New ES250 Trophy Project!

[Puffs]

OK, then it's 1 possible cause off the list - it's not the condenser.

Coil dated '77? Or maybe '72, the year of manufacture of the bike?

You keep mentioning 'backfire', which would suggest a fuel mixture igniting in the exhaust. This happens sometimes in 4T's, with a very hot exhaust, but I don't think it's very common for 2T's. I think there it might require a very much offset ignition timing. Is it possible there is something wrong in the cam for the contact breaker / points?

[Blurredman]

Yep, the coil is definitely 77, not 72. So replacement coil already it seems..

Backfire. This occurs after the stall by a split second. Actually, I have had coil failures in the past (on both my 251) and my brother's etz250 and both bikes exibited the same symptoms, including a backfire after a stall (depending on conditions (normally when you're stopped and you try to save the stall by opening the throttle but you fail)).

I remember once I bought a replacement coil (which was defective) that also was the same.

Anyway, the condenser is out of the question definitely because again i'm back to work on my ETZ250 with no problems. But as for replacement 6v coil, I should hope to pick one up on Sunday.

[Puffs]

Well, thinking a bit more about it, yes, it does seem possible that a break/rupture in the wire that makes up the coil's primary, could cause a spark at odd times. It could make or break contact, with the points closed, and that could cause a spark. If that happens with the exhaust port open, you get a nice backfire. Particularly as you just fed the system by opening the throttle.

[dave47]

I once had a problem which I was certain was electrical, having to rev the engine at traffic lights to keep from stalling.
It turned out to be a broken float.
Having said that, it seems more likely that your problem is with the coil or battery.

[Blurredman]

Dave: I've already installed a new float a few months ago, after I found the original was split.

Well, good news. After picking up replacement coil on Sunday from a friend at the MZ club meet local to me, I installed it last night and went for a quick 10 mile ride. There were absolutely no problems.

I've taken the bike to work today and all is happy. Idling well, with lights and all.

Now onto some different problems...

- For a start in a high gear, low rpm situation, pulling back the throttle still gives a sort of mis-fire type of feel. I think this one might actually be down to compression, as the bike will not surpass 65 (indicated) MPH. The engine is quite noticably quite 'slappy'.

- Charging, it still seems to vary between a max of 6.4v and 7.2v with the lights on so even on the lower end so long as i'm moving it seems just about to sustain itself (i've got this far), but I really aught to go through the wiring in the headlamp bowl to clean all electrical contacts. There may be a voltage loss at the contacts to the lamps themselves for example, but also potentially the switch itself.


Additionally, there is some good news however. The other night I took apart the horn again to see if I could get it to work, and disasembling it further, and cleaning all the contacts I could has got me a horn! Simple things really... It's not bad off the bike directly off the battery (and I suppose in an enclosed space). But it's pretty puny on the bike - once again, I could clean the contacts in the handlebar switch (and the junction to the main loom is via the ignition switch so see above).
EDIT: Added video of horn on bench if anyone so happened to be interested in the tone.
Additionally number two, the above coil fix means that I can swap back the condensers on the ETZ and ES after some more riding/testing, if I so wanted... I might.

[Puffs]

Success!!!

With 'pulling back the throttle' you mean giving full throttle? You feel it runs & pulls better at half throttle, or even less?
Two strokes (even cautiously tuned ones like the ES) do not really like low rev full throttle; you get blow-back into the intake. This is where reeds help. Without reeds, at low revs a 2T is often stronger at half throttle or even less. In long-stroke 2T's like the Jawa that is less of an issue (but that might also be because those are 180° twins with a shared intake - the flow continues), but these MZ's are square singles. My ETZ does it too.

Yes you may have piston slap & may need a rebore, but normally that doesn't impacts top speed much. At top speed/high rev there is little time for gasses to leak past the piston. And also, the rings should still seal & prevent major blow-by.

Fluctuating charge voltage: brushes (which I know you checked), V drop @ contacts, or maybe still the mechanical regulator...

OK, so the 6V horn takes a lot of current... V drop @ contacts? The one on my 6V Jawa is quite loud though, much comparable to the ETZ's original horn (12V); so it is possible to get a decent horn on 6V.

