Supa 5. Stuttering at low revs.

[Pete Lamb]

I’m looking for some advice to hopefully eradicate a flat spot/stuttering when riding my late Supa 5 at smaller throttle openings. The bike runs fine when under load and at higher revs, and ticksover nicely too. It is just when cruising and trickling along at low speeds that the problem appears.

I have been experimenting with the needle height, as it seemed that it was maybe running too lean, but that doesn’t seem to affect the stuttering issue. The Haynes manual says that the late Supa 5s’ should run with needle on the no;1 (top) slot.

This is my first MZ, and the issue is spoiling what is otherwise a great little bike. So, any experienced advice or ideas would be very welcome.

Pete.

[Matthew Nichols]

I’ve had stuttering issues due to HT lead breaking down. Initially it was only an issue when the bike was cold, and wouldn’t seem to rev cleanly under load then it got gradually worse whereby the bike would simply stall at a stop when it had electrical load on, and then later including when it didn’t have electrical load. Just saying this as it may not be fuel related at all…

[Pete Lamb]

Thanks Matthew

Possible electrical issues had crossed my mind too. The bike has a VAPE kit fitted, and appears to have anew HT lead, so I was excluding that for now. But of course all this troubleshooting is a process of elimination, so I will certainly look into that further. I’ll post again if/when I find the solution.

I wouldn’t have thought that it would be a characteristic of the bike, as it is quite intrusive whilst riding.

Pete.

[Matthew Nichols]

What I would add is that I would rather follow the original MZ manual, which actually states the below:

Main jet: 140,
Needle jet: 70,
Partial load needle: C6 with 5 notches,
Needle position FROM TOP: 4 to 5 (5 just for running in),
Starting jet: 110,
Slow running jet: 35,
Float needle valve: 20,
Slow running air screw: 1.5 revolutions open (from fully seated).

I don’t think any manufacturer would recommend a needle to be in anything other than the middle or close to it for a needle in order to allow for altitude and tuning and other things. So I would double check the above wording. I have found mistakes in Haynes books concerning both MZ’s and other makes, so always cross reference with the original service books first! 🙂

[Pete Lamb]

Thanks Matthew

I will check my carb against that data.

I was running the needle on 3rd slot on the needle, but then I found supplementary info towards the back of the manual for the late Supa 5s’ which advised using the top slot.

When I bought the bike it was running on the top slot, but of course it was displaying the problem on that position.

[Matthew Nichols]

I’ve never had a TS250, so I have never bothered to look through the manuals. But I know what you mean regarding supplimentary information, MZ did that a few times, so it is possible to go wrong if you don’t know exactly what revision model (as you say, early or late etc) bike you have.

[Garth Howarth]

Have you resolved this yet? If the problem is at very small throttle openings then the needle should have little effect. For most slide type carbs it mostly controls mixture from about quarter throttle up to nearly full throttle, and I gather your bike runs fine in that range. The mixture at smaller openings is mostly controlled by the cutaway and the idle system. Cutaway should be correct as BVF never offered an alternative size, but it could have been modified with a file! The 1982 edition of the parts list shows the idle jet as 35, or 40 “if required”, main jet 135 and needle jet as 70 (without cross-hole). Back in the day many riders found the larger pilot jet gave better running. I did, and also found that correct setting of the float height was important. Good luck!

[Pete Lamb]

Thanks Matthew

Sorry for the delay, I’ve been busy with other stuff.

I have completely dis-assembled the carb, so that I can check all the jet sizes and the other specs.

I have found a few anomalies.

1. I notice that my carb is a BVF30N2-5. Other info I have says it should be either a BVF30N2-3 or a BVF30N2-4. I’d like to confirm which one it should be, what the difference is and, whether it’s likely to matter.

2. The slow running (pilot jet) fitted to mine is a 45. The data you kindly provided (MZ manual) and the Haynes manual state 35. Any thoughts on this with respect to the running issue.

3. Do you know what the slide cutaway dimension means, and how do I measure it. The Haynes manual states “50-5mm”.

Jet sizes.
Main Jet is 135
Pilot is 45
Float needele jet 20
Needle jet 70

At this point, I think I essentially have 3 options.
1. Change the pilot and main jet sizes, to match the manual, 140 and 35 respectively. Also change the needle as a precaution, as there are no visible markings on mine.

2. Source a replacement carb, to see if that creates any improvement.

I would quite like to sort out the one I have, preferably, as it seems in good condition. But don’t want to waste time chasing shadows.

Any advice you can offer would be much appreciated.

Pete

[Matthew Nichols]

I have replied to you on the forum 🙂

[Keith Angus]

When I ran a Supa5 it was always a bit rough at low revs – it only runs happily over 3000. There is negligible oomph below that. When I complained to the dealer about surging on overrun he opened up the idle jet to 0.5 mm, so presumably a 50 jet. I didn’t notice any difference. Other jets should be corrected. The proper manual for the 250, in the supplement for the Supa5 says main jet 135, needle jet 70 (without cross bore). For the older four speed the main jet is given as 140, needle jet as 70 (no mention of cross drilling), starting jet 110, slow running jet 35, float needle valve 20, slow running screw 1½ revolutions open. I assume the difference in the main and needle jets is significant – the rest remain the same.

For the needle position:

“Needle position from top 4 to 5 (5 for running in). Besides the driving habit, the sparking plug appearance is decisive for the adjustment.”

Have you checked the cold start plunger? If the rubber stop in the end of it doesn’t seal on the jet it can cause odd problems with the running. When mine was a bit worn I prised it out and turned it over. That improved things a bit.

If I had one now I would look at one of the Bing carbs offered by some of the German spares dealers. It’s a better carb.

Also bin the Haynes manual and get a proper one. I bought a genuine MZ one and found it so much better. It left with one of my Supa5s, but I since downloaded one from somewhere – can’t remember where but they are out there.

[William Dunstan]

Have you tried the mixture screw as this can affect small throttle mixture as well as tick over . Supa 5 carbs are now all over 40 years old and worn so standard settings are mainly only good as starting points. Most supa5’s I have set up in recent years respond best with the needle on the leanest or leanest but one position and MZ’s carb parts vary I have seen two needles marked C6 but have visually different taper and lengths and some carbs with slides marked 70 and others not marked . Most of the carbs I am talking about were 30n2-4’s but parts may been changed over the years . Could your carb be off an early ETZ as I have never seen 30n2-5 on Supa5.

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