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Old 07-05-2012, 11:25 AM   #1
pthomas OP
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New Transmission?

What to look for when buying a new to me transmission? What fits a 1974 R90/6? I'm aware that the 74 is a bad year, but is there anything to "fix" the problem should I open things up and fix it myself? I've never opened a transmission, so I'll be in unknown territory.

I was ridding home last night (1974 R90/6) and broke down around 10:00 p.m. I was clearly ready for the trip with nothing more than the clothe on my back -a polo and shorts-, my wallet, and a cellphone running out of charge.

I was on I-75 when, cruising along when I lost propulsion. RPM didn't change but I was clearly losing speed and more throttle didn't change anything. Shifting down didn't help but I could see that I putting some power to the ground; sadly that was short lived. Being the only tool in sight, I spent the night on top of the bike, and waited for U-Haul to open up and finished the "ride" home this morning.

I have yet to open things up to diagnose the problem, but my belief is that it's a single failure point at the transmission or beyond (driveshaft). I heard no noise or rattling, so I don't have much to go on but I believe I'm going to be in for a new transmission.

Thanks for the help.
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Old 07-05-2012, 11:42 AM   #2
SOLO LOBO
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Sounds like a clutch to me...

That said, if it is a trans I would always spend the extra $$ on having my trans rebuilt by on ok of the known experts (not just anyone who says they can do it) as opposed to buying a used and unknown one.

You may pay $400 for a replacement or (if you get lucky) $600 for a rebuild of yours
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Old 07-05-2012, 11:57 AM   #3
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I agree - sounds like a clutch issue to me. Perhaps the clutch got oil soaked - they don't normally go south that quickly.

When transmissions go they tend to make lots of bad noises.
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Old 07-05-2012, 02:26 PM   #4
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Quote:
I lost propulsion. RPM didn't change but I was clearly losing speed and more throttle didn't change anything.
Ran out of gas?

Quote:
nothing more than the clothe on my back -a polo and shorts-
Don't do that. It's dangerous.
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Old 07-05-2012, 02:49 PM   #5
disston
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Another vote for this is not a transmission problem. True there are problems with the 1974 transmission but this current problem does not seem to be one of them.

Now that you are home can you start the bike? (good call Bill)

But I also think clutch. So sitting on the bike with the engine running can you put it in gear and what happens when you let the clutch lever out?

Look on the rear of the trans for the clutch throw out arm. The clutch cable goes to it. And when the cable pulls the arm pushes in on the throw out parts under the small rubber boot. Is the throw out arm working? Sometimes the pin holding it in place can fall out.

I hope you wear a helmet. And no sandals, no slip on shoes. Boots preferred. And, and, and....Oh never mind.
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Old 07-06-2012, 07:41 PM   #6
pthomas OP
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I normally wear more gear... and have more in the saddlebags; the one time I'm not ready and I get burned.

The engine still still has gas... I already know that feels like

The engine still run and I thought it might be a clutch issue but don't think it is. There is slippage in the drivetrain but pulling the clutch in removes it all. I would think that a clutch failure would mean that I can't disengage the driveline, not that it all of the sudden disengage. Does anyone know the failure mode on these?

The more I've been thinking about it, I agree that it should not be a transmission issue but something else in the driveline... investigatory work will tell the story.
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Old 07-07-2012, 07:28 AM   #7
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Sounds like maybe you popped the taper loose on your trans output flange. Still a little friction to give you a little go, but nowhere near a solid mechanical connection. If you spun that taper for any length of time you'll need to pull it apart and see if it's even salvageable.
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Old 07-08-2012, 09:21 AM   #8
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Sounds like you are confusing the two failure modes for the clutch.

Your stated presumption that clutch failure would stop you from disengaging the driveline would in reality mean a failure in the clutch cable, lever, or the other lever at the transmission. That would keep the driveline from disengaging.

The other failure mode would be as described by several folks above - the clutch has started slipping and no longer transmits power through the drivetrain to the rear wheel.

You stated that you were able to downshift (without results) and that it seemed some power still made it to the ground. But you also stated that pulling in the clutch removes all the slippage????? Do you mean that pulling in the clutch removes all driving force (as it should) and thus the slippage goes away along with the driving force? Or that pulling in the clutch transmits full driving force to the rear wheel?
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Old 07-08-2012, 10:13 AM   #9
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I think you must pull the tranny to do an inspection of the several clutch parts that can have wear that explains your symptom. If the clutch parts meet specs, then look for the output flange problem. If that's the culprit, I would look for a 76 through 79 tranny for replacement. With a good clutch and updated tranny, you will be good to go for a loooong time. Bite the bullet, fix it right once and be done with it.
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Old 07-08-2012, 01:09 PM   #10
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Quote:
Originally Posted by DoktorT View Post
I think you must pull the tranny to do an inspection of the several clutch parts that can have wear that explains your symptom. If the clutch parts meet specs, then look for the output flange problem. If that's the culprit, I would look for a 76 through 79 tranny for replacement. With a good clutch and updated tranny, you will be good to go for a loooong time. Bite the bullet, fix it right once and be done with it.
actually, 1970-through 1980 all interchange
I prefer to find transmissions from R65 bikes. They don't seem to be pounded inside as hard as R100 Transmissions. They're all the same fitment wise in the proper year range.
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Old 07-08-2012, 08:28 PM   #11
pthomas OP
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Quote:
Originally Posted by mark1305 View Post
Sounds like you are confusing the two failure modes for the clutch.

Your stated presumption that clutch failure would stop you from disengaging the driveline would in reality mean a failure in the clutch cable, lever, or the other lever at the transmission. That would keep the driveline from disengaging.

The other failure mode would be as described by several folks above - the clutch has started slipping and no longer transmits power through the drivetrain to the rear wheel.

You stated that you were able to downshift (without results) and that it seemed some power still made it to the ground. But you also stated that pulling in the clutch removes all the slippage????? Do you mean that pulling in the clutch removes all driving force (as it should) and thus the slippage goes away along with the driving force? Or that pulling in the clutch transmits full driving force to the rear wheel?
Pulling the clutch removes all driving force.

I'll read up on the clutch and takes things apart; I didn't have time to do it this weekend as my car also broke down this week and I need it for work... it was one of those weeks.

The good news is that this will force me to take the bike apart and learn more about it, do a top on the engine, and see what else needs to be done. It will move me further along the restoration I want to do on this bike but don't have the funds for.
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Old 07-09-2012, 10:37 AM   #12
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It is a good way to learn Airheads to learn how to repair it as you go along. Then when it's working you ride. Then when it's broken you repair. After a few years you will know more about what you are up against as far as this "restoration" goes. And you should have the bike working properly by then.

So here's part of the deal. Right now we are going to repair what is broken right now. It seems there is a problem with the clutch or (have we eliminated this one?) the output taper. If you haven't done this yet I think you should first take off the swing arm and with the drive shaft out of the way see if the universal flange is firmly attached to the rear of the trans, to the output taper. If it is it is time to remove trans and look at clutch. Taking the swing arm off is part of removing the trans so you are proceeding in a logical manner.

If top end repair is what you mean by, "do a top on the engine"? Then don't do this. It is another part of the engine, it is at the other end of the bike, it has nothing to do with the current problem. Fix the current problem. If you want, after this is done, and have any money left, then we'll go on to other repairs needed.

Since I brought it up maybe I should mention that any of the systems on these bikes is not cheap to repair. Transmissions can go over a thousand dollars, a clutch can easily cost five hundred, or more. I think you should stick with one thing at a time.

Charlie
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Old 07-09-2012, 08:11 PM   #13
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pthomas, peruse this thread I wrote a while back. It should give you some insight into the inner marvels of the BMW 5 speed boxes. Also check the links in which I credit some other folks with lots of valuable info that helped me.

I picked up an Ebay box to replace my shot transmission and rebuilt it myself. Still works great.
http://www.advrider.com/forums/showthread.php?t=435511
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Old 07-13-2012, 04:57 PM   #14
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Quote:
Originally Posted by mark1305 View Post
pthomas, peruse this thread I wrote a while back. It should give you some insight into the inner marvels of the BMW 5 speed boxes. Also check the links in which I credit some other folks with lots of valuable info that helped me.

I picked up an Ebay box to replace my shot transmission and rebuilt it myself. Still works great.
http://www.advrider.com/forums/showthread.php?t=435511
Reading your thread is making me think I need to farm out that work; I've feel okay doing an engine but the trans scares me for some reason. Worst case, I open it up and find out I need to send it away because I can't do it.

I'll be working on the bike this weekend. I've been putting long days at the office all week but I should have a free weekend
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Old 07-14-2012, 12:10 PM   #15
pthomas OP
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Given the amount of metal out of the transmission, it needs to be opened
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