MZ/MuZ Riders

Ah well, a common problem on higher mileage bikes that I suppose get more smaller distance use than long distance use. The centre stand butts up against the footrest bar and the footrest bar bolts to the bottom of the frame, which is somewhat box-sectioned but quite open really with little strength support.

It happened as I parked the bike for work, I didn't even feel anything was wrong- but coming back to it to ride home later on in the day, the bike was more difficult to get off the stand, and the footrests vibrated/wobbled a bit, and of course the rear brake required more distance (as that rests neutral against the bottom of the RHS footrest ofc).


Anyway, here are some pictures. Yet another bike that I need to do some work on. Thankfully it should be a bad job-, as there is box section at the top of the mount which has bent as it has given way- so should be able to just bend it back into position and weld.. However, It's a time consuming process as It would be best to remove the engine (and obviously clean up the area thoroughly) in order to properly repair this problem. I'm not entirely sure why there are two holes in the box section- I don't remember them being there before, but as you can see the holes are what helped enabled the fracture.

Adding a strip of steel to the bottom, to close that box section would help a fair bit to prevent this from happening again.

No, the portion of the frame that holds the kickstand and footrests is a separate piece welded onto the main frame.
The main frame has a cap at the bottom and a small hole for water drainage. Pretty sure the engine does not intake its air from the bottom where all the dirt is. Not sure if the 251 even intakes its air trough the frame like the older models.

Either way, no reason why that box section couldn't be reinforced and closeded off.

Hi Guys,
Sorry about the problem M hope you get it sorted. To answer Kruh, about the air travel on the 251, I would imagine it probably does come
through the top frame because as you said it does, on my 77 250/1 and on my 95 Saxons so I would imagine it will on the 251. I am trying to
remember the area at the bottom of the frame and I do remember a fair bit of crud and rust in that area, so if I was welding in this area
I don't think I would close it completely just in case it's pos to collect water. Anyway, just me thoughts.
Stay safe Alan.

This is an interesting side way move.

But you're all making interesting observations. There is a hole at the bottom of the frame for residual moisture and is independant from the box section piece that makes up the footrest mount. The 251 takes it's air from the hole above the airbox entrance itself as Puffs say which is interesting as I have never acknowledged this before, the ETZ 250 takes it's air from holes in the side of the frame tubing (just as the predecesors do) as is shown in this picture of my bike:

http://blurredmanswebsite.ddns.net/pictures/bikes/mz/cylinderoff.jpg


And in the picture of another person's 251 bike from the internet conveniently of the area I wanted to show, you can see there is no hole above the airbox mount for air intake as there is in the 250. I wonder why they changed it...


https://jbmoto.pl/wp-content/uploads/2018/03/P1030592.jpg

https://jbmoto.pl/wp-content/uploads/2018/03/P1030621.jpg




And back to the main stands, here you can see the same person as above has taken a picture of the stand of the 250 and how it interacts with the frame... I wonder if this will happen to the 250's too after sustained use.
Either way, interfereing with the footrest or the bottom bracket centre stand rest it will do the same result with the 251's.



https://jbmoto.pl/wp-content/uploads/2018/03/P1030584.jpg



So there is no real difference in structual integrity between the 250 and 251's except that the 251's have these seemingly random holes then I suspect encourages the very fracture I have acheived, and have seen evidence of stress in the same area cutting accross these holes in another picture..... What on earth are these holes for!? Will have to check my 250 and see if that has holes there - I think they do not.

maybe some strange holes are for jigs for alignment of parts to be welded during assembly

I checked my ETZ 250 - no holes down there where the footrest is, but yes the holes on the frame for air flow, I don't know how many (but they are both sides) must be adequate enough for the needs of the motor, and of course although there is a hole at the bottom of the frame for water and foreign debree disposal, but it must be said that air will obviously come in there too.

the frame is very much like my rotax powered MZ500
but the engine oil is in the frame down tube on it

i will get some pictures
dave

Well, after i've sorted one project, this has now become my second whilst I have time to be able to do this sort of work.

After taking the footrest off, I had an attempt to see how bendable this piece of the frame would be to get back into place. And the answer is not easily. It's quite thick metal actually, and I still stand that if it wasn't for the seemingly needless holes I don't think this breakage would be an issue (after all, this seems to only be an issue with the 251 models, which seem to be the only bikes with these holes and I can't find any accounts of this being occuring on 250 model bikes). So I decided the best course of action would be as I originally suspected at this stage and that would be to take the engine out, if not for accessibility alone. And here we go:



The stuff on the floor is dirt/mud, not rust. Infact despite the condition of this particular machine, including non-painted parts of the frame elsewhere that have flaked off, there seems to still be nothing more than surface rust all over it even in these concentrated areas which find a lot of neglect and general use.
I can also confirm that there is no air-inlet through the section of the frame as previously discussed.

Here we go.
After getting it back as far as I could to original location I had a good idea to attach the footrests and see if I could bend it that little bit more, but too much- it was like cheese.. I have no torch gun, so heat wasn't an option for me to get the piece aligned back exactly like it should have been.


But that's okay, it meant I could actually align things easier in the absence of the torch for proper bending and make it perfect original angle, trajectory wise. And fill in and grind down the bent top section with weld and the cracks on the side. I then welded in the holes themselves that I think really promoted the breakage in the first instance.


Welding the seams inside was harder.


Although I didn't want to weld a lower plate on first and thought welding in the holes might suffice, I decided to reverse that thought. Personally I think I should have made it a tad longer, but I think it should be good. I was thinking about the access to the bolts. But there we go.


Plate fully welded in.


And everything ground down a bit and painted. Paint is still wet and blobby but it'll even out. I'm happy with leaving everything like this for now. Tomorrow I hope to attach the footrests and fully test out my endeavours.