play.
First thing I'd do, is sort that. It does affect the gear change enormously.

Ensure the setup is as it should be by sorting the easy things to exclude the difficult ones, eg lever, cable, oil, gearchange technique...what's left maybe be a perfectly fine gearbox, or you confirm a real problem. I agree the cable is unlikely to be a culprit here, but it's vital to be setup well for the lot to work well, as with the lever.


The gearchange technique for someone not familiar with the trans has often solved a problem with changing gears.
..boot touching the lever with slight upwards pressure to preload it, clutch in, snick, clutch out...all in a nanosecond. Practice it, it does help.
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Old 11-25-2011, 08:16 AM   #12
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I too think you might jsut have something sticking if it has been sitting a long long time. I would go with a light oil, take it for a short ride, shift in and out of 4th a lot, get back, drain put in real trans oil and go for a ride shifting in and out of 4th a bunch
and yes practice your preloading

I would use big caution using a synthetic on a bike that has been sitting a long time,
synthetic and old dry seals dont go well
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Old 11-25-2011, 02:41 PM   #13
spo123
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Cool2

Synthetic formulations, for at least the last 10 years have included very nice additives, including SEAL CONDITIONERS.
The synthetic (whatever you choose) will help......
You can even run the "non spec" 75/140 weight gear lube......I do....gearbox, differential, driveshaft......In Winter, I am careful to WARM UP the box before pushing the bike "hard".
Others will either agree or disagree........Do whatever you wish.
Good Luck!
I like the idea of the marvel mystery/ gearbox lube mix.....Heat.....repeat.....heat...repeate many times.....Then go with the synthetic...or not.
Please post your experience(s) concerning this box.
Best wishes always......worst case....REBUILD THE BOX.......Ride and Enjoy! REPEAT the ride and enjoy THOUSANDS of times.
Ride Fast and Safe!
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Old 11-25-2011, 03:45 PM   #14
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May I make you a suggestion that some will not like, but does work!

Stop using the clutch for anything other than getting on the move and very low speed changes. It's a dog box and doesn't need the clutch for higher speed changes, when going up the box roll the throttle off gently 'til the load comes off and change up before engine braking commences, when changing down roll off gently then gently blip the throttle as you change down, your changes will speed up and become much more positive as well as getting rid of false neutrals. It took me a little practice to get into it but it's now second nature.

You should not use synthetic oils in BMW boxes, they're too thin, you should only use a GL5 spec 80/90 Gear Oil, GL4 is not capable of standing up to the stresses of BMW boxes, use the same oil in the Bevel Drive (and the drive shaft if required).

(I have been involved in rebuilding one or two of these boxes, including both of mine, because I can!)

And of course as previously mentioned, sort the lever and the linkage out.

cheers,
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Old 11-25-2011, 04:02 PM   #15
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Quote:
Originally Posted by Arkwright2007 View Post
May I make you a suggestion that some will not like, but does work!

Stop using the clutch for anything other than getting on the move and very low speed changes. It's a dog box and doesn't need the clutch for higher speed changes, when going up the box roll the throttle off gently 'til the load comes off and change up before engine braking commences, when changing down roll off gently then gently blip the throttle as you change down, your changes will speed up and become much more positive as well as getting rid of false neutrals. It took me a little practice to get into it but it's now second nature.
Years ago when I rode a different R90/6 I shifted this way most times. Don't remember where I heard about this, I don't think I thought it up myself, but it was common amongst many Airheads. Some how during my long period of not riding a motorcycle I forgot about this. Will try to get back into the habit. Thanks.

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