TS 250/1 amal concentric

[DAVIDGUEST123]

My recently seized 250 is now back on the road and running ...sort of. The cold starting is a real pain but once warm runs fine, all be it with a little piston slap from the oversized rebore. I'm now looking into the carb. When I orginally bought the bike it came fitted with a Amal Concentric Mk2 carb and after a bit of digging the carbs mainjet had been drilled and float sticks. Another issue is the brass 'spray' is a four stroke one. I have tried the setting from the 'Sweller' carb web page but nothing seems to work.
I'm now a great beliver in standard is best, so I'm weighing up the options for a replacement carb. The options seem to be..
1) BVF Orginal ... If you know any about let me know
2) BVF non orginal copy cheap but are they any good?
3) Mukuni vm34 Seems to be difficult to get the rights jets without ordering fron the states
4) Change the spray tube in the amal...how's it done?

Any input greatfully taken on board

Thanks Dave

[Puffs]

Dave, good to hear the bike's back in business!

I understand that historically Amal have quite a good name, but that history would mainly be for 4T. I have never had one, 0 experience.

Jetting between 4T applications & 2T are different: a 4T sucks longer & harder, but less often, and a 2T may spit back more. Jetting for a 4T is often more towards the leaner side, as compared to a 2T.

Your points:
1) Original BVF: there are different types. Mechanically appears to be very resilient, IMO, but on my 251 I have a type with a separate idle channel (30 N 3-1), which is not my first preference.
2) Copy BVF: don't know, no experience. A copy is not necessarily bad though, it depends. There could be differences in alloys used & workmanship/precision of manufacturing, but also minor deviations of the original design, due to newer insights.
3) Mikuni is good (never tried on the MZ), but is it really a good idea to go for a 34mm carb on a historic bike like the TS 250 (with 19hp originally)? That would be quite a bit bigger than the standard 30 N 2-3, so the 34mm carb is 28% bigger in venturi x-section. Surely 250cc race bikes go up to 38mm (& beyond), but is it needed? A 30mm carb would be closer to the standard, might be easier to jet properly (due to the higher velocities in the venturi), and might give a better power curve for touring/road applications.
4) Is what you call 'spray tube' what I know as the 'needle guide', so the tube that sticks out into the venturi, in which the needle moves, and which has the main jet at the bottom? Surely those can be replaced, but how that's done in an Amal I don't know. In some carbs they are screwed in, in others pressed (then temperature might help). Sorry, no more help there. But it sounds like an interesting way to go, and that carb does look good. Needle & guide need to be matched, and possibly you may get the same result by using a different needle.

[DAVIDGUEST123]

Hi Puffs. You really are the font of all mz knowledge.
I've decided to have another go at the Amal. This is what 'Sweller' recommends

Amal Concentric Mk II - Type L2 - 903 - 303 Throttle Slide Part 2928 No. 3 (28mm carb?)
Needle Type 2D1 - middle groove
Needle Jet 105
Main Jet 210
Float Height Factory Set (nylon type float)

When I bought the bike it came with a hole box of various Amal concentric bits and bobs and a complete
spare carb(2930 30mm I think). I have however stopped using any of these spares as I've decovered that a few of
the jets are only for four stokes, the others have been butchered by them being drilled through. So I'm starting
from scratch and buying in the above jets. The carbs that's on the bike, after a dig around the net is a 2032-303
whichis slightly bigger than above, some I'm guessing I'll need slightly bigger jets.

I believe I'm travelling down the same route as a prevoius owner. If I can't get to the bottom of it by
Chrsitmas then the only option will be to get Father Christmas to buy me a new Mukuni/Bvf/OKO.

I will not be beaten.

Once again many thanks for the help.... Dave

[Puffs]

Ta, but no, not really.

Amal seems to be a high quality product, and if I had the stock you have I would definitely try to use that.

I dug a little bit deeper into their website, and I find their naming bloody confusing. Wrt naming of their Mark 1, Mark1 1/2 & Mark 2 Concentrics, they say:
Carburetter type number, followed by internal bore diameter, followed by the following specification number, according to adjuster screw handling and fuel type e.g.
928/300 is a Mark 1 900 series, with a 28mm bore, right hand adjustment, for a 4 stroke petrol engine.
1930/301 is a Mark 1 1/2 series with a 30mm bore, left hand adjustment, for a 4 stroke petrol with flange mounting.
2932/302 is a Mark 2 series with a 32mm bore, right hand adjustment, 2 stoke petrol with spigot mounting.
2934/303 is a Mark 2 series with a 34mm bore, left hand adjustment, 2 stoke petrol with spigot mounting.
2036/304 is a Mark 2 series with a 36mm bore, right hand adjustment, 4 stoke alcohol fuelled with spigot mounting.

From the pics, your carb appears to be a MK II carb, either 2000 or otherwise. See for instance http://amalcarb.co.uk/generic-2000-series-mk11.html, and looking at the component list, apparently what was called 'spray tube' or 'needle guide' in my previous post, in Amal speak that's called: 'Needle Jet', item #15. It is screwed in #16, the Jet Holder. A whole variety of needle jets can be bought, for alcohol, 2T & 4T (which might be different in length). For the needle, #6, there will be a difference both in OD & in taper, depending on application.

