Clunky First to Second Gear Change

[IandJ]

Greetings All

I have a conundrum that I hope someone will have an idea for a cure

I had the gearbox on my 1993 R100GSPD rebuilt before we went away on an Americas trip last year. The bike had done 120,000km and this was the first time the gearbox had been taken apart. All the usual parts (Bearings, return spring etc) were replaced. It was done through a dealer and the guy who did it has many years experiences on airhead bikes. He has diagnosed a couple of other obscure problems that I've had in the past so I trust (famous last words) he would have done a good job. I specifically asked that he replace anything to make it as near new as possible.

Before it went in for the rebuild changing from from first to second gear was usually very clunky, other gear changes were fair

After the rebuild it changes gear beautifully in all gears when the engine/bike is cold but after it warms up I still have a clunky change from first to second gear but all other changes are smooth.

My question is what could be cause the poor first-second gear change after the bike warms up? Worn Selector or other part? Incorrect shimming somewhere? Something else?

I agree it could be poor gear change technique on my part but it is fine when cold. Also very occasionally (1 out of 20) it will change smoothly when hot and I'm not doing anything different.

I intend to go back and see if it can be fixed but I'm looking for ideas and suggestions before spending more money.

Thanks for any help

Ian J

[BMW Kurt]

Try applying slight upward pressure on the shifter before you pull in the clutch for the shift. Works for me!

[jtwind]

I think every airhead I've owned has been "clunky" especially from 1rst to 2nd. That being said preloading the shifter as mentioned by Kurt does help. My /5 practically required this to shift well, but the cush driveshaft I installed was a major improvement on that bike. Try it!!

[IandJ]

Thanks for the quick replies,

I do apply pressure to the gear lever (preload) before changing gear so I don't think that is the issue. I've also tried fast changes, slow changes, speed matching and just about anything else up to sacrificial offerings.

I don't think it's so much technique as some sort of subtle mechanical issue as it does change beautifully (for a bmw gearbox) when the bike is first ridden then gets worse when it warms up.

I know these gearboxs have a reputation for being rough so I'm not looking for perfection.

But thanks again for the ideas.

Ian J

[Wirespokes]

I think the shimming may be just a bit tight. Not sure which shaft, but that's what I think. Funny thing is, I thought that if it's tight it would be easier shifting when warmed up.

One thing not mentioned in shifting techniques is something that took me years to discover. My first airhead shifted horribly and there was nothing I could do to make it shift right - eventually took it apart and discovered it was shimmed very sloppily.

Anyway, back on topic, first off, don't wind out first gear - short shift instead. The tech in our area who's worked on these bikes his whole life and raced them as well, gave me that advice. I'm not sure why that is, but I assume it's easier on the gear box that way. First seems to shift easier at lower RPMs as well.

The second thing is to back off the throttle slightly before loading the shift lever or pulling in the clutch. What this does is unload the rear suspension. Otherwise it'll dump the stored energy back into the trans (the rear rises on acceleration, remember - let off the gas, it comes down) - jamming things up. Doing this makes for butter-smooth shifts in all gears.

This was the only thing that helped me with my /6 before finally reshimming the box.

[Beemerboff]

When I replaced the rear oil seal on my box I replaced the bearing and had the shaft reshimmed - it was to tight previously.

I also had the clutch off to replace the starter ring , and I greased the splines and the pressure points on the clutch.

I used Mercury 101 special spline lube , as recommended for Ted Porters Bronze bushes on the paralever.

The box was refilled with Mobil synthetic gear oil , and the change was just perfect .

Straight after the plot was assembled, that is.

As the new bits have bedded in the change has gone off a little, which leads me to suspect that he poor hot change may have as much to do with the clutch as the gearbox.

[Uncle Ernie]

Just a hunch, but try changing the oil. I use Redline MTL.

(doesn't everyone let off the gas some when they shift these bikes?)

[kixtand]

Originally Posted by Wirespokes

I think the shimming may be just a bit tight. Not sure which shaft, but that's what I think. Funny thing is, I thought that if it's tight it would be easier shifting when warmed up.

...

Incorrect shimming was my initial thought as well. If the bike shifts good when cold, but not as good when warmed up it is too tight. If it shifts good when warm, but not when cold, its too loose.

It does not take much a deviation from spec for these boxes to be improperly shimmed. Even if someone has years of experience, they still might not get it just right. One thing to remember here is we're talking about tolerances down to .001". Unfortunately, "good enough" is not really good enough in these gearboxes if you want them to shift at the optimum level (and they CAN be built to do so.)

kix

[Wirespokes]

Originally Posted by Uncle Ernie

(doesn't everyone let off the gas some when they shift these bikes?)

Yeah, I sure do! But the point I was making was to back off the throttle BEFORE shifting, while it's still in gear. That's what's different. I've had it explained to me like speed shifting - though I've never done that - trying to match RPMs and not using a clutch.

It does not take much deviation from spec for these boxes to be improperly shimmed. Even if someone has years of experience, they still might not get it just right. One thing to remember here is we're talking about tolerances down to .001". Unfortunately, "good enough" is not really good enough in these gearboxes if you want them to shift at the optimum level (and they CAN be built to do so.)

You definitely have that one right, kix! The measuring equipment doesn't normally measure that fine, so it's more art than science getting these things right.

[IandJ]

Thanks for all the replies,

When I had the gearbox out I replaced the clutch at the same time and lubed the splines etc.

In the many methods I tried changing gear I think I've tried backing off the power prior to the actual change as you suggested but I can't remember if it helped much, ditto short shifting (I've had the bike on blocks for 6 months) but I'll bolt it back together soon and try that as well.

I'm tempted to buy a second hand gearbox and have a go at rebuilding it for the experience, I'm just looking up what tools, parts and jigs I would need. It'll keep me off the streets and out of trouble for a while!

I did see a post somewhere with some nice pics and descriptions of a rebuild.

Regards

Ian J

[hogmolly]

Your gearbox sounds normal to me. Cold gear lube is thick gear lube which helps synchronize the gear dogs. Your best bet is to make the 2nd gear shift occur as soon as possible without too much acceleration.