uncontrollable revs - etz 251

[Diligaffer]

I posted this thread on 'Mechanical talk' too. My recently bought 251 revs uncontrollably past 6K. Bike starts easily - often after first kick but as soon as I disengage the choke revs go out of control.
Tried to change the carb settings when bike is running but it doesn't do anything at all (I just got a brand new carb - original BING 84 from Ost2rad.de, new intake hose, air filter appears to be fine and battery is fully charged).
Engaging choke around halfway makes rev go down and then she idles around 1K or slightly below that and ticks over nicely. I tried opening the throttle to see what happens but it just makes induction noise for few moments and revs very very slowly climb up.
Spark plug colour is black - so it runs rich...no surprise here thou as it was running on choke. I just don't want it to keep running around 6k as it sounds like its gonna disintegrate.
Any ideas what might be the problem?

[Cerberus73]

oh i also see you are a local fellow as well, and we have similar issues with our bikes.. now that is funny.. if it wasn't such an annoying issue. if it helps i have copies of the manuals for your bike.. and mines (early etz 250) if you would like a copy of the PDF just ask.

[Tubaman]

Just a guess, but are the jet sizes all correct? The diameters on mine are stamped into the jet - I had to replace one of mine as it was too large.

Also if you adjust the pilot and air bypass screws while it's running does it not make any difference?

[Diligaffer]

Thanks guys, if I get some time away from work then I'm going to check carb jetting and look out for any air bypassing filter.

[Diligaffer]

Adjusting both screws on bing while running doesn't make any difference.
I also noticed it makes occasional metallic 'knocking' noise while revving high.

[Cerberus73]

sounds like it is deffo sucking air from somewhere then.. mines did that with the new carb.. tick over was fine with my old one except it was knackered at anything over 4k rpm.. gutless, misfiring, and conking out. mines is fitted with a BVF 30N3/1 carb.. the one i returned was a 30N2 type.. the replacement is another 30N3

Today i had the clutch cover off and noticed the kickstart spring was buggered.. so ordered a new one, and will fit it tomorrow or wed when it arrives, then fit the new carb. hopefully it will run ok then.. and i can finally really enjoy using this bike. if i can offer any help id be glad to. a simple way to test if bikes sucking air is use some soapy water around the air filter rubber and the carb manifold.. theres usually a bubbly "tell" if its drawing air. or simply listen around carb and a hissing sound can be heard.. but our bikes are a bit noisy so harder to tell this way.

[Tubaman]

Yeah, if the jet sizes are ok this does like too much air being sucked through the carb to give such high revs. As the adjustment screws are making no difference, the obvious path for the air is straight past the main throttle valve - is this definately closed? Perhaps the throttle cable is holding it open and needs slackening or it's getting stuck. On mine (30N3) you can hear a metallic click as the throlle is released and it springs back down to touch the bottom of the carb chamber.

If not, one thought you could try is disconnect the air intake rubber and (making sure everything's clean to avoid sucking any bits of clag into the engine) use your hand against the carb air intake to restrict the flow. If you block the smaller holes at the bottom and thre's no change then this I think suggests the pilot/bypass is blocked. If instead blocking the main hole slows things down, sounds like the throttle valve letting too much air through - if you undo the carb from the cyclinder you should be able to see inside whle adjusting the throttle to make sure it's opening/closing fully.

Should say above is just a guess based on my limited understanding fronm taking the carb apart to sort mine, but hope it helps track down the problem!

[Diligaffer]

I know for sure that throttle slide is moving smoothly inside the carb.
Gonna try taking hose off like you suggested and cover air intake side on the carb to see if it changes anything.

More worrying is that occasional metallic 'knock' sound at very high revs- it seems like source of it is inside the engine.
Sounds like piston is smashing into spark plug or cylinder wall, or maybe one of the bearings went.
Its a weird kind of sound - not typical ping, more like hammering block of lead - muffled but still metallic.

