October 13, 2020 at 5:34 pm #13398
Has anyone any experience of fitting a Mikuni VM carb to an etz251? Good, bad, indifferent?
October 13, 2020 at 7:37 pm #13399
Good questo Skorpy. I would like to know as well.
My prob.with the standard carb is that it never gets the plug wet even. So I’m thinking that’s why its such a bugger to start. Done everything to it even new crank seals. Tried a brand new carb – just the same. I have kicked it till I’m blue in the face. One good
point it really has strengthened my right leg !
Oh for a regular strangler/choke
So whats your beef Skorpy.
October 13, 2020 at 7:40 pm #13400
The shame of it all is that I really enjoy riding it, possibly my favourite bike.
October 14, 2020 at 7:06 am #13401
My problem is a Chinese copy of the Bing Carb which I think is probably junk. If I’m buying a new carb I want to spend my money well, either on a genuine Bing or the ever popular Mikuni VM. I was fishing for the experience of someone who has fitted a Mikuni.
October 14, 2020 at 1:37 pm #13402
Hmmmn. Did it start up ok with it. I would kinda be satisfied with that.
Are they on Ebay.
October 19, 2020 at 5:34 pm #13419RussParticipant
I have the Mikuni carb and it is excellent as the bike came with a Mikuni I wanted to put it to the original which is a BVF, I tried the BVF and immediately changed it back to the Mikuni, I have run thousands of miles on the Mikuni and never felt I need the original BVF which is laid in a box SOMEWHERE.. You can fit the Mikuni without changing or looking for anything, it fits straight onto the intake and airbox.
November 9, 2020 at 10:19 am #13470
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.
November 9, 2020 at 7:36 pm #13471RussParticipant
Knowledgeable information helps in all manners.
Excellent much appreciated.
November 10, 2020 at 8:47 am #13472
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.
November 11, 2020 at 4:31 pm #13475
Very interesting Ian. I live in hope that a Mikuni would improve the starting of my Supa5 which is covered up and languishing in the shed as I had virtually given up on it but after reading your post, hope springs anew !
So could I ask the model/size of your Mikuni and who the supplier was.
PS I fitted the Vape system to it some years ago and found it to be great bit of kit.
Not one kickback since.
November 12, 2020 at 7:17 pm #13478BrianParticipant
Here’s an article from the old Real Classics mag archive. Should have all the info you need.
MZ ETZ 250 Tuning
Home -> Tech -> Techniques ->
Derek Pickard looks at the stock MZ250 carb situation, tries the alternatives, finds what works and gives the details of how to make your ETZ run super sweet…
Let’s start with a few hard facts: The BVF carb as fitted to the MZ two-strokes can only be considered ‘average’ in its metallurgy and machining. It is the main contributor to bad starting and rough running. As the carb wears, that bad behaviour gets worse. This can be corrected by replacement. The trick is to know which carb and how to make it fit and work.
We all like the unique MZ 250. The bike works well as a good basic rider but it does mean accepting a few compromises. As the years pass and various engine bits are replaced, few owners go to the expense of fitting a new carb. Yet the BVF inevitably wear and with it goes any accuracy of settings which means missing out on easy starting, best pick-up, smooth over-run and maximum economy.
My ETZ250 had done over 70,000 km and like all 25 year old bikes has had major engine and transmission parts replaced. But the original East German BVF was simply a worn old carb. Rather than putting money into renewing it, I decided I’d take on a project of finding which alternative did what; albeit from a position of considerable carb tuning experience over the years, including Amal and Mikuni.
The most obvious replacement is to find a good Bing and mounting stub from a later model MZ 251. But those carbs have probably covered a high mileage and are worn which means they are unlikely to be a good upgrade.
See something different? This is what a Mikuni VM30 looks like on an ETZ250. Not much change visually but the performance is a big upgrade.
