Mikuni VM30 ?

[alexxx]

I hate to replace the original designed carb on my 1991 MZ ETZ251 but it may have to go as the bike is unpleasant to ride with the surging on overrun which can be so bad I have to pull in the clutch
Has anyone tried a VM30 Mikuni carb conversion? was it a good improvement?
I have ridden lots of two stroke singles and never had any that felt as bad as this, I had a variety of CZ and Maico motocross bikes up to a 490 two stroke single, I even had that ticking over, pulling clean from low and never had surging....on any bike I ever had.
Any thoughts on what causes the MZ to surge so much on the overrun, even the cheapest two stroke moped behaves better than this.

[Blurredman]

I think it's possibly because of the lack of reed valve in the MZ? I know it does help performance on most two strokes, but also allows a smoothing out action, esspecially on over-run when the reeds tend to be more in the closed position and thus not giving the engine fuel?????


If you weren't in Edinburgh, I would come to you with my MZ and we could compare.. In my experience all my MZ's have a certain amount of surge, but more so when something is up with the carburettor (for example, one time I crashed my MZ, and as soon as I got it started I noticed a much better low response and smoother ride - odd!), and also when I had a completely degraded contacts set, whereby the contact material had completely worn away on one of the arms, it was giving me surging and poor idle- as you'd expect really..


As for the VM30- Never actually tried one.. I was seriously considering it when I had issues with my 30N3, but I managed to get the carburettor working very nicely for me now so I had no need..

I wonder- if you did buy a VM30, would you sell the BVF carb? I would be interested- It would also be interesting to see whether I have the surging you currently get.. That's what I like about having two MZ's, and my brother having an MZ.. I can swap parts over to diagnose.. Quick and cheap!

[alexxx]

Thanks Blurredman,

Not bought a Mikuni yet, trying to avoid it until I can get more info.
Don't fancy crashing my MZ though
I may try to make a one way valve to fit in an adapted manifold...how hard can it be.
I have never ridden another MZ so no comparison, I was tempted to buy a Saxon in Scotland on ebay today as it had the bing carb on it...it went for £360 but I dont want to start a mad collection

[breakwellmz]

CZs and Jawas i`ve had don`t do it and their carbs(Jikovs?)are even cruder.
Saying that, years ago i fitted a keihin carb from a Suzuki 250 to my Supa 5 (As well as a boost bottle!) and the carb conversion was well worth it; no more surging.

[Andy_C]

Have a read of:

http://www.realclassic.co.uk/techfiles/ ... tz250.html

You can get a VM30 kit from Allens for around £160.

[fatnslow]

I have just completed fitting a VM30 to my 1988 ETZ 250.
The bike now starts first or second kick, hot or cold. It has a normal tickover screw and a carb body mounted choke that only needs to be on for a short time. It's easy to reach.
Surging on the over-run is vastly reduced. I would say 70% better. If you stir the gearbox and ride in a slightly spirited manner you will not really notice it. The bike is much, much better to ride and no more weird starting sequences are needed .
It feels more lively, with a distinct little power band. I might play with the needle height as it's a little flat at 3-4k. From 4-6 it positively zings round

Firstly you don't need the spacer as per the Real Classic link given here. Just slice half the carb to intake rubber away. This is not from the length, it's removed from the inner part if you know what I mean to increase it's opening to suit the 57mm of the VM30 Mikuni. It doesn't have to be pretty as this part sits in the outside of the VM30 just like the original.

This is it fitted to the VM30. You can use the original clip to hold it in place, but I used a jubilee type which will grip it better.

I then removed the intake rubber to refit once the carb was mounted.
The Real classic guide says to remove a thou so that the carb will fit the original MZ intake stub. I removed the rough casting with some 400 grit paper. It still wouldn't slot into the carb to engine mount and I was reluctant to remove to much material from my expensive new carb. The seal needs to be airtight of course. So I removed the intake metal stub and put it in the oven for 15mins at 125 deg C. It then slotted straight on to the carb and will shrink to form a good seal. Make sure you reverse and refit the two pinch bolts. The pic should show this as well as how far the carb must be pushed onto the stub.

Refit to the bike as a single unit, ie, metal intake stub and carb.
Now refit the rubber boot. It's a struggle, so worth investing £8 in a new one as it's nice and soft. Secure it with the jubilee clip. The boot doesn't sit perfectly, but it's going nowhere and is a good seal.
On my ETZ the existing throttle cable can be reused. My carb was the later BVF with the clip that holds the float bowl on. Hope this helps.

[alexxx]

Thank you for the very helpful posting with the pics, great work!
I am now convinced to follow your method, I am sure this post will be helpful for many people, well done and thank you to everyone who has contributed.

[burudi]

hi. please let mi know settings for VM30.
Thks !

[parrbd]

On my 1985 ETZ250 surging can be eliminated by richening up the idle and needle settings. Also check for manifold leaks or crank seal leaks as these can cause lean running issues.

[greens8]

I also just put a Vm30 on mine (ts250) and its so much better. Easier to tune and just better.

[burudi]

someone so kind to give the VM30 setting data in ETZ 250 ?
thanks

[dirtsurfer]

someone so kind to give the VM30 setting data in ETZ 250 ?
thanks

I haven't tried it but Derrick Prichards recommendations are at the bottom of this page http://www.realclassic.co.uk/techfiles/ ... tz250.html
If I recall correctly Derrick and his ETZ were in Melbourne Australia at time of him writing this , so pretty much sea level

[burudi]

many thks dirtsurfer

[arthurf]

Dear All, Recently I have been speaking with Derek Pickard (out here in Oz), and he mentioned that his article on the VM30 had one error - the size of the pilot jet - it should be 30 or 35, not 40.

Now however I cannot get onto the "Realclassic" site to write down the specifications/jetting/and conversion process and would like a bit of help from all/any of you in this area.

My BVF carb is quite worn (the slide can be "rattled" from side to side within the carb), and occasionally sticks and does not return the carb to "off'''.

So I am looking at using a Mikuni. Which Mikuni model has the spigot mounting (so that I can use the existing manifold)? Can anyone supply pictures of changes to both the manifold and the air box connections?.
Many thanks, Arthur

[burudi]

many thks for your mikuni help !!