[Ian Richards]

In addition to fitting the VM30 carburettor, I had three possible engine ‘improvements’ planned for my ETZ 250.
These were:
1. Increase the capacity to 300 cc or perhaps beyond
2. By one means or another, alter the porting to something near that of the TS
3. Fit a reed valve

Each of these modifications has attendant downsides, one of which is that the modification might not work as well as anticipated (perhaps that should be ‘hoped’). Another is that each of them is difficult or impossible to reverse, except by replacing the modified parts.

When I bought my MZ it was always intended to be an interesting and inexpensive bike that I could modify as I wished without worrying too much about originality. Unfortunately the supply of cheap secondhand parts has largely dried up and the Covid pandemic seems to have accelerated the trend towards these bikes becoming ‘classics’. I don’t need another one of those.
So what should I do? It rather looks as though I should throw a dust sheet over it until the spring and find something else to occupy my time.

Ian

[Ian Richards]

Thanks for this, Brian. I had assumed that the architecture of the TS engine changed when the horizontally-finned head was introduced. I can now see why quite complicated modifications to the ETZ barrel are sometimes recommended rather than a barrel swap.
I shall certainly follow your advice to proceed with caution as I am generally very pleased with the bike as it is and don’t want to spoil it.

Now, I may have got this wrong as well but I understood that the ETZs made for the domestic market were substantially less powerful than the model we are familiar with. Was this achieved with milder porting or simply by a restrictive, I wonder?
Ian

[Ian Richards]

Hello Rob,
I am sorry I took my eye of the website after my original post; I hadn’t expected such a welter of responses.
I had refrained from mentioning specific companies but, yes, my VM30 was supplied by Allen’s. Overall the service was OK but the man who answered the phone always seemed pretty grumpy, as if he didn’t want to be disturbed. Very knowledgeable though.
Most people seem to have found that light dressing of the OD of the Mikuni inlet stub will enable it to fit the MZ inlet manifold. I don’t think that the difference is as much as 0.010″. This would be best done in a lathe but I don’t have a 4-jaw chuck so did it gradually with 240 grade wet and dry, with frequent trial-fits. It’s almost impossible to get the stub both round and parallel by hand but I obviously did it well enough though, I confess, I used a smear of silicon just to make sure that there would be no air leaks. So little metal needed to be removed that I am sure the carburettor could still be used with the usual moulded-rubber connector.
The standard connector to the airbox is of a decent wall thickness (5-6mm). I followed the advice I had read elsewhere and gradually whittled sufficient rubber from the ID using a craft knife. It sounds crude but with care it can be enlarged fairly accurately. A decent worm-type hose clip clamps the hose snugly around the carb inlet. A stepped adaptor would have been more elegant but I didn’t have the necessary diameter aluminium bar or tube to hand and, besides, I think that it might have taken several goes (and a lot of swarf) before I got it right.
Incidentally, as everyone else probably knows this, backing off the nuts on the top engine mount allows the engine to pivot forwards and downwards (don’t forget to slacken the exhaust hanger first). This makes it much easier to fit the carb and, if necessary, to remove the throttle slide and needle with the carb in situ (although I found that the two manifold screws still needed to be removed to allow the carb to twist).
Good Luck
Ian

[Ian Richards]

Oops. When I wrote that I fitted a leaner PILOT jet as an alternative to a throttle slide with a greater cutaway, I actually meant a leaner NEEDLE jet.
Apologies for not reading this properly before posting.
Ian

[Ian Richards]

Hello,
I am coming rather late to this particular party but as I fitted a new VM30 to my ETZ 250 a few weeks ago, I may have some useful observations.
Firstly, fitting the Mikuni did make a big improvement, both to the way the bike runs and also to the ease of starting. However the comparison is between a brand new Mikuni carburettor and a BVF carburettor that was certainly not new, though it didn’t seem to be appreciably worn.

Secondly, the cost of jets, needles and throttle slides for the Mikuni is very high and the number of possible permutations is huge. It would be very easy to spend the original cost of the carburettor several times over. The carburettor that I bought was was supposed to be jetted for this engine (and in fairness, the jetting was very close to that recommended by others). I found that the engine was reluctant to run cleanly at about 1/4 throttle, stuttering badly at as the engine speed increased to about 3000 rpm. Other combinations of revs/throttle opening seemed to be fine. Lowering the needle to its weakest setting made a considerable improvment but it still, to me, felt too rich. What to do? My instinct and previous experience (both unreliable) suggested that a throttle slide with more cutaway was needed. the cost of a throttlre slide (delivered) is about £50 so instead – and on the advice of the supplier – I ordered a leaner pilot jet at just less than half the cost. The result is a definite improvement, though the needle still needed to be lowered one notch from standard. Is the jetting now optimum? I don’t know and I probably never will.

My advice to anyone thinking of fitting a Mikuni would certainly be to discuss in advance with the supplier what would happen if the jetting supplied did not seem to be correct. A jet-exchange scheme would be very desirable. My (highly reputable) supplier did not offer this service and, in fairness, I can see pitfalls, with customers trying to exchange damaged parts for new, or blaming the carburettor for unrelated faults.
I have a ‘spare’ exhaust gas temperature (EGT) setup from a microlight that I intend to try on the MZ. EGT offers a very good guide to the margin-to-detonation but it is normally used to to check the mixture and timing at either ‘cruise’ or wide-open throttle. Ii don’t know what it will tell me about light-throttle running. Watch this space.

Incidentally, at about the same time as fitting the Mikuni I also fitted a VAPE electronic ignition. In combination, this has made the bike is now a genuine first-kick starter, which was a huge improvement. I did notice however that when the voltage regulator was playing up, the idle speed would rise almost uncontrollably. I believe that this is a known ‘feature’ of the VAPE ignition. The cure was a new regulator, which was needed anyway. I wonder whether this might be something of interest to other members.
Ian Richards

[Ian Richards]

I see exactly where you are coming from Gerry; most attempts to ‘improve’ MZs seem to end with a qualified success at best – and those are the ones you here about.
On the other hand I’ve no reason to believe that the brief given to the MZ designers 40 years ago matches my requirements or indeed what sort of bike the company would have liked to design had it not been constrained by cost, lack of modern machine tools and materials.
So, I shall continue to try to improve my ETZ. At the very least it will provide a future owner with an interesting project should he choose to return it to its original condition.
Ian

[Ian Richards]

Thanks for advice, Andy. Ideally I would like to move the footrests backwards rather than downwards but I shall just have to see what’s practicable. I had read about the idea of using a smaller final drive sprocket but, as I have not yet seen the chain, I don’t know whether a previous owner may have already done this.
BW
Ian

[Author]

Posts