Just about everybody these days knows that the carbs which hardly ever wear are the top-rated Mikunis. These Japanese carbs have the best materials and machining which is why they go on for decades with a fantastic slide/body fit to match their top design. Even old ones show up as being in great condition — whereas at the other end of the carb rating scale, old Amals are usually best left for keeping fishing lines tight! Here’s the results of my investigation:-
For the MZ250, 30mm is the correct size but it is nearly unobtainable from breakers since very few old Jap bikes used that size. So for the bargain hunter, the choice is to go the next size down and use a VM28 as fitted to older Jap bikes such as the 1970s Yamaha RD350 and trail 250s. The problem is they are a smaller bodied carb so special sleeves will have to be made for the front and rear to fit into an MZ. These convert the VM28 to exactly the right length and diameters – but be careful to make the front one an exact fit or an air leak could result. (Don’t worry too much about the 2mm drop in venturi size as such a reduction may well lose a couple of bhp at the top end but there shouldn’t be much difference in the effective torque around town. And don’t fit a VM32 onto a 30mm inlet as that will work worse than a VM28.)
My ETZ 250 runs well on such a VM28 conversion which was my first carb upgrade and I was delighted at the improvements. If you have access to someone who can make alloy sleeves then this conversion would make a very cost-effective improvement for a commuter or tourer.
Amal Concentric Mk1
This British carb may well have been part of the Empire but it is rather crude and famous for not giving a reliable tickover. Fitting a chrome plated brass slide from Surrey Cycles usually gives a tickover but there’s also the additional expense of buying the necessary front fitting that converts it from a flange to a push-on spigot mount. Beware buying old Amal Mk1s as most were made with a pressed-in size 25 pilot which can’t be removed.
While I accept the Mk1 has the advantage of offering a simple tickler flooding device for cold starting and can be very cheap to buy a used one, all-in-all I’m not a fan of this carb. The Mk1 metallurgy and machining is worse than the BVF and it cannot be an improvement on a 250 Zed; so I didn’t test one on my ETZ.
Amal Concentric Mk2
Since Burlen (the UK maker of the Amal Carb Company) has reduced the new price of this carb it is worth considering, and for a little more money the Teflon-coated slide can be bought. Fitting means making a very thin (14 thou wall thickness) sleeve for the front so it accurately fits into the MZ female fitting by being just under 40mm. Also, it involves turning down the large rear inlet trumpet from 60 to 52mm which may look ugly by exposing a couple of holes but these are covered again by the MZ air filter sleeve rubber. And remember when ordering to request a ‘left’ one so the side adjustment screws are more easily accessed.
An interesting comparison between this and the stock carb is the incredible difference in weight. MZ must have cast their carbs out of the heftiest alloys with lots of zinc or lead which is illustrated by holding a BVF in one hand and an Amal Mk2 in the other. The immediate comparison is something like between a house brick and an empty box of tissues. Presumably, Amal use plain light aluminium. (Be advised that this Amal needs a two-stroke spray tube where the top is angled forward at 30 degrees. Also, the pilot jet must be fitted in the front of the two positions.)
For fast open road work, the ETZ250 is only a 250 and so needs every last bhp which comes from the full size 30mm carb. A VM30 is the best and this may be the most expensive at £105 from the UK but it has the advantage of being superb at everything in carburetion. It definitely optimises the engine for the best in starting, pick-up and economy. If it is possible to come close to bolting on the precision of fuel injection, then a new Mikuni is it. These VM Mikunis are a very sophisticated instrument and a heck of a lot of variations in needles and jets are available so some may find the tuning difficult and expensive. But I bought mine from Mikuni specialist Allens Performance who supplied it correctly jetted.
However, again, a sleeve needs making as the rear bellmouth can’t be turned down. This means making a difficult stepped spacer for the rear that takes the push-on end from 57 to 52mm OD for the MZ air attachment; I made mine 2mm thick out of aluminium. A difficult thing to make but vital. The fit is tight and a couple of things must be remembered. Firstly it is necessary to carefully take off a thou or so from the Mikuni’s front OD by hand with say 180 emery paper which will allow the stock VM to slide into the MZ mount. This is done for fit as much as the polishing off the as-cast semi-rough finish for an air-tight seal.