The confusing Amal naming would suggest Sweller recommends a 2928/303, a Mark 2 series with a 28mm bore, left hand adjustment, 2 stoke petrol with spigot mounting. In the pics, I do not see numbers on your carb, but you can easily measure the venturi size with a calliper, and it does seem to be a Mark 2 series. But watch out: the jetting very much depends on the venturi size. A smaller venturi will cause higher flow velocities, necessitating smaller jets to make the same concentration fuel/air mixture (though less of it, in terms of volume). So before you order, check the venturi size.

The other thing is: why would Sweller recommend a carb smaller than the standard? The standard is 30mm, and you have that 2930, so that would be another possibility (presuming it fits in your manifold), and actually be my preference as it has the same venturi size as the original.

But indeed: jetting for that 30mm carb should be bigger than in the Sweller recommendation for the 28mm, and for the 32mm one you have now (?) it should be yet again richer. But I do not know by how much; maybe Sweller could advise for the 30mm carb?

[DAVIDGUEST123]

Thanks again Puffs.
Well after a day of fiddling I found the answer. It may be of interest to anyone else think of going down the same route. The carb I have installed is
a 2930-303 with a Norton type Needle 'Spray tube. I managed to dig out from the bits that came with the bike a small plastic container with variuos jets. On dismantling the
carb the main jet and the needle jet were more or less correct 220/ 160 this is where the fun starts. the difference between the two stroke version and the four are.
Spray tube
Hole in in the side of the needle jet.( from top to bottom it's..Spray tube thingy. Needle jet. jet holder then main jet)
Main jet holder collar
And finally Two positions for the idle jet to sit.
This is where the problem was with my carb. The previous owner had installed two pilot jets one in the main body and another in the bowl. The reason for the wto position
is so that if neede the jet can be placed nearer the venturi exit which would be required for a two stroke.
So the only part which is incorrect now is the 'Spray thingy' and see that the bike now starts on the first kick and appears to run ok I may well leave alone.
When the weather improve here I'll give it a decent run and fine tune it.
Thanks Again Dave

[Puffs]

Good you found it!

Some more comments, questions & things I don't understand:
- The 2930 seems like the right carb for this bike. Following the Amal naming, the 303 has "left hand adjustment" (and the 302 RH), would you know what this "left hand adjustment" means?
- Following the Sweller recommendation of a 210 MJ in a 28mm carb, and presuming the number is proportional to the size, I'd expect a 225 in a 30mm carb. So 220 is indeed not far off, but might be a bit on the lean side. However, looking at the Amal site, those jet numbers do not seem proportional to size, and 225 does not exist... Anyway, regardless of all that, have a close look at your plug. Right after a good full speed blast, weather permitting, and also how the take-up is, near full throttle.
- Other manufacturers indicate their jet sizes in hundredths of mm, for instance my 250cc ETZ has standard a 130 main jet (I run a 133), indicating 1.30mm. Looking at the jet sizes on their site, Amal does not seem to do something as simple & understandable as that.
- Needle jet = needle guide = spray tube = 160 does not seem to exist, maybe it is 106? Anyway, here the number means something quite different, as the hole in this item should be bigger than in the main jet below it.
- I see, yes - ta. Indeed, on the Amal site the 4T needle jets seem to have a small hole, which the 2T ones don't show. Don't quite know what that would do, but it might make the mixture slightly leaner? Odd. It would seem to upset the venturi effect.
- Yes, I now see they indeed indicate 2 pilot jets, one placed at the air filter side (#50), and one on the side of the barrel (#13). On the website they don't give any guidance, but it makes sense to only install 1, I guess, and they also provide a 'jet blanking screw' (#14).
- On the jet holder (#16): they only indicate a single model on their site, only the one on the right, with the collar, which you suggest is for 4T. Could someone have filed off the collar? I don't see what the collar would do anyway.

Good luck with tuning your carb!

[DAVID THOMPSON]

i find carbs and hydraulic to be a bit of a problem they are both a mystery and a help
and may GOD help you if you use the wrong part or turn the adjustments the wrong way
and i have been helping a friend with his mess in a car ...things have not improved
there is now a computer involved
Dave

[DAVIDGUEST123]

Hi. Here some answers, or at least what I found out

Amal mk L2932-303 Two stoke carb left handed which means the adjustment screws are on the left.Makes life easier for something like a twin to have left and right adjustment 'Sides'
Swellers Amal mk2 carb number don't make sense.

#13 Pilot jet 25 in the front position ie near the barrel for a two stroke. Apparently two stroke don't suck as sharply as four stroke
#50 Small blocking screw shuts the other hole off. One and a quarter turns out on the air screw.

#17 Main jet is a 220. the number is not a physical hole size but a flow rate.ml/minute or something like that. Might try a 225 if it wasn't so bloody cold out.

# 7Slide is a two stroke 2928

#6 Needle a 2d1 matched with above.

