Timing a modified bike

[Puffs]

As promised, I've taken my barrel off & pencil-printed it. The picture below should be properly scaled, lengths are mm measured from the top of the barrel. Note that the standard 251 barrel has a 2.0mm high rim there, with an OD = 73.0mm, which falls inside the head. So it's measured from the top of that rim, and if you drilled the barrel up to a 76.00mm bore, that rim is gone & you need to subtract 2.0mm from the numbers. Also note that at TDC the edge of my piston is about 0.8mm below the top of that rim.

Port map ETZ251 @ 70.50mm.jpg (32.15 KiB) Viewed 239 times

These numbers match quite well (+/-0.5mm) with other maps I have seen, of both ETZ250 & ETZ251.

Widths you can measure yourself, but realise that what you're seeing is a piece of paper rolled inside the bore, so the actual width of, for instance, the exhaust port is less than the length of the arc you see on the paper.

I've also had a look at how the ports go inward from the surface of the bore; if they do that at an angle, the height will change if you increase the bore. But actually only the bottom of the exhaust port is at quite an angle, the rest is largely perpendicular to the surface of the bore. So you don't need the 2mm base gasket to correct port timings, in fact you disrupt all the timings of port opening/closing by that 2mm (for ° try http://www.race-base.com/2t-tools/steuerzeit/). But you do have the issue of preventing the piston hitting the head (as it can not enter the head, without machining that).

On the backwards running: it does so because your ignition allows it to. When it kicks back (due to too high pressure, or possibly the port timing issue) & rotates in reverse direction, the points close at 22° after TDC, and bouncing causes another spark. Way too late, so it runs like sh*t, but still good enough to ruin your rev cable. Maybe try the electronic breaker/ignition you have on the other bike?

[Blurredman]

Thanks very much for that Puffs. Quite insightful.. One quesiton.. Why didn't you join the forum years ago...

The head is machined to accomodate the fact that the piston has the possibility of interfereing with it. Though this is removed with the thicker headgaskets required.




I had a mess around tonight with the machine. Whatever I did, whether it was 5 degrees btdc or otherwise. It would still start backwards at least half the time.


I have forgotten completely, have I changed the points!? I don't know.. They could provide the 'bounce' that is giving a spark. At this point we are inferring that the ignition is at fault here, for whatever reason.


You're right, I could try the electronic ignition. Obviously that will not provide a spark for the backwards running. I did however notice tonight that the lobe is quite scored, lengthways to which the points would rest. I don't know how they got so damaged so, I will try a new set of points and a spare lobe I have before I go into replacing the points with electronic, if needed.


Thanks.

[Puffs]

I don't particularly care much for Donald Rumsfeld, but one thing he said is very true: you don't know what you don't know. I just didn't know of this forum. Anyway, just as a summary & for completeness the following:

Your backward running issue has to be ignition: the engine stops running when you switch off the ignition.

But how exactly that works is a bit of an enigma, as an ignition with a battery + coil + points/contact breakers only gives a spark when the points open. Not when they close, not when they remain open, and not when they remain closed. When the engine runs backwards, the points open +/- at BDC, and they stay open till a bit (21° - 22°) after TDC, and then they close. So you should only get a proper spark somewhere around BDC, and that can not support the engine running.

Nevertheless, yours is not the only one that runs backwards. I mentioned my brother's '67(?) Husky, and also in other MZ's it is a relatively common phenomenon. In the paper by the German MZ guru I sent you earlier, the explanation he gives is that shortly after the points close, the contact/continuity is broken again because of either bouncing, or rugosity/scoring of the cam/lobe, or maybe because of some springing in the points follower when running in the wrong direction. When that happens, you do get a spark, typically 22° after TDC. So it runs lousy, but it does run.

It would start running backwards because of high compression pressure, helped by early ignition: a kick back. But you should not change the standard 21°-22° preignition, because that's what it needs to run well in the normal direction. Mine also kicks back sometimes, if I don't kick it hard enough, even with the lousy compression it had. But it has never run backwards, apparently my cam is too smooth for that, or my points don't bounce, at any rate, I do not get a useful spark running backwards.

So if it runs backwards: polish your cam = lobe, lube the follower, try other points, or try an electronic ignition.

[Blurredman]

Thanks puff for the condensed info.


Well, I tested a spare lobe and points. And at first the bike ran better. But still, at idle, it mis-fires like it wants to run backwards. It didn't do so whilst It was running, but did run backwards on an unsufficient kick-start kick.