Edit: FWIW, I have a decibel measuring app on my phone, and I measured 85dB for the 6V Jawa horn, and 80dB for the 12V ETZ251 horn. Of course it depends on the distance too, and that app is not necessarily very reliable.

[Blurredman]

Yes, when pulling back the throttle that is indeed what I mean- by opening it. It does indeed run better at half throttle, or to change down. I don't get this with my other MZ's.... hmm. Would timing do this? 3mm BTDC is set for spark, and unless it is out extremely I doubt i'd feel much difference between a slightly incorrect vs correct setting anyway.

Maybe the top speed thing is related to the throttle thing though... perhaps..


I fully intend to go over all the contacts on the front end at some point! At the minute, all that has been done is the middle and rear section, IE regulator, stator, fuse box, coil, battery and rear light unit and brake light switch - the terminals on the items, as well as the contacts on the wiring.

A decibel meter might be interesting for me to download yes. But also: Where did you test? Outside or in the garage? I imagine that make a difference, it certainly does to my ears.

[Puffs]

I do get lesser pull with my 251 at low revs & full throttle (a sign to shift down), but maybe it's not quite the same as on your ES. I tried it yesterday, and it does not give a 'sort of mis-fire type of feel'.

A little while back (on a geological time scale), the relationship between ignition advance timing & Compression Ratio was discussed:

When the fuel/air mixture is more compressed, its temperature is higher and the flame front moves quicker through it. The mixture ignites at the plug, when that sparks, and that flame moves through the mixture in the combustion chamber, heating it up and thereby greatly increasing it's pressure. You would want the maximum pressure to be reached at or shortly after the piston is at TDC, in order to maximise the engine's efficiency. At a low CR, it takes longer for the entire mixture to heat up, hence at a low CR you need more pre-ignition, or more ignition advance. Older engines had a lower CR because the fuels used at the time required that; at higher CR they would pre-ignite/knock/detonate & cause engine damage. Also, remember that the speed of the flame front in the combustion chamber is independent of RPM, but at high RPM the piston moves quicker (or maybe faster), for that reason many classic motorbikes had a manual advance system (while automatic systems are in place in some modern engines).

Yours has a rather low CR, would a little more advance help? Or reduce squish --> increase CR (at the risk of higher loads)?

Well, I tested the horns with the bikes parked inside the garage. And the 6V Jawa is closer to the (reflecting) walls, yes, that too will impact the measured SPL. Anyway, the point is: a horn can work well at 6V too, also on the bike.

[Blurredman]

I have a mirror!

First made out of aluminium bar in my brother's 1948 lathe to see if the concept would work, and it seemed to, and didn't look too bad. So then made it out of steel and then oil hardened. I don't think it looks too bad!

First picture is finished product. Second picture is of the original aluminium, but I put the picture up because it shows what the whole bike looks like.

In place of the pivot bolt for the brake lever, in lieu of finding anywhere else to easily and without modification of original parts, to fit a mirror. The mirror itself is an original ETZ one. I originally meant to have the bar itself about half the height that it now actually is, but I didn't have a full depth m10 tap, only a tapered one so I had to make the length an inch long. I intended to later on buy a full depth tap and cut the threads fully, and then chop the rod by half an inch, but I'm seeing how the positioning is for now. It looks a bit funny, but not too awkward. But it is certainly nice to see the smoke behind me! And dangers of course...

The pivot where the brake lever itself rides on is of course not tapped, but the bottom half of the clamp is threaded, and on the end of that we have a lock nut as well. So this adapter doesn't make any adaptation to items to make them non-standard. I like it.

[Puffs]

The pivot part is probably 6mm, with M6 at the bottom, in toughened steel that should be strong enough for normal use. I have quite a bit of experience in riding without a mirror, but really, in today's traffic a mirror is indispensable. You must have eyes everywhere, hang the looks. Congrats on finding & implementing this elegant solution!

[Blurredman]

Another quick update. I finally, FINALLY, 2,000 miles after the mileage in which I wanted to actually install them, put on the new chain gaiters. They don't look half bad. The old ones are broken and frayed at the rear (where they surround the rear sprocket cover), but were usable enough until now - maybe I should have taken a picture - but alas you couldn't really say that they looked very good or even may not actually be even doing their job as they once had done.

One of the worst things about MZ's is messing around with the individual tubes for the chain. They're great- but also annoying, fiddly and very messy! Easiest way is to fish a rod through the engine entrance of the gaiter, and tie something (in my case fishing line) around the end of the rod and the end link of the chain and thread it all through. I actually used the brake actuating rod from my ETZ250, what with it's handy hook on the end. I hope these gaiters last, as there is a lot of disapointment with the quality of aftermarket chain gaiters and that they break through (or crack and then break through quickly), in just a few hundred/few thousand miles. I have been assured by the person I bought these from last August (?) that these are the best quality aftermarket gaiters so they should be good! And I hope so too!

I put in a good half a (small) tube of grease in each gaiter before threading each end of the chain through so hopefully that staves off the reason why the cheaper ones break early. I think they look smart. Also greased the speedo drive (via the nipple) whilst I was messing with the grease tube and gun. Don't think I've done that for 3,000 miles.

[Puffs]

Yes they do look smart. Lets hope they last.

So that triangle gizmo over the sprocket is your speedo drive. And I hear you did at least 3k miles with the bike already!

[Blurredman]

Old ones for comparison.

[Blurredman]

Well, after acquiring a rack, I spent a good 10 hours modifying it to fit.

In the end, I don't actually like it. The hinge on the seat on one side got in the way, as did the locking system for the seat on the other side (which I didn't actually manage to align properly). It slopes backwards too but it should be good enough for now I think but I'll re-design it in the future. It has pretty poor welding, and indeed I haven't even bothered to paint the thing because I know I'm not happy with it and will likely chop it up again.

I should have gone with my original idea of copying the way the original rack mounts to the bike, using long tubes that slot straight into the frame for about a foot.
As it is, I decided to go down the route of making up the rack with spare scrap, including using old handlebars to reconnect the rear sections where I had to cut through them to widen the rack as a whole.

As you can see on the third photo, the bit of tube acts as what I thought might be a handy handle, but more so to re-inforce the support section. Not sure how that will hold up in the long term. But as a handle it's pretty useless- easier instead is to just use the lower rear of the original rack section itself.












I've just put some cardboard under the box in order to reduce the vibration noise as it isn't held down the most securely as I didn't bother flattening the extension bars.

[Blurredman]

Spent some more time to-day in order to check out the speedometer.

For about 1.5k miles now it had been running about 10mph too fast. It used to seem fairly accurate within 5mph in the first 1,800 miles of running, but it got worse suddenly and it had stayed the same running too fast for all that 1.5k, until a few hundred miles ago the speedometer needle was vibrating all over the place by up to 10 mph variation in needle swing, but what's more it was making an annoying buzzing sound and also sometimes the needle was jumping and staying at 85mph, which was the needle tip hitting the bottom of the speedo.

I took it apart to-day and there was nothing broken, but there were a lot of metal fragments stuck to the magnet inside. I cleaned up old grease of the magnet riding bearing and re-oiled, removed the metal from the magnet and cleaned the cup at the top side.

What I really liked about the design is that I didn't need to remove the needle and speedo face in order to get the unit apart, which I like because I hate trying to recalculate the speed by pushing the needle in the right position.

Put it back together and did a quick test in the garage with the bike in gear and the whirring/buzzing sound and and the jumpy needle is gone. I haven't yet gone for a proper test ride but it's good progress thus far so can't wait to check that out. Only problem is that I manged to break two of the 6v 1.2w bulbs probably when I initially tested before assembling the speedometer properly. No problem, they only are the background illumination- but I don't have any spare 6v bulbs so it'll have to stay as it is for now. Unfortunate but there you go. I have spare 12v bulbs but they would probably be so dim they'd not be worth even putting in.

Picture of the magnet (I did remove the metal surround for cleaning) on the one half and on the other half the cup which then spins the needle.


I've decided not to crimp over the speedo glass surround incase it needs to come apart yet again. This is as you may know from reading the thread the 3rd time I've opened up the speedometer, the first time around was fixing the the main mile drum counter, and the same on the 2nd too. This means I've now (hopefully) rejuivinated both main facilities of the speedometer.