MZ/MuZ Riders

So I inherited an early 1982 ETZ 250 to which I recommissioned it three years ago but never got round to using till now.
New oil seals etc but it smokes like a magic show with mirrors! Anyway I did a crank leak pressure test and wasn’t happy as it was gently blowing out of the gearbox level screw hole. So from that I totally stripped the crank cases and started again. I leak tested the crank case for hairline cracks but nothing so I was happy and both unhappy for obvious reason. I prepped the case halves and reassembled then made a bespoke crank pressure tester.
It held good pressure for a few minutes so I then built up the whole engine and refitted it to the bike.
Whilst during the build I noticed the piston was a massive 75mm and that equates to 300cc. Anyway back on track…. The bike still smokes well past expected and covers the whole road in oil mist turning it black.
Question 1, what carb should a 300 have?
2, what spark plug should it have?
3, is there any indication on the barrel to say 300cc if at all it’s std?


The bike needs choke but won’t run any more than a few seconds without as it dies. I think it’s not sending enough petrol through hence the smoke as it’s trying to burn more oil than fuel. Can’t get any info from previous owner as it came from a deceased estate

3, is there any indication on the barrel to say 300cc if at all it’s std?

There was a post by Guesi on Apr 15, 2019, 2.48pm ETZ 250-251 diffrences which might help.

75mm would I guess be 287cc piston if stroke is still 65.

Probably likely if there aren't any markings that the barrel has just been over-bored. What do the linings look like? Are they thick still, or has it been bored out so much that tapered edges to help piston ring installation are missing?

I myself have an 76mm piston (making 295cc) but on an overbored 250 barrel. No markings etc- it's just been done, and I've continued to use it.

I use the standard jet settings (found on my ftp for etz250 official service manual). And have done so for.....9 years and about 12,000 miles.

Anyway, few months down the line and the oil burner/smoke/n/mirrors magic show is finally over as it was found that the LHS crank case gasket was too thick therefore further out and there is a minute oil seal and brass tube on the early etz’s that transfers oil to the crank and barrel but as it wasn’t a tight fit anymore it also drew in ALL the gearbox oil with it!! For anyone living in West Yorkshire namely Huddersfield it was Moll Springs M/Cs in Honley that found the fault. Well done to Andy!!

I thought an O ring fit there? Anyway- Congrats- these annoying problems can actually be the simplest. I recall tracing down a stalling at idle for many many months and changed all electrical components more than once EXCEPT for the HT lead, which I eventually changed.

Tis an oil ring although it relies on the crankcase cover to complete the seal.

Crankshaft is the same on 250 and 300 ccm.
But the 300 ccm engine was not produced before 1989, so your 300 ccm engine is a 250 ccm engine tuned to 300 ccm. The piston should be around 75,45 mm.

Carb. is the same on 250 or 300 ccm. If it is an original GDR carb (BVF) then it should be a 30 N2-5 type.

Yes that’s the one!