MZ 250 causing no end of greif

[Blurredman]

Hello there!

New member, and also first owner of an MZ, and indeed a two stroke, should I really have started with an MZ?


I bought a D reg MZ ETZ 250 in October last year. It's caused me no end of greif since I've had it for various issues. Along with very poor MPG figures as low as 37 (due to carburettor slide catching), and having to buy a new clutch plate set after putting GL5 grade transmission oil in it, I finally had it back together after a few weeks of waiting on MZ-B for parts like a new silencer and the clutch unit.

Now, I must say, since I purchased the bike, I have always had it on pre-mix, due to the oil pump cable being frayed and not working properly, so I decided to play it safe and since then never really got around to using the pump.

It ran fine, after realising it needed a 7 grade plug in (instead of an 8 grade) on a particularly cold day after it refused to start.

Jump ahead to this moment, It was running fine on pre-mix, until I decided that it was the perfect time to set up and use the pump instead, bad decision, I have been having trouble since, so much to the extent that I extracted the oil from the oil resevoire and put a ratio amount into the tank and adjuster the carburettor to what it was before I changed it after putting it on the pump.

I found, that i'd be experiencing the power surge of an MZ, lower than where i'd have previously been getting, it was if the engine couldn't exceed more than 4,000 rpm, and when I did, the bike jolted and didn't feel right at all.

However, it's still being a right royal pain in the arse! It can start cold, oddly enough with just one kick, and it will idle for a minute or two, but then, even when in idle or otherwise it will simply cut out, I truely expect due to carburation mixture.

I have taken the carburettor appart (more than a couple of times) to inspect the jets, and clean every crevace of the device, to find the same issue occurs. So I doubt if it is anything inside the carburettor of issue (not like when I bought the bike to find that it didn't idle - pilot jet loose in bowl ).

Usually, the bike would start up again after cleaning the spark plug thoroughly, and when I take a look at the plug after it stalls, it doesn't seem any different to when I put it in, other than small droplets (of I presume fuel). It definately doesn't look fouled, but then it wouldn't at such a small running time, that doesn't rule out that it might be running rich.

My carburettor model is the BVF 30N-2, from a 1987 MZ ETZ 250 (not 251), and bearing in mind i'm currently using pre-mix what should the air/fuel mix screw setting be? and likewise the idle. The jet is a normal 70, and the needle is currently on the 3rd notch from top (according to the lower of the two supporting plates).

I've looked at what it might be in various books, and have adjusted it anything from 1 turn from in to 3, to no avail. I'm starting to want to turn back time. The choke plunger and rubber are fine, and intact.

[Skorpion]

Hi Blurredman

The carburetor setting are the same for both pre-mix or oil pump use, and you seem to have the settings about right, you don't mention checking the float height, this is the main cause of week or rich mixture.

Are you saying the engine will not run past 4000rpm without jerking (engine cutting out then firing again), you may have an electrical issue and not fueling.

Sorry I can't be of more help.

[Blurredman]

Float height is correct, main jet is normal 70, the haynes manual recommends 2-2.5 turns from in, I must try (again) when I get home tonight.

I cannot get the engine to fire for more than a minute or two, but when I could, the engine's peak performance seemed to be pre-mature to what it used to be, (I found myself quite happily riding in 5th gear at 30mph, which is obviously incorrect).

This suggests the mixture must be too rich. It was indeed jerking in a motion of backfire (but I heard none).

I will have another look at connections etc, I did replace the original fuse box "continental fuses", which often became corroded at the contacts resulting in a prompt loss of anything electrical, or indeed a total shaking loose of the fuses once . I have a much better spade type fuse system on it at the moment.

I did check the ignition cables leading from the points and all seemed fine there (all three black wires lead to the same place anyway right?)

When I start the bike, it seems to readily accept fuel, and seems quite happy idling, but after maybe a minute, whether it is idling or I am applying throttle it will just stall, it is not a gradual thing, it simply stops.

Air filter and air box are dry.

Thanks.

[Skorpion]

The needle jet is a 70, the main jet should be 130 although some have a 135, is this still fitted.

I found on my ETZ that the air screw was better out one turn, don't believe everything in the Haynes book of lies. carburetion alters through time due to wear, try to get hold of a genuine works manual, I think you may be able to get one from the MZ Riders Club.

http://www.mzridersclub.com/html/mz_regalia_shop.html

If when its running and you blip the throttle the rev's are slow to return to tick over, you need to screw in the air screw until it comes down normally.

[Blurredman]

Thanks for your help, I am unsure what the main jet is inside the carburettor, I presume it to be the original, and the fact that it has been running for neigh on 800 odd miles in my possetion with such a carburettor set up that it just can't be the jet.

However, after getting home from work tonight, I did manage to get it started, after many failed attempts at getting it to run (same problems as before), I decided to change the choke cable (had a slit on it), and replace that, as well as re-seating the rubber plug at the bottom of the choke slide, and just trying to get the damn thing started.

I found that the bike lasted longer while in reserve, and I kept the lever in that position. Oddly enough, after several minutes of idling I changed to the ON position and the bike continued to idle and rev quite freely, and did not stall!