I've kinda given up. I'm not even going to be putting electronic ignition on it, because for me- Putting on the original 250 barrel and piston is less effort as it stands, as odd as that sounds.


And in honesty, I'm not really sure I was quite happy with the power delivery. It seemed quite low end, as you say puffs, and although would quite happily get to the higher RPM, it was not lightening. Disapointed.

I have read a lot regarding overboring and timing issues/engine expansion in Hungary, a very supportive populace of the MZ. No one can get it right there, either..

[Puffs]

No, that doesn't sound odd. Replacing a barrel maybe takes an hour, and is easy, while hassling with the ignition (on 2 bikes) will take a lot longer - and risk both bikes not working. And I understand the gain of the extra 40cc is not all that big: just 23hp vs 21hp.

On power delivery: if you drove it with the extra 2mm base gasket + 1.25mm head gasket, I don't think you saw it's full potential. Surely an original ETZ301 can run well, but indeed those are not bored-out 250 barrels.

FWIW, I needed a rebore, and I considered going bigger than the standard 0.50 up, but decided against it. Not worth the hassle or the money. I also considered buying a complete aftermarket 300 barrel (which are quite cheap, typically just €80 over the price of a rebore + Almot piston), but for the same reason I didn't bother. If you want a quick bike, better ride something else. Your Honda will be faster.

That said, the other day I noticed my old 251 was clearly faster than a Bullet.

[Blurredman]

Tonight I put the original 250 barrel and piston that came off the bike on the engine..

I made careful measurements of each item and found them within spec (regarding gasket thickness etc).

At first ran fine.. Ran pretty well.

But my usual test route of only 1 mile, soon I found that it was mis-firing on the idle.

I'm very confused and disheartened. It was a very unsuccessful test as I had to push the bike back to the house. When I got back it would start but idle and then cut out every half second.. Like it was counting a second. Stalling, then another second, then stall. Repeat..

I tried 3x spark plugs to rule those out. The cylinder was quite warm in my opinion. But.. It is air cooled...




It's a possibility that this replacement crank has the timing cam seat off. I am pretty sure I compared this (I have a memory back there somewhere), however between the old crank and the replacement, before I installed it.

Spark plug colour is nice tan brown. It doesn't sound like metal on metal, so I don't 'THINK' the piston is expanding and seizing. Either way the piston has 30k and you can definately hear the piston slap in it so I don't think that is the cause.

Don't be fooled however. This above problem has only recently reared it's head. Infact... Before I put the motorcycle away for a few months, the bike ran backwards if It started incorrectly, sure... But I did 700 miles on it and when it was idling, the engine characteristics would not change.

I have changed the cam, changed the points, capacitor, coil, (and i can confirm that it is charging!)..

Grrrrrrrrrrr!!!

[Puffs]

But on the upside: I don't hear you saying it's still running backwards!

What you now have sounds like a loose electrical connection somewhere. Does the charge light come on directly when it stalls (so with the engine still turning)? Can your HT lead leak somewhere? Plug cap?

Sorry, no clear solution from me... Good luck...

[Blurredman]

Well.. Unfortunately. It did - Just the once. But I can't determine whether this was a byproduct of the general crap idle, which I am also adament is electrical.
I will be going over the wiring soon.

Im not sure whether the charge light was on or not. I am going to say yes. It doesn't help on the old clock set I have on this bike, that the charge and neutral light are both orange. But.. When I came to a junction, and it started 'missing', I did see an orange light. So yes. The charge light does come on when this occurs. But presumably that is only because the engine RPM is not satisfying the requirement of the regulator. Nothing more to read into that, I think.
The charge light stays on when the engine stops.


HT lead and plug are new- as part of the rebuild.

[Puffs]

Mine had a loose connection in the fuse box, and when the ignition stopped, the charge light came on. Have a look there.

[Blurredman]

Mine had a loose connection in the fuse box, and when the ignition stopped, the charge light came on. Have a look there.

I bloody hope I didn't go through all this trouble and (breaking a 300cc ring, btw ) for one faulty connection...

I've always found the MZ wires and connections to be fairly good. That is... so long as they only involve things that aren't compulsory for the bike to run, ie indicators. Otherwise, they're crap.

[Blurredman]

Just to update.

Still not solved.
Rectifier, regulator, coil and stator have been changed again to rule any electrical thing out.
It is not a coincidence I feel that this has started since the re-build- surely?


