MZ/MuZ Riders

Hi
I am trying to resolve a slow running/flat spot issue on my Supa 5. I have the carb apart to check the jet sizes fitted to it etc.

1. I notice that the carb is a BVF30N2-5. Other info I have says it should be either a BVF30N2-3 or alternatively an N2-4. I'd like to in down which one it should be, what the difference is and whether it is likely to matter.

2. The slow running (pilot jet) fitted to mine is a 45. The MZ manual and the Haynes manual state 35. Any advice on this would be appreciated.

3. What does the slide cutaway dimension mean, and how do I measure it. The Haynes manual states "50-5mm".

Any informed tips would be welcome. The bikes runs very well under load, but is very hesitatant and flat on partial throttle openings, spoiling what is otherwise a great bike.

Thanks
Pete.

Hi

Thanks for that useful information.

I'm wondering whether they may have been a crossover with the -5 carb from the late Supa 5's to the ETZ? This would explain the carb number and some of the jet sizes being the same. Of course I have no idea whether this is the original carb. The bike does have a low mileage of around 14,000 miles, which I can verify with documents back to 2005. It is in nice original condition too.

I have tried playing with the needle height, but it makes no difference to this particular 'stuttering' problem.

I agree that it seems as though it's running too lean with this type of issue. It is very similar to modern 4 stroke bikes, and the issues they have with them running too lean at small throttle openings to comply with modern emission regulations.

My only thought is whether it is an ignition timing issue. It has a VAPE unit fitted, and I'm not sure how the advance system works and whether there is sudden jump to the more advanced setting as the engine speed increases, or whether it is a smooth curve. I have ridden bikes where the engine is very 'lumpy' at certain revs as it transitions from one advance setting to the next.

I will persevere with the carb settings and do some more checks.

Pete.

Thanks for the tip about checking for air leaks. I have heard of that before but haven't tried it yet.

I didn't know about the lack of an advance curve. I will check the timing I think.

Pete.

Hi Pete,

Matthew here...


Yes Puffs is onto a good point concerning air leaks. However your more likely than not to feel the problems of an air leak at idle and above idle. Giving symptoms of an idle that just won't stay in a single place but wander. But of course, it can also give lean conditions on low rpm high gear situations. But yes it sounds like it could be this problem.

As puffs says, spray brake cleaner or some other material around various locations such as the carburettor to inlet join, the inlet to barrel join and also the base of the barrel all around. I've had such leaks there.


I think I would work on that first, because actually I think the pilot jet and main jet although different are minimally different.

Indeed I have never actually had the TS250 - maybe one day, might be fun - but was always under the assumption 30n2-5 is on ETZs. I don't know about any crossover period though.

I think the cutouts in the slide are all the same in design, but the valve opening of 5mm or 6mm (I don't take notice of this, I just get the bike to idle where I want it) merely identifies the gap opening in height for idle purposes I suppose.


That said- where do you live? Just curious if you're anywhere near where I could help directly maybe.


But also, did you look into the spark plug lead resistance check? It may not be the carburettor configuration, or air leaks at all!


And yes, check timing.

Blurredman wrote:Hi Pete,

Matthew here...


Yes Puffs is onto a good point concerning air leaks. However your more likely than not to feel the problems of an air leak at idle and above idle. Giving symptoms of an idle that just won't stay in a single place but wander. But of course, it can also give lean conditions on low rpm high gear situations. But yes it sounds like it could be this problem.

As puffs says, spray brake cleaner or some other material around various locations such as the carburettor to inlet join, the inlet to barrel join and also the base of the barrel all around. I've had such leaks there.


I think I would work on that first, because actually I think the pilot jet and main jet although different are minimally different.

Indeed I have never actually had the TS250 - maybe one day, might be fun - but was always under the assumption 30n2-5 is on ETZs. I don't know about any crossover period though.

I think the cutouts in the slide are all the same in design, but the valve opening of 5mm or 6mm (I don't take notice of this, I just get the bike to idle where I want it) merely identifies the gap opening in height for idle purposes I suppose.


That said- where do you live? Just curious if you're anywhere near where I could help directly maybe.


But also, did you look into the spark plug lead resistance check? It may not be the carburettor configuration, or air leaks at all!


And yes, check timing.

Hi Matthew

You get around?!

Thanks for coming back on this. Even if I'm not making much progress for far, I'm learning a lot.

The bike runs really well both at idle, and on the main jet/under load. I had a ride last week for around 60 miles, and it ran really well apart from the annoying slow running when riding at small throttle settings.

I haven't done any checks on the HT lead, as it does look new, and the cap is moulded on to the lead. But I will try the intake leak test.

It doesn't seem that there are different slides, as you suggest.

Thanks for the offer of help. I am in sunny East Devon, near Honiton, if that's anywhere near you? Which model/s do you have?

Pete.

Ah yes, I get around! The bikes I own are in my signature, and I live in South Wales so not exactly local to you unfortunately...

Maybe the slow running is just one of those things that are actually normal for the bikes? Is this the first MZ you've owned- they do have a surge in over run especially at the lower rpm. Maybe you're just not used to it. Is it possible you could make a video?

What cap do you have? Pictures?

Thanks for your replies and info.

I'm still 'working on it'.

Matthew.
It is my first MZ, but I've had a lot of different two-strokes over the years, so I don't think that this is a normal characteristic for this engine. I'm aware of the surging on overrun, and to be honest that's not a problem for me. It's the stuttering at lower throttle openings which I'm looking to resolve. I'll be taking it out tomorrow, so I'll get back with an update.

Pete.

I've still been unable to resolve the running issue with the bike so far, but have been riding it occasionally regardless. It runs very well at wider throttle opening and idles perfectly.

Since my last posting, I have tried every needle height available and checked the ignition timing, which makes no difference at all to the flat spot at marginal throttle openings. I made a short video which shows the issue quite well, as I filmed it with the rev counter and my throttle hand in view. So you can see how I have to open the throttle past the unresponsive area to get past it before it starts to respond again.

I won't be able to upload it to the forum, but I am able to send it as a text or a Whatsapp message if anyone feels they might be able to offer some advice.

My next step will probably will be to source a replacement carb to try, to see if there will be any differences.

Pete.

Thanks Puffs.

I will take a look at reducing the file size of the video. But I think you need the pictures with the sound, so you can see how when gradually opening the throttle up to 3,000rpm, the bogging starts and despite opening the throttle further the revs stay the same for a while until they get past the bogging range and then climb normally. It doesn't look or sound so dramatic in the video, but when I'm riding I am in that throttle range a lot of the time, so it's pretty frustrating.

I will take a look at that link too. I think you're right about the replacement carb, even if it's just one to try. I'll also check the pressure at the end of the exhaust as you suggested.

I found some useful tuning/jetting info during my research too. Link below.

https://motocrosshideout.com/dirt-bike-jetting-101/

Pete.

An Ex MZ Owner here, but still have a couple of 2 strokes.

Bogging down is usually down to a weak mixture at the particular throttle opening - how much throttle are you on whhen it bogs ?

Depending on the throttle opening you need to look at the jetting / throttle slide / air screw.

Not read the entire thread but are you sure that all jets / carb airways are clear?

Might also be worth checking the air filter if you have not already done so.

Just my 2 penneth worth.