Then it is necessary to invert the two MZ carb clamping bolts after thinning down the heads so when in the inverted position these heads clear the front of large VM float chamber that unavoidably locates very close. And because this VM with the rear stepper spacer is longer than the stock carb, heaving it into place is a tight squeeze but it can be done. When compared to all other carbs the VM32 Mikuni is a delight. My ETZ250 is now far more user-friendly usually requiring no more than two kicks to start and runs spot on every time. Definitely a better bike.
As the stock BVF is inevitably old, worn and best replaced, the 250 Zedder has a few options. Finding an old VM28 (bottom left) can be cheap but front & rear fitting sleeves must be made. An Amal Concentric Mk2 (top right) with a Teflon slide works okay but both a thin front sleeve must be made and the inlet trumpet must be turned down to fit onto the MZ air filter attachment. The VM30 Mikuni (bottom right) is the last word in optimising an engine¹s carburetion but the rear needs a stepper sleeve for the air filter join. Any one of these three carbs will improve the ETZ250 starting and running over an old BVF.
In terms of outright power, just about all 30mm carbs can produce about the same top end if the main jet is right. But not exactly the same because as the Mk2 casting has a lot in common with other Amal sizes, the all-important trumpet effect into the venturi has too many compromising angles and cannot flow air as well as the VM30. But in reality we’re only talking a tiny difference at the very top end. Interestingly, the BVF has a superb air flow shape pity the rest of it is so bad. Yet more proof that the East German engineers knew the correct designs but couldn¹t get the right materials and machining.
Cable Versus Lever
While both Mikuni and Amal carbs have chokes that can be operated by lever or cable, the latter is better . The reason is a simple on or off is inadequate in not providing a setting of only partially on for warm-up for a winter morning. It all adds to the user-friendliness of a good system.
It is definitely possible to transform the running of an old MZ two-stroke: fit a new Mikuni for 105 quid and have the new inlet sleeve made. You won’t believe how much better your bike will run.
• 40 pilot 1 turn out
• 5F3 needle second notch
• N-8 or P-5 needle jet
• 2.5 slide
• 140 main
• 2 air
Amal Concentric Mk2 30mm:
• 25 pilot front position 1 turn out
• 2D1 needle mid or number 2 notch
• 107 needle jet
• 3 slide
• 210 main
• 50 starter
• 2.5 air
• 150 main
• 45 pilot 1.5 turns out
• 6DH3 needle mid-notch
• P-5 needle jet
• 2.5 slide
Curiously, I later converted the 250 to a 300 and found the same jetting worked in both the Mk2 and VM30.
November 13, 2020 at 9:05 am #13479
Strewth ! ! !
In fact I’ll say it again STREWTH ! ! !
What a reply.
Must be a record breaker, many thanks.
I will now sit back and absorb.
I might be some time.
November 13, 2020 at 9:16 am #13480
Right. Excellent, my cup runneth over as they used to say. Now to track “Allens” down. I expected the price has tripled in the intervene but if it gives as he says
it will be cheap at the price. I mean what use is a bike that will not start.
Thanks again. Rob.
November 14, 2020 at 8:12 pm #13484BrianParticipant
Allens will have their own thoughts on jetting, having done their own research. It might be a case of decisions, decisions. The price is comparable with a Vape conversion which is a price many are prepared to pay. There are VM30s on eBay from America for around £100 last time I looked. But as always, beware of fakes on eBay. Allens have a very good rep.
November 17, 2020 at 2:18 pm #13492
Hi all, Well Allens did have from what I’ve heard elsewhere. This was confirmed to me by not even replying to my enquiry.
I had however by searching the web come across Steve of Motocarb Liverpool who I subsequently found from another Forum was reckoned to be Ace and so it has proved, answering my enquiry from Saturday on Sunday would you believe.
