Help with 251 carb issue

[treehouse]

Edited to add: I've had a look at the carb and it doesn't match any of the pictures in Haynes. The guy who sold it to me said it's got a Bing carb, being a later model, but it's not - I've just looked up the markings and it's a Ken Tse 84 Taiwanese one. I presume they're not the same thing, and also that this "engineer for decades" doesn't know is A from his E. And I'm not sure I dare ride this bike except to the nearest garage to get it repaired.

I'm completely new to MZs and only have a little experience with carburettors. As described in an earlier post, I bought a 1993 ETZ 251 Saxon Tour a while ago, I don't think it was running right on the way home (excessive judder on the overrun). The previous owner had removed a spring from the carb (I think it can only be the main return spring, which he put in a goodie-bag for me along with some spare plugs and spanners). I'd like to pick your brains before I start trying to fix it. I haven't dared ride it since (also due to the snow), and it's due for its MOT in a couple of weeks. One other point is that the tickover is set far too low, I believe. He said it should tick over at around 500 or 600 RPM, and although it did when I went to look at it, it cut out after my 35 mile ride home (I didn't dare let it idle at any other stage of the journey!). The Haynes manual says 1200 +/- 100, and others have said something similar.

So, questions on my mind are:
Would the low tickover cause the judder? (I know these bikes do a bit of juddering, but it was serious enough to start worrying about the engine, frame and forks parting company. It was there - although not very severe - even with very slight engine braking. With full engine braking it was occasionally ok, other times really bad.) Should I just increase tickover and try it?

Could it also be the other thing he said it needed - some good runs to clear out the muck after standing and being occasionally started and ticking over, etc.?

Would removing the spring cause the judder? He said he did it because when you let go, it reduces the gas gradually instead of all in one go, suggesting it's actually smoother on overrun (although he didn't spell this out in detail), and because it reduces the tension on the throttle, so you're not having to grip it quite so hard. My experience was that it was nice and light and it closed ok, apart from the juddering. Should I replace the spring and try it again? I've been advised here that I shouldn't ride it without, and even the previous owner admitted there was a risk that if you ever gave it full throttle the needle might come out of the jet and not go back or something. This was long after the deal was done, money in his account, and I was about to take it home.

If I replace the spring, should I attempt to open the carb top and remove the throttle valve to reinsert the spring (i.e. without taking the whole carb off)? Would I need to adjust anything afterwards, or would everything else stay as it was? Speaking of which, are there any settings you'd suggest instead of the ones in the Book of Lies?

Sorry if these are dumb questions - writing this they seem like no-brainers - yes, increase the tickover and replace the spring, duh - but I might as well use your collective wisdom in case there's something I've missed. Is there anything else it's more likely to be - air filter, plug, some electrical gizmo - something I should check out first?

If I start taking the carb apart (even the top), should I replace O-rings and gaskets, and are they pretty standard? How do I know what to buy?

Finally, in your experience, do non-MZ bike mechanics generally do a good job with MZs (if I chicken out or decide to get the whole bike checked over)? Are they likely to be able to get their hands on any parts it requires? I'm hoping it doesn't come to that. I guess I'll try the two things above and then if it's still rocking and rolling, take the carb apart.

[breakwellmz]

Have you come across this?-

http://www.ttnet.net/ttnet/gotohtm/0/13 ... 831363.htm

I would suggest that the juddering could be down to to the slide spring not being fitted,it is there for a reason!

You SHOULD be able to refit without taking the carb off,you may need to loosen and rotate the carb to do this,it has been a VERY long time since i worked on one of these!
If you are taking the slide out to put the spring in,there are no gaskets or`o`rings to worry about.
I would suggest you get a manual,if you haven`t got one,read it,and take your time.

Cheers

[treehouse]

Have you come across this?-

http://www.ttnet.net/ttnet/gotohtm/0/13 ... 831363.htm

Yes I did, thanks - unfortunately it's none of those, and I don't have anything on how to tune it or how it behaves when stuck on a 251. I don't even know what the screws do. If I were more used to messing with bikes I might even be able to identify them. But even if I can apply the Haynes instructions to this, I'll probably still have weeks of tweaking to set it up well, maybe months, since I'll be interested to get the best mpg I can. I'm getting MOTs on this and my old bike, so I'll ask the guys there if they can help me set it up.

What's what?

It's got a rather long, thin return spring, quite unlike those in the standard carbs. I imagine it would make the throttle a bit heavy by the time I squeeze that in there - it's as long as the whole carb.

I would suggest that the juddering could be down to to the slide spring not being fitted,it is there for a reason!

You SHOULD be able to refit without taking the carb off,you may need to loosen and rotate the carb to do this,it has been a VERY long time since i worked on one of these!
If you are taking the slide out to put the spring in,there are no gaskets or`o`rings to worry about.
I would suggest you get a manual,if you haven`t got one,read it,and take your time.

Cheers

Yeah, that's what I'm less than pleased about - I bought the bike with a manual (even if it's a Haynes book of lies) because I wanted to do the work on the bike myself - now I haven't got a manual for the carb, one of the most important bits I need, and the first job turned out to be fixing the bloody thing. The better MZ manual won't help either. But many thanks for your pointers - it's helpful to know that the spring could be causing the judder. I guess it'll get sorted - or I might replace it with the correct one sometime.

[breakwellmz]

Hi

That carb does seem like it belongs there,are you sure that it is non standard?
If no one here can tell you try the UK MZ riders-

http://www.mzridersclub.com/

These things are not complicated!

Throttle stop screw,and low speed mixture screw you can see,other adjustment is needle position within the slide.Assume your float height is correct as it`s unlikely to have been altered really.

That spring looks awfully small(Diameter)for a slide spring!

Cheers

JUST came across this-

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

[Skorpion]

You may be better buying a genuine carb as you are on a hit & miss session.

Below is a link to a seller offering an ETZ Carb, but you will also need it's inlet manifold, as your manifold is for the Bing carb.

You never know the seller may have both as he is breaking a 300 engine.

Check out,

ETZ 250 parts.

BVF 30N 3-1
http://www.mzridersclub.com/html/for_sale_-_wanted.html

[treehouse]

Cheers guys, sorry I'm being a big wuss, great info. I've not had time to roll my sleeves up and find out what's in there, so I've got those fleabay jitters about it. But I was getting an MOT on my CBR today and discovered that my local bike shop owner is well into MZs and used to sell them, so I'm pretty sure he'll help me out if I get stuck.

UK MZ riders - right - I haven't quite got my head round the different sites yet.

"These things are not complicated!" -

[cmtstu]

HI

i am in the process of putting my 251 saxon tour back on the road at the moment and ive had all kinds of problems with the carb, The carb you pictured there looks to be correct there to me, it should be a bing type 84 carb. the slide spring you have pictured is completeley wrong. it should be a much wider shorter spring wich wouldnt be as hard to twist the throttle, bing are still manufacturing and you can buy any spares right up to a full new carb for your bike.

The two adjusting screws on the side:
the larger one with the spring is the air by pass screw and the smaller one is the air idle screw.

[treehouse]

Thanks cmtstu, I have wondered which screw is which, so that helps. But it's not a Bing, it's a Ken Tse. That's one of the problems - the manuals aren't much good when it's a non-standard carb. On the other hand, it may be perfectly fine, even better, I just don't know yet, as I haven't had time or inclination to do anything with the bike since I got it home, including riding it. Might need to get it into the garage.