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I’d be tempted to clean the exhaust. Probably won’t sort it but narrows the field down and it’s never a bad idea to have a clear exhaust.
I’m with Rob on never underestimating the capacity of bikes to mislead. If you’ve got them to hand try a “new” carb, coil, exhaust etc.
It could be an issue with the ignition module, even just a bad earth.
Have a word with Kevin Bacon. He knows someone, who knows someone, who knows someone, who knows someone, who knows Roger.
Peter
Thanks for that I thought i’d seen it in a Real Classics article, no wonder I couldn’t find it. It’s Issue 174 July August 2008. I have a copy so if anyone wants a scan let me know.
Brian
Ian
Don’t know about reduced power on early ETZ’s but they were offered with the option of drum front brake and premix ie no oil pump. I think this was to reduce the price differential between the ETZ and the TS for buyers, maybe to appease the likes of Wilf, but perhaps there wasn’t sufficient sales to encourage then to continue. I believe that the early disk brakes were imported Brembo, rather than the licenced copy they used later and the oil pump was from Mikuni in Japan so it could of just been economic/currency difficulties with importing stuff.
Oh I forgot to mention the 12V electrics on the ETZ changes/improvements.Brian
Hi Ian
I have a TS250/1, Supa5, and only have experience of a short ride on a ETZ250. Esentially I found that the TS250/1 pulls sensibly from around 3,000rpm whilst the ETZ needs around 3,500 to give the same sort of pull. The difference was nowhere near what I was expecting from reading the same sort of things you have. Definately recommed trying a TS before you make any decision.
Just so you know the cylinders are more different than just the porting. Stud size and spacing is different, crankcases are different with the start of the transfer ports being bigger in the ETZ engine. The cylinder is a different height due to the revised crankcase. It’s possible to fit a TS cylinder to an ETZ but requires a 10mm (I think) spacer under the cylinder scalloped out to merge the ETZ crancase transfer area to the TS cylinder, the cylinder studs of the correct size and spacing need fitting to the spacer. I thought I had a copy of the article I saw on this conversion but I can’t see it just now, if I find it I will let you know.
I think they modded the ETZ to match the characteristics of the then current Japanese bikes. Whilst Wilf wanted to stay with a cheap and cheerful commuter bike the Factory deceided to challenge the Japanese, so it got a peakier engine, oil pump, disk brake, 18 inch rear wheel and more current styling. Plus of course an increased price which definately didn’t please Wilf. Was this the correct decision? Answers on a postcard please.Brian
Hi John
A look at the Polaris site may help. Polaris do loads of two stroke engines for snowmobiles, outboards etc and I believe they list them all somewhere with tech specs. You may find a suitable piston for a project or find out the industry standard for measurements.
Hi rob
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.
BrianHere’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.
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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:-
Mikuni VM28
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.)
Mikuni VM30
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.
After-Thoughts: Power
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.
Bottom Line
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.
Numbers
Mikuni VM28:
• 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
Mikuni VM30:
• 150 main
• 45 pilot 1.5 turns out
• 6DH3 needle mid-notch
• P-5 needle jet
• 2.5 slideCuriously, I later converted the 250 to a 300 and found the same jetting worked in both the Mk2 and VM30.
Gordon
Bearing pullers are available on eBay from £20 upwards. They aren’t just for blind bearings you can use them on,say, wheel bearings and it saves having to drift them out with a hammer.
Here’s a cheap one. There are many others just put “bearing puller” into the search box.
Brian
Hi Neil
I had this issue with a Honda XL250 and put it down to the float valve in the carb leaking ever so slightly and over time the fuel level rose in the carb untill it gave a too rich mixture to start.
BrianHi
Can I just clarify something from my previous response.
This is where a picture is worth a thousand words but here goes:-
I beveled the inside of the exhaust nut using the bench grinder. This was done by rotating the exhaust nut by 45 degrees horizontally on the vertical axis and pressing the 12 and 6 o’clock positions of the exhaust nut onto the corner of the grinding stone, applying gentle pressure and rotating the exhaust nut.
This will bevel the inside of the exhaust nut to about 45 degrees which was close enough for me. If you was a better match then rough it out using this method and then hand finish by whichever method works for you.
You must use gentle pressure and keep the exhaust nut rotating as the grinding stone is not designed to take side pressure. So if you are in any doubt don’t do it.
Eye protection should be worn as a minimum, but a full face mask is better, and a filter over the nose and mouth.
Now whilst I think the trouble and expense of getting the thread reclaimed on a BMW head is probably justified I think with an MZ barrel it would be more cost effective and easier to get a barrel from a vehicle dismantlers.
Brian
Hi
Sorry for the delay I’ve just seen the post.
This happened to me on a Supa 5 where the best engagement I could get was about 10 to 15 degrees without a gasket, somehow it worked like this and stayed on whilst I was fettling it. The thread at the front of the port was chewed up but it looked OK at the back. The exhaust nut was in a bad way as well.
I removed an exhaust nut from another Supa 5 and this was undoubtedly better but not good enough, maybe half to three quarters of a turn thread engagement with a gasket.
Looking at the exhaust nut it was obvious that it wasn’t threaded to the end so I basically ground it back on the bench grinder until there was a good full thread at the end. I also bevelled the inside to match the exhaust pipe. I must have ground away about 6mm/quarter of an inch.
This now screws on about 3-4 full turns with a gasket and is rock solid.
Try it, you’ve got nothing to loose as you need to be fitting a new exhaust nut and gasket if you get the thread reclaimed.
Brian