[Cerberus73]

sounds like piston slap.. or pre-detonation.. has the bike got high mileage? was it looked after (from best of your knowledge) by the previous owner? is the oil pump adjusted correctly? good quality 2 stroke oil?

If the barrel is all carboned up, it could cause the fuel to detonate early when the bike is hot.. as the carbon can literally glow red and like a glow plug in a diesel engine.. will ignite the fuel to early. either that or the piston rings are worn, and it is causing piston slap.. a compression test would tell you at least a decent indication if the piston is worn or not, without having to lift the head and barrel. no worries a piston on a MZ aint expensive.. falconers out in blackburn can do a rebore.. never used them for a MZ but they did my Kawasaki KH 2 stroke.

[MichaelTorre]

to find this source of noise is real real easy......... take a set of cheap mechanic
stethoscope and pull off the probe and the diaphragm so you have an
open hose to your ear.

Now you have a unidirectional microphone ......... and if pointed at the sound it
will tell you definitely the location of the sound.

If the stethoscope its too pricey a length of small hose close to your ear canal and pointed at
suspected sound (like the air leak) will show up right away it will be amazing how you can
pinpoint a hissing air leak even if engine is running.

The drawback of the hose only, is you will not have hands free, but need a
mate to goose the engine or, " cause the noise".

[Diligaffer]

Phoned Martin at Burwins, from what he said it might be carb jetting/setup or air leak around intake area like you guys predicted....or worse - crank seals being damaged (weird knocking noise?) which would also cause an air leak in process.

[Cerberus73]

Funny you should say that.. i got the carb onto my bike, dialled in as per factory settings, bike starts, but i have a cloud of white smoke which settles down a bit, then appears again whenever you give it some throttle.. the crank seals are shot.. so bugger it, I'm just gonna haul the damn engine out and rebuild it when i can be arsed.. never has a damn bike in so good a cosmetic condition caused me as much grief as this damn ETZ.. i traded a Honda C90 for this thing.. the C90 started first kick every damn time.. and ran sweet as a nut.. this thing pretty much from day 1 has broken down.. i think once i get the motor fixed i will be sticking this MZ up for sale.. and look for another Honda of some sort.. yours truly… 1 sickened MZ ETZ 250 owner.

[DAVID THOMPSON]

if a bike is for sale the owner is tired of it
if its a truck or van that is for sale he wants it GONE..

[Diligaffer]

Even though Martin doesn't race tune MZs for Burnwins anymore he said to give him a shout and he might be able to help.
There's something nice about those MZs and once properly set-up they are good, fun bikes to ride.

[davegsm82]

I'd recognise those red rear springs anywhere, this is my old girl I'm Dave, the guy who trailed it up to you all the way from the northeast

Sorry to hear you're having trouble with her, I hope you manage to get it sorted in good time. The 'Clack' noise that you're hearing is pre-ignition on the overrun, quite common on the 2-stroke machines. They can sound horrendous popping and banging on the overrun, the usual solution is to have virtually zero idle so there's no fuel getting into the chamber, the only downside of this is the plug will oil up quite quickly.

I'm surprised you're having trouble with the bing carb though, they're very reliable, the one on my 301 is some 40k miles old and has only ever needed a new needle and jet housing.

I can't remember if it's possible with the Bing 84, but have you got the carb slide in the wrong way round? this has happened to me before, obviously the throttle is then stuck about 25% open!!!

Other things to check are the gaskets on the Stub-inlet to the engine and Carb. There's a round one between the Stub and the carb, check the carb is pushed right the way home then tighten the clamp. Also make sure the one between the Stub and the Cylinder barrel is positioned correctly, you can get it 4 different ways round and only one of them is correct! Sometimes it drops down and you bolt right through mid-air without noticing!

Finally, I mentioned at the time but it's worth saying again, the Electronic ignition is of my own design and although I'm confident in my abilities, I can't guarantee it. Go back to points and see how that goes.

I look forward to seeing how you get on.

Dave.

P.s. I've lost your address and still have the old spare BVF carb to send to you along with a couple of other bits. I haven't found the side panel transfers yet though.