MZ/MuZ Riders

Hello, it's me again with another TS125 question as I slowly learn about this bike.

First a bit of background.
I've had the bike rebored as part of the rebuild and measurement showed the next piston size up would be 57mm. This is too large for a 125 and so I think at some point someone has fitted a 150cc barrel to the bike.
So I have a 57mm piston in there now and with the carb the bike came with it ran well. It was reliable and nippy.
However this carb had the idle set with the throttle cable tension and I couldn't get this to work. The bike would always die when idling.
Adding some tension to the cable lifted the revs but them they would keep rising until the whole things was about to go bang . I tried many times to get it to hold a steady idle but it would either die or race away.
All very odd.
Meanwhile - I read that as a 1979 bike, mine would originally have had a later carb with the throttle stop screw.
A friend had a spare one from his TS150 and so I have fitted that.
I can now set an idle easily and it starts right up.

However, as the bike warms up it starts to misfire, and chug, and will only travel with the throttle wide open, anything below this and it dies.
There is more smoke than there used to be and I thought the bike was running rich and so I have moved the needle down.

That brings me to today.
Another test ride and it is still the same.
After about 2 miles it chugs and dies.
Then I wait by the road side for a bit, perhaps clean the plug. The spark is good.
Then it restarts, but this time only gets about 1/2 a mile before dying.
So I do the same again with the plug, spark is still good, after a while it restarts and I nearly get home. I push it the last 100 yards.

Sorry for the long message but I wanted to explain that the bike ran fine until I changed the carb, so that points to the culprit.
I also think the bike is a 150cc.
So some questions I have are - does it sound like it is running rich?
If yes, what jets should I fit for a 150cc bike?
Or could it be the float level?
I have checked that the float needle works and that the floats don't leak.
I also checked that the enricher fully shuts.
This is the current main jet, but I can't see a 90 or 95 on it.

Grateful as always for your collective wisdom.

It sounds like a defect ignition coil...

Hi Guesi,
That's interesting, I was set on the carb because I had changed it, but I'll see if I can run some checks on the coil.
Thanks.

old BMW's are bad about carbs adjustment
till you get the magneto tuned up correct..
DAVE

I would directly compare both the old carb jets and the replacement carb jets.

I know your symptoms also point out to what Guesi says in that defective coil (overheating) or even something as simple as a clogged fuel tank cap vent can cause these issues but if the bike was working before the carb change, then perhaps the first step is to put the original carburettor back on and see what happens. If you can ride it successfully for more than a mile or two as of late then you have your culprit. The checking for spark etc can be a bit of a red herring when investigating coil problems because you think it's the spark plug or whatever but by that time the coil has cooled down a little bit enough to give some more usage for another half an hour. Been there, done that.

This is what I would do..

Take off replacement carb.
Open up both carbs simultaniously, but carefully not to mix the individual parts up !!!
Compare jets.
Put back on original carb to test. If all is good, I'd probably transfer the jets to the replacement carb and see how that felt??????


I see 115 on that picture btw. You are expecting a 90?

Ah ha! success!

I was missing an obvious experiment, to swap back to the old carb, to confirm if the new one was the problem.
So thank you for that suggestion.

The old carb returned the bike to a good state of performance.
Longer rides were possible again with a normal level of smoke but of course no idle to speak of.
Both carbs are 24mm.

Looking at the jets, the old carb had a main of 95 and the new one 115.
Swapping the smaller jet into the new carb has got the best of both worlds.
A fine riding bike and the ability to set a steady idle.
The needle jet in the new carb is 70 whereas the old one was 65 but these are not interchangeable, otherwise I'd be tempted to swap them as well.

For the time being it seems to run well. So I will put some miles on it and gain some confidence.
Hope to meet some of you at the next MZ meet.

Cheers.

Congratulations. It looks like your spare carb was jetted for an ETZ 150, rather than a TS.