Spoke to him on Monday and he is indeed a fair bloke. Ordered the carb there and then. Delivered 11.45am today – Tuesday !!! I have it here at my elbow.
Very nice it is too. Came set up for the Supa5 with spare jets and the connecting
rubber & clips and still had change from £200.00 ! Which I thought was pretty fair. I think that article must have been printed some time ago.
Now I just have to make up a mounting to the cylinder flange and sort out the air filter arrangement.
Any suggestions most welcome.
Hope the forgoing will be of help to the Brotherhood.
Ole I am a Bandit ! ! !
- This reply was modified 10 months, 1 week ago by Rob Lane.
November 17, 2020 at 5:31 pm #13494
I sometimes wonder about what we pay in the UK for our bike parts. I’ve just looked on MikuniOz website (Australian) and see they are selling Mikuni VM30 carburettors for just 154 Australian dollars or £85. They also list lots of adapters, cables, twist grips etc. Now I know you can’t buy advice on eBay but it does seem we are living in what is commonly known as “rip off Britain”. As far as I understand it anything imported valued at less than £135 will not attract any import tax etc you just need to pay the carriage but I stand to be corrected. Incidentally, my own experiences with Allen’s have been good doing it the old fashioned way with a phone call.
November 18, 2020 at 4:46 am #13495
Hi Skorpy, glad your still here.
Yes to all you say, will be interesting if you do go ahead to hear your experiences. We don’t seem to have a Mikuni website in the UK. Motocarb imports them from the U.S.
Now look at the time of this message.
Never assume anything. I just woke up and the first thought that came into my head was that I hadn’t actually checked the sizes of the Mikuni. I mean they couldn’t possible be the same could they. Well I have just checked and amazingly they virtually are. The overall length is shorter by 6mm – good. The inlet dia is somewhat larger and will need a reducing collar to fit the filter hose and the outlet dia. is about ten thou bigger so will be able to use the existing inlet manifold, once bored out to match.
Interestingly the main obvious difference in the two carbs is the fuel enrichment chamber is at the engine end of the body so I’m hoping it will be more reactive.
Time for a cup of tea.
November 18, 2020 at 3:08 pm #13497
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).
November 19, 2020 at 8:58 am #13498
Ithangyou Ian, like the idea of paring out the air filter hose to fit if unable to get a reducing collar in there. Hoping to get the carb up in the lathe to skim down to suit the manifold. A bit too much to take of by hand unless you are really desperate.
As for jets the carb came with two spare main & ditto pilot jets as part of the deal, for fine tuning.
Looking forward to give it a go. Regards.
December 2, 2020 at 2:46 pm #13568
Hi, well the Mikuni is fitted (yesterday) And she fired up on the ninth kick which to me was incredible. Fitted but not sorted out the intake hose yet so seems a bit weak low down but of course as said it came with spare jets so can change if necessary. Did a slight miss service to Motocarb, the price actually was £165.00 + del. Do like the feel of it. For your interest the enrichment chamber is at the engine end of the body so must make it more responsive.
Skimmed a few thou. of each end dia. (see pic) to give a good fit in the manifold.
Still have to sort out the inlet hose. Would you believe it had split just sitting there. Twas only eight month old. Old stock do you think ? Trying Supaglue, well they are over thirty quid now. Did half try your suggestion of paring the hose out to suit the flange dia Ian but wasn’t happy with the way it was going. Still pondering on’t.
Below should be a pic of it in the lathe. Used a boring tool so as to miss the choke lever.
Hasta la Vista Amigos.
- This reply was modified 9 months, 3 weeks ago by Rob Lane.
December 2, 2020 at 5:18 pm #13570
Re. the comment about old stock rubber items, I have experienced problems of old rubber on 500R airbox side rubbers from both a UK supplier and a German supplier, maybe of the same origin? They were so bad they were cracked half depth when supplied new.
December 3, 2020 at 9:59 am #13571
Interesting Mr Bond. I haven’t the heart to tell Burlens – yet.
- You must be logged in to reply to this topic.