I have my reservations about the fuel, which although is fresh, I just have an odd feeling about. As well as the choke operation.


I will take it for a ride tomorrow after work, and hope (fingers crossed) it will start, idle, and ride perfectly fine. And rest assured I will not mess about with the oil pump until the summer at least. I can't be having this annoyance when I wish to ride the stroker!


But, in light of your comment about the carburettor not needing to be adjusted after being put on the oil pump, this means thus that I should not change the idle nor air/fuel mixture?

[radiograf]

I've had simular problems in the past, to find that they where electrical, not the carburretor. I suggest you change the points for a new set as the spring will loose strength over the years, also check your battery. I had a simular problem with the engine cutting at a fixed rev range. Changed the battery, problem solved.

[Blurredman]

Well,

I went out on it yesterday for a while, it took a while to settle down. For a start, it took an age to get running and pull away (that could be down to the dragging clutch (new plates)), and once it did run on it's own, it misfired at the higher revs.

I then set out to:

-Clean/sand the points just to make sure they were fresh(ish) temporarily.
-Took the bowl off the bottom of the fuel tap an inspected the debree of (whatever-rust?) collected at the bottom of it, and cleared that.
-Cleaned the immediate surface that the brushes contact in the points area.
-Took the fuses out to inspect surfaces and cleaned.
-Charged the battery a little bit.


Eventually after a bit of a run it acted like the motorcycle It was a few days prior. I believe the air screw is at one and a half turns from in. The usual clogging in the lower range, the slight over-run at just 3.5k at exactly 30mph in 3rd on flat ground (using 16t front sprocket). And it didn't misfire over 4k.

This has been such a difficult and I think the most un-reliable bike i've owned.

[DAVID THOMPSON]

the spring in old points can give you a very bad running bike as they loose there spring
new old stock points can also be a problem
i have a whole box of r75/5 points that are not worth installing
they run good to about 3500 rpm then its like the carb is running on bad gas
dave

[Skorpion]

Blurredman.

Strange why are you are using a 16t front sprocket, 19t is standard for a solo, 15t for sidecar use,
Do you live in the Alps.

[Blurredman]

I found 19 tooth was way overgeared, and i was using only first and second gear around town, I couldn't comfortably use third gear at 30mph. But, with a 16 tooth sprocket, it was a perfect balance (3.5k depending on road gradient) for the speed, and indeed, 4th is also great for 40mph. I think I might buy a 17 tooth sprocket next time though.


Points will be on my list of things to buy, I am constantly buying replacement things for this bike it seems.

[mr_luke]

I might be stating the obvious, but I'd definitely change the condensor at the same time as the points if I were you. Another possibility is the coil. Sometimes when they're getting weak they they can cause a high revs misfire. I've not had a lot of success with original MZ coils...

How much rusty crud was in the water trap in the fuel tap? If there's enough sludge floating around in the tank it can start to restrict the flow through the tap, meaning demand can outstrip design as you start to speed up!

[Blurredman]

So i've ridden it for a good 80 miles since I've had issues, and it seems there isn't an issue anymore. I will put points and condensor on my list of things for service.
It hasn't misfired, it's started fairly easily, no issues at all currently it seems touch wood. There was a bit of rusty residue at the bottom of the bowl, the tank innards are somewhat rusty, but not the worst i've seen. It doesn't have a petrol filter in-line through the petrol pipe from tap to carburettor, It did have one when I bought the bike, but it was a larger car type filter and it just looked stupid, but it continued to work for neigh on hundreds of miles without.

A slight change of topic though, I've been wondering about iridium plugs. I wouldn't normally think about iridium plugs at all on a four stroke, but being as this is a two stroke motorcycle, and one of currently a pre-mixed nature, would an iridium sparking plug produce the claimed performance and efficiency increase?

Have any other two stroke users (esspecially of the TS250 varient) used an iridium spark plug? If so, how did it effect the bike? Would you have to use a lower spark grade to counteract the efficiency of a cleaner spark?

[Skorpion]

Hi Blurredman.

I use the Iridium plugs in all my bikes two and four stroke, never had one foul, 45,000 miles on an ETZ250.

Last year I tried Wurth two stroke engine cleaner additive in my 17,000 mile RD350, when I checked the plugs 1,000 miles later they were a mass of carbon deposits, never seen anything like it before, but the bike was running normally.

So yes give them a try there not that more expensive.

http://www.ebay.co.uk/sch/i.html?_from= ... ix&_sop=15

[radiograf]

I agree with using Iridium plugs, no problems whatsoever, same set in Emzie for 5 years with no need for adjustment, and no fouling.

[DAVID THOMPSON]

as a reference my cousin had a Yamaha 250 that he used same plugs in it as my bmw r50s this was 1976 to about 1980
and they worked real good my ym1 1966 ran good on KLG and LODGE but have not seen them in years
do compair cross referance charts befor using these in your MZ
http://www.bmbikes.co.uk/specpages/R50s.htm
http://www.bmbikes.co.uk/PDF%20Downloads/R50sSpec.pdf