Timing a modified bike

[Puffs]

I have since seen that there are several companies offering the service of increasing the volume of the combustion chamber in a 250 head to fit a 300, possibly to combat this problem of running backwards.

In principle, the higher compression you get from the standard 250 head would increase the engine's output, and there are other ways to solve the problem of running backwards, while keeping the high compression. For instance, some electronic ignitions do not give a usefully timed spark when running backwards. But also with a conventional breaker system, running backwards can be prevented, if the breaker closes sufficiently late after TDC (as seen when running in the normal direction). For instance, in the Jawa 634 the breaker closes around BDC, making it quite impossible that such an engine would ever run backwards. Now that's with quite a different cam profile, but you could try it with the standard cam, while setting the lift height very high. Set the contact breaker baseplate as late as possible, and adjust the timing to a standard 21°-22° pre-ignition by setting breaker gap. Then see when the breakers close; that will probably be quite late, say 40° - 50° after TDC, so too early to allow running backwards. Of course such a big gap will increase wear on the breakers, so if that works & if you want to go that way you can consider another cam (or use a compromise gap).

Of course a higher compression will also stress the system a little more, particularly the small end bearing (hotter & more load).

[Blurredman]

Hi Puffs.

I've used the rebuilt engine for approximately 600 miles now but recently i've been getting symptoms of coil overheat. I've therefore taken this opportunity to use my other MZ for now and take the backwards running bike to my lockup where I will be measuring combustion chamber space and acting appropriately with thicker base gaskets. This will also allow me to investigate any problems which may have resulted in the high compression.

So thanks for your help, but the first thing I want to do is to measure how much oil goes into cylinder with piston at TDC.


As far as the lob and points are concerned, I have noticed that this particular lobe does have an extended amount of time open when the points are resting on the lobe. For as much as I can see, the initial firing point is almost immediately after the points are open, which continue to open to 0.3mm as standard, then start to close about half way, before opening again breifly before closing completely for the second half of the revolution. I understand what you say in perhaps machining the lobe so that the second raise is removed at least, but.. Like I said. Now is a good time to investigate the engine.

I am wondering whether the coil failing is anything to do with the initial compression problem, but then it has 40k on this coil and so far Within a short space of time, all three (with the third being this recent coil break down) MZ's in my household have had a coil replacement. Perhaps just one of those things, unrelated?

[Puffs]

Overheating coil: haven't a clue. As far as I can see, should have no link with whatever goes on in the cylinder/head/compression.

It could be caused by the breaker being closed too long.

I'm sorry, it's been more than 13years since I worked on the ignition of my MZ (haven't run that bike since I live here), and I don't remember what that cam profile is like. But the Jawa I mentioned has a cam profile with a long open time. From closed, an initial rise (and you get the spark the moment the points open), then it stays open for a long time (about 160°), then it opens a bit further, in order to close with a bang, thus allowing good contact. Then the coil starts to charge, for about 170°, and some 2.8mm before TDC the breaker opens again & you get a spark at the plug. But that's a 180° parallel twin, with a breaker + coil per cylinder, and I always figured that they kept the breaker open for so long, in