#15Nozzle jet . My mistake not a 160 but a 106. Again this is flow rate but with the needle in.

#16The main jet holder without the collar. They are shown on the website and is sometime to do with how deep the main jet is in the float bowl

#2 And last but not least the 'Spray nozzle thingymebob' This surround the nozzle jet and is there to spread the fuel. Two strokes have a relativly slow suck as compared to a four stoke
There are three type One slopeing cut two stroke another flat as per#2 and one with a stepped cut I think this the one I have and can't see it's going to make that much difference.

p.s #31 is for an alcohol 4t

Any how with the settings and the carb above bike starts first kick and ticks over at 1500 rev without problem.

Hopefully this is of use to anyone

[Puffs]

Yes, great, ta for clarifying Dave!

OK, so Amal's numbers, on jets etc, refer to flow rates. In a way that makes sense for a carb manufacturer. I don't doubt you & it's not up to you to clarify, but in my mind that raises questions like 'at what ΔP' & 'at which viscosity'. Never mind.

Indeed, a 2T doesn't suck very sharply, not compared to a 4T. The reason is that there is always quite a bit of 'dead' volume in the crankcase, below the piston. With the piston at BDC, there is the volume of the inside of the piston, the volume between the piston base & the crankshaft, and the volumes in & around the crankshaft. Altogether that can be quite a bit, in the order of the stroke volume. Consequently when the piston moves upwards, the change in pressure (ΔP) isn't so much & doesn't rise so sharply. In a 4T by contrast everything happens above the piston, and the dead volume is very small (+ a tuned exhaust sucks).

Btw, when tuning elderly 2T engines for speed, a first approach is to minimise this 'dead' volume, for instance by filling-up space in the crankshaft. In an MZ there is little to be gained there, at least in the ETZ.

It was -4°C here last night. While my old bones don't like that, another reason to wait is that jetting also depends on temperature.

[DAVIDGUEST123]

Hi Puffs,
Yep that's me done for a while. It starts really well and quick run up the road all seems well, any further and my old bones won't take it....done my winter riding a good few years ago.
The piston rattle still bothers me a bit and as the orginal barrel has a corner missing from one of the fins ,I'm gonna keep an eye out over the winter months for a good un and rebore that on
to the correct size. Now if anyone one knows of such and item........
Many thanks Dave

[Puffs]

Well, I too have a rattling 250. Like yours it was re-bored to 0.08mm (3 thou) clearance, and the first rattles occurred some 2 miles after starting it up for the first time (after the assembly lube cleared). I am still in a struggle with the guy who did it, as I asked him explicitly and very clearly: 0.05mm clearance, maximum 0.06mm. Problem is: there is no way back. While in the standard bore (69mm) there are various sizes of pistons, in oversize there are not. All proper (K20) pistons are 0.05mm under the oversize (at least for the 250cc ETZ), like in my case 70.95mm (4th re-bore = last normal re-bore). You might be tempted to go the way of trying lower spec alloy pistons (recognised by their cheaper price, and by being marked with the bore size rather than the piston size, so '70.0' in my case), but you'll find those are actually smaller in OD (to cater for more thermal expansion), and rattle even more.

MZ specify a maximum of 0.10mm clearance, but you can probably go above that, to at least 0.12mm. But too much clearance can cause the piston to crack, presumably with quit dramatic consequences. For now I'm just enduring it & lubing it a bit richer, which might reduce the slap a bit.

As you're missing part of a fin, on eBay Germany you might run into a suitable 2nd hand barrel.

[Blurredman]

I'm glad you are getting the performance out of your bike as you hoped..!

[DAVIDGUEST123]

Hi again,
Test run done, inbetween the rain showers. All seems to be good, probabally should go to 225 on the main as the plug is a more milk chocolate than dark choc, but for now it's ok.
Yep the rebore is a bit of a pain, kind of blame my self for letting the chap decide what was the right gap. Saying that, the rattle sound bad when it's ticking over and on the overun but out on
the road under load it sounds ok. just need to keep an eye out for a barrel then get it done correctly to spec. I've had a quick look on ebay Germany but they're all a little salty. There's one listing for
a barrel with half the top fin missing and the chap want 220 euro for it..suppose there's no harm in him trying.

[Puffs]

Yes, and why do you think he wants to sell that barrel?

Because now he has a better one. Anyway, there's a market there.

I don't think they list a 225. The next one up might be 230, or have a funny name. Anyway, if it runs well, & takes-up well on the throttle, also near full throttle, but the plug is a bit light now, it's probably OK. Certainly for summer time. Maybe raise the needle 1 notch.

Air, like all gasses, shrinks when it cools down (and expands when it heats up), and consequently cold air has a higher density & contains more grammes O2 per litre. Though differences are normally small, cold air requires a bigger MJ, and warm air a smaller one. Similarly, higher altitude requires a (slightly) smaller jet, and sea level a bigger one. Computer-controlled injection, as modern engines have, corrects for a.o. these things, and that's why those engines normally run so well.

[DAVIDGUEST123]

https://www.ebay.co.uk/itm/MZ-250-2-196 ... Sw~ERd2rK9

Enough said