Anyway- I've had enough. I've decided to purchase a 'Muller' electronic ignition from Hungary which is just the backing plate and sendor- it is completely without the black box- so it's really simple to set up, and relatively cheap- If it doesn't work- It will be something I will always carry around with me- as it is just the electronic adjustment plate, two wires for coil and positive power, and the sendor disc- incase my MZ with electronic ignition (different type) should ever fail.

[Blurredman]

So...disapointment AND Revelation in one..

The electronic ignition didn't work.. At least that rules out the possibility of points chatter, bad points, or cam.

The bike reacted EXACTLY as other times. Runs well when cold. 10 minutes riding and you find power is low. You stop, and the idle will be settled at 1k RPM and not the 1300 or even 1500 it was set at. It runs lumpy. Then eventually the bike will fail to even run at all and keep stalling. You adjust the plate to retard to get you home. Very slowly as lacking power.


Luckily however- before I did all that work, I compared both my MZ's. Specifically the 'key' in the rotor which controls where the timing point would be and I did find a difference, albeit small- but likely a breakthrough in my problem, and what I had a feeling might be the issue. (after all- the only thing different was the crankshaft).

I measured as best I could, what the difference was in MM between the edge of the 'key' in the rotor to the, although unreliable- the best place I could achieve at that time, the edge of the timing adjustment plate base. (not the timing plate itself). I found a 1mm difference! The Working bike (and I will measure tomorrow what my brother's MZ's is like to confirm) had a 6mm gap. The bike with the problems had 5mm! Obviously the timing key on the crank is off! Which explains why the bike ran so much better when I seemed to be setting firing at '0.5mm' before TDC... It is too pre-mature a spark, and although it doesn't run backwards, this is likely the problem of the compression on previous 300cc cylinder.

This gap was measured with pistons on both machines measured at 2.5(ish)mm BTDC. 1MM gap in that small revolution has a significant amount of difference! But.. I need to find out how much that equates to the 'mm measurement' taken from the spark plug with the piston. I can quite likely find that it 'would seem artificially' that the best firing point would be '0mm BTDC/Fire at TDC'. of course this is artificial and not actually the case, but as long as I remember what the actual firing point is of this machine then the problem should be solved, right????

The pin for the stator rotor has obviously been put in the wrong place.., if not by the smallest amount...


I'm hoping, that adjustment via the plate is still available, and getting the true 22degree firing point is not out of range of the adjustment plate..



We're getting somewhere Puffs....

[Puffs]

I'm getting confused a bit, so a summary - please correct me if I'm wrong.

From your other thread I understand that the electronic ignition works well, and from this thread I understand that your new ignition did not solve the problems you had with the bike. With an old 250cc barrel on, it doesn't actually run backwards, but after warming up it still runs poorly, and with this new ignition in very much the same way as with the points before.

And you have found a difference in how the key is located in the crank pin, between your 2 ETZs. But what I do not understand is how that can explain the poor running. You set the ignition timing (on either ignition system, points or electronic) with reference to the piston position, right? Some 2.7mm BTDC? Then that's all that matters. If the key is rotated a bit, it just means that the cam for the points (or the timing disk for the electronic ignition) is rotated a bit, and that consequently the base plate for the points (or for the electronic ignition) is also rotated a bit. But that doesn't matter, the spark still comes when the piston is at 2.7mm BTDC. (The crankshaft is then some 21°-22° BTDC, regardless of the position of the key.)

How do you measure the piston position when you set the ignition timing?

Apparently the orientation of crank pin, that has the key in it, did not receive too much attention on assembly in the factory. They didn't press it in very accurately, rotation-wise, and as long as it is within the adjustment range for the ignition, that doesn't really matter. (It is not something that could go wrong when you, for instance, take the crankshaft apart to replace the big end: for that, those crank pins just stay in place. So it has to be in the factory.)

[Blurredman]

Yes. the two posts are different. The fact that the bike doesn't work properly is inconsequential to the electronic ignition in my opinion.


I'm finding it very hard to look at aventues as to what could be wrong.. Very difficult indeed. Perhaps I am grasping at straws. I agree however that the adjustment of the plate should remove this problem of key placement.


I measure the position of the piston with a timing tool with mm increments identified.

I must say though, that the crank I put in the bike is not the same crank that blew a big end. Merely a replacement that I picked up on my travels..

[parrbd]

Try running the bike with a hot wire. Run a wire from the battery positive to the coil positive. This way all other switching systems are bypassed. As long as your wiring from the coil to the points and condensor are good and the battery is fully charged the motor should run with the ignition turned off.