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.