125SM: Shifter won't re-center

[SwanAustin]

Hi, all.

I finally got around to getting my poor little MZ running again after cutting my own engine cover gasket out of manilla envelope paper. Even after sitting for years, it fired up pretty willingly (I had to push start it of course, because of the freewheel issue), but a test ride around the block revealed that the shifter no longer centers--if you shift up, the lever stays up. Shift down, it stays down. If you put the lever back to center with your foot, you can then shift again.

Before I crack this thing open again, have any of you seen this before? I'm thinking maybe there's a spring somewhere that's either broken or become unseated somehow. There's a likely-looking candidate in the parts book, but I figured I'd at least consult the foremost authorities before I start.

The bike has sat a long time in a garage with no oil in it. It was partially disassembled when I bought it (freewheel clutch) and is an ex-racebike that I am hoping to put back on the track at some point.

Thanks a bunch,
-S

[DAVID THOMPSON]

item 17 is the best guess
its behind the clutch plate carrier at the bottom rear of the engine on the right side
check the no 22
but 17 may just be back in there wrong

WARNING THE NUT ON THE CRANK IS ANTICLOCKWISE OR BACKWARD IF I REMEMBER CORRECT
THE CORNERS OF THE WRENCH FACE HAVE A SMALL CUT TO INDICATE REVERSE THREAD
LOOK AT THE HOSE NUTS ON A GAS WELDING BRAISING TORCH YOU WILL SEE THE SAME MARKS
SO THE HOSE IS ON PROPER
YOU MUST REMOVE THE CLUTCH BASKET TO REPLACE THE CENTERING SPRING
GOOD LUCK
DAVE
17 3747022000 RETURN SPRING

[SwanAustin]

Thanks for the insight, David. That's the spring I was thinking about.

I took the side cover off again today, but (without further disassembly) everything seems to be unbroken and properly seated. What's more, the shifter seems to move okay and return to center with the cover off, but of course I can't ride it like this to test so I can only wiggle it while rolling the bike back and forth.

I notice on the inside of the cover there is a pad (for lack of a better word) in the casting that seems to control end play for the shifter shaft. The shaft can move freely about a quarter of an inch with the cover removed.

The idea has occurred to me that perhaps the combination of gasket (and the Hylomar I used for good measure) allowed too much end play and led to the problem. I didn't see the oil light come on or anything (I read in another thread that gasket thickness might affect the oil pressure control spring/ball setup), so I didn't think the gasket thickness might otherwise be a factor here. But I could be wrong.

Unfortunately the possibility didn't occur to me until after I tore the thing apart again, so I'm not exactly sure what end play the shaft had while it was assembled. If you (or anybody else out there) has an assembled and properly shifting 125 on hand, do you know (or could you check) if the shifter shaft has any end-to-end movement? No disassembly required--one could just reach down to where the lever attaches to the shaft and push/pull gently to see if the shaft moves in or out. I'd greatly appreciate the data on a "known good" example.

The other possibility is that there isn't enough end play and the mechanism is binding. The pad in the casting is flush with the gasket mating surface, which (based on my bike) would allow only the gasket's thickness as end play. So if my gasket is too thin, the shaft would be harder to turn, which as I write this starts to seem the more likely explanation. Unfortunately I have never had the OEM gasket to measure and/or compare with what I had made myself--it had been thrown out by the previous owner. If anybody has a stock gasket and a micrometer (or even a "best guess" as to whether it is thicker or thinner than plain yellow manilla envelope paper), I'd really appreciate a measurement.

I guess the real solution is an OEM gasket. But I'm trying to minimize the money spent until I am sure the thing is more-or-less fully functional (of course the freewheel thing is toast, but push-start is okay for a race bike, and it will enable me to shed some additional weight from the bike). And that doesn't even address the US Parts availability situation. If I can get away with cutting my own, I'd prefer to do so.

Thanks in advance for any help you can give me.

-S

[DAVID THOMPSON]

item 21and spring 22 work against item 7 a shiftier pin wheel
make sure its all in order
i do not want to take the clutch basket out of my spare mot0r to see how it all works

and do remember to watch the spring and ball bearing that sets the oil pressure
as it can fall out of place and then oil is all messed up
also too thick a gasket can cause pressure problems and sidecSE IS OFF ALLOWS BEARING AND SPRING TO GO missing
item 7 may be binding with item 21

if you ever decide to fix the freewheel contact me first for important info on its problems and some cures for them
dave