ADVrider

Go Back   ADVrider > Bikes > Old's Cool > Airheads
User Name
Password
Register Inmates Photos Site Rules Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
Old 06-23-2015, 06:27 PM   #1
Mikepotter86 OP
Gnarly Adventurer
 
Joined: Sep 2010
Location: Alexandria VA
Oddometer: 181
R75/5 Transmission Slipping between 3rd and 4th

My R75/5 has about 35k miles on it and over the weekend on the way to the Festival of fives, while accelerating on an uphill in 4th, the bike slipped down into 3rd - I thought it might just be a fluke, so I rode on.

On the way back it slipped from 4th to 3rd a few more times, all under heavy load, and once found a false neutral between 3rd and 4th. The bike is behaving normal in other situations, but I know somethings wrong, and I am wondering where I can start.

I'm not the most experienced airhead, but I'd like to try to sort this out on my own. Any suggestions as to where I should start?
__________________
08 R1200 GS
81 R100 RT
72 R75/5 Toaster
Mikepotter86 is offline   Reply With Quote
Old 06-23-2015, 06:45 PM   #2
Solo Lobo
airhead or nothing
 
Solo Lobo's Avatar
 
Joined: Mar 2004
Location: Shoreline, WA
Oddometer: 9,113
So you are saying your bike changed from 4th to 3rd gear by itself? That's a new one for me.
__________________
"punk rocks what it's all about" - J. Strummer

Quote:
Originally Posted by Stagehand
your bike is suitably dirty. Well done.
R65LazaruS
So I sold my GS and went shopping for a G/S!
Solo Lobo is offline   Reply With Quote
Old 06-23-2015, 07:01 PM   #3
bmwrench
Beastly Adventurer
 
Joined: Nov 2006
Oddometer: 2,355
This is a common problem with 4-Speed gearboxes. It is possible that the output shaft needs to be re-shimmed, but more likely that the engagement ring or dogs are worn. The engagement rings are NLA. You'll need to open the gearbox to see what's going on.

If you don't mind going away from original, install a good used 5-Speed.
bmwrench is offline   Reply With Quote
Old 06-23-2015, 07:35 PM   #4
disston
ShadeTreeExpert
 
disston's Avatar
 
Joined: Mar 2011
Location: Silver Spring, Md
Oddometer: 10,218
What bmwrench says. A good 5 speed is easier to find than another 4 speed. And rebuilding the 4 speed is not an inexperienced even if eager type job. Tools needed will run several hundred dollars and the first box you do may not work so well. The next three or four boxes may not work much better. Transmissions are best done by the pros.

There's plenty for you to do swapping transmissions over. There will be a few parts and the clutch should be checked for wear. And the rear main oil seal should be changed if it has never been done.
__________________
.
Never memorize something you can look up.
---Albert Einstein

Pay your debt, piratejohn.http://www.advrider.com/forums/showthread.php?t=949341
disston is online now   Reply With Quote
Old 06-24-2015, 06:22 AM   #5
Mikepotter86 OP
Gnarly Adventurer
 
Joined: Sep 2010
Location: Alexandria VA
Oddometer: 181
Thanks for the feedback, guys. I reached out to a local airhead mechanic to see if he has any used gearboxes.
__________________
08 R1200 GS
81 R100 RT
72 R75/5 Toaster
Mikepotter86 is offline   Reply With Quote
Old 06-24-2015, 08:49 AM   #6
Mikepotter86 OP
Gnarly Adventurer
 
Joined: Sep 2010
Location: Alexandria VA
Oddometer: 181
Can I swap in any pre-1980 gearbox or should I be looking for something more specific?

Thanks,
Mike
__________________
08 R1200 GS
81 R100 RT
72 R75/5 Toaster
Mikepotter86 is offline   Reply With Quote
Old 06-24-2015, 10:26 AM   #7
disston
ShadeTreeExpert
 
disston's Avatar
 
Joined: Mar 2011
Location: Silver Spring, Md
Oddometer: 10,218
You can use any gearbox from 1970 thru 1980. That is either 4 or 5 speed. These are the long input shaft gearboxes that mate with the heavy flywheels. In 1981 BMW goes to the clutch carrier system and the gearbox uses a short input shaft.

This in a nutshell is what you are dealing with.

Try to avoid and 5 speed from a 1974 bike but if the price is just too good and it works it may be a good deal. The problem with 1974 5 speeds is that it was the first year of the 5 speed and they have proven to be more problematic than other years. However there are many 1974 boxes on the road so, again, if the price is really low you may want to chance it.

Your best choice seems to be a 1979 or 1980 5 speed if one is available. These boxes will have the reinforced ribs to them like later boxes but they have the long input which you need.

I think the box you have is fixable but you should have it done by somebody that has done a number of 4 speeds. Not all BMW transmission mechanics do 4 speeds. The 5 speeds are really a better transmission in a lot of minds. They get more attention and there are many more mechanics that do them. But a 4 speed has a lot of differences in it and is not even much related to what is happening in the 5 speed. many riders do like the 4 speed. A lot of them think it is a heavier duty trans than the 5.

And it will be an advantage to have two transmissions.
__________________
.
Never memorize something you can look up.
---Albert Einstein

Pay your debt, piratejohn.http://www.advrider.com/forums/showthread.php?t=949341
disston is online now   Reply With Quote
Old 06-24-2015, 10:32 AM   #8
Mikepotter86 OP
Gnarly Adventurer
 
Joined: Sep 2010
Location: Alexandria VA
Oddometer: 181
A lot think it is a heavier duty trans? Is this the case, or does it just feel like a heavier duty trans on account of its clunkiness? I will look for a newer pre 9/80 transmission - haven't seen one in any of my typical go-tos yet, but I figure I have time.

I like the feel of my 81 RT's transmission, and I like having 5 gears, so I will pursue that route. Any suggestions on finding tutorials on performing the swap?

Thanks,
Mike
__________________
08 R1200 GS
81 R100 RT
72 R75/5 Toaster
Mikepotter86 is offline   Reply With Quote
Old 06-24-2015, 10:36 AM   #9
AirGsPd
Dream Rider
 
AirGsPd's Avatar
 
Joined: Jul 2012
Location: Carmel, IN
Oddometer: 119
Guenther Wuest

Call Guenther Wuest in Indiana! He is a great BMW wrench and I know that he has four speed gear boxes in his shop. (812) 472-three seven three nine
__________________
Life is too short not to live it...
AirGsPd is offline   Reply With Quote
Old 06-24-2015, 01:07 PM   #10
disston
ShadeTreeExpert
 
disston's Avatar
 
Joined: Mar 2011
Location: Silver Spring, Md
Oddometer: 10,218
A lot of what you "feel" when shifting is do to the clutch.
__________________
.
Never memorize something you can look up.
---Albert Einstein

Pay your debt, piratejohn.http://www.advrider.com/forums/showthread.php?t=949341
disston is online now   Reply With Quote
Old 06-24-2015, 01:19 PM   #11
kixtand
Beastly Adventurer
 
kixtand's Avatar
 
Joined: Apr 2004
Location: Lexington, KY
Oddometer: 2,178
Quote:
Originally Posted by AirGsPd View Post
Call Guenther Wuest in Indiana! He is a great BMW wrench and I know that he has four speed gear boxes in his shop. (812) 472-three seven three nine
This^^

In the end it might cost you a bit more than a used tranny, but you can also know it will be right...
__________________
Bacon...

it makes everything better.
kixtand is offline   Reply With Quote
Old 06-24-2015, 02:33 PM   #12
bmwrench
Beastly Adventurer
 
Joined: Nov 2006
Oddometer: 2,355
If you get a 5-Speed, you'll need a clutch pushrod and release mechanism; thrust bearing, piston, and lever.

Chris Hodgson, Mr San Jose BMW, ran 4-Speeds in his race bikes for years. He felt that the 5-Speed was relatively fragile.
bmwrench is offline   Reply With Quote
Old 06-25-2015, 04:15 AM   #13
Stan_R80/7
Beastly Gnarly
 
Stan_R80/7's Avatar
 
Joined: May 2012
Location: VA
Oddometer: 1,631
If you want to diagnose this yourself the gearset will need to be removed - you will need a special tool to take off the rear flange without damaging the gearbox case. This is one version of the tool as sold by cycleworks (http://www.cycleworks.net/index.php?...ducts_id=321):




I use an impact wrench and 15/16" socket for the 24mm nut on the flange, then an impact wrench on the puller bolt - but that is personal preference (I find it easier). The rear cover will need to be heated to ~210 degF with a heat gun or propane torch. The propane torch is faster but must be used with care or seals may melt and require replacement.

Once the rear cover is carefully removed (wear gloves) the gears can be viewed. However, it requires removing the gearset and shifter before examining the dogs, shift forks, gears, etc. I highly recommend taking plenty of photographs during this activity to aid in correct reassembly. Heating the opposite side of the gearbox case to 210 degF is required to release the layshaft and output shaft bearings - it helps to have the shifter assembly bolts removed before removing the gearset. A new rear cover gasket will be required.

Basically, you will go through the majority of the steps to install new bearings in the gearbox for inspection. That's how these gearboxes were built. There are sources of information on the internet including IBMWR (http://www.ibmwr.org/r-tech/airheads/trans.shtml) and youtube videos. Cycleworks also sells a DVD on rebuilding transmissions which cover 1955-1995 years (http://www.cycleworks.net/index.php?...roducts_id=243)
Stan_R80/7 is offline   Reply With Quote
Old 06-26-2015, 08:32 PM   #14
Mikepotter86 OP
Gnarly Adventurer
 
Joined: Sep 2010
Location: Alexandria VA
Oddometer: 181
I drained the transmission today and there was very little fuzz, and no slivers of metal to be found, that was kind of a relief.

I'll let you guys know.
__________________
08 R1200 GS
81 R100 RT
72 R75/5 Toaster
Mikepotter86 is offline   Reply With Quote
Old 06-27-2015, 04:11 AM   #15
Dark Helmet
Studly Adventurer
 
Dark Helmet's Avatar
 
Joined: Nov 2006
Location: You'll know when I know...
Oddometer: 729
Don't forget to check with Beater on the airhead forum. He came into a bunch of used Airhead parts. I know he had a bunch of engine blocks but not as sure about tranny's.
__________________
You only need two tools in life - WD-40 and Duct Tape. If it doesn't move and should, use the WD -40. If it shouldn't move and does, use the duct tape
BMW 1200 GS Adventure
BMW 1970 R75/5
Suzuki DRZ 400S
Dark Helmet is offline   Reply With Quote
Reply

Share

Thread Tools Search this Thread
Search this Thread:

.
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump


Times are GMT -7.   It's 08:27 PM.


Powered by vBulletin® Version 3.8.5
Copyright ©2000 - 2015, Jelsoft Enterprises Ltd.
Copyright ADVrider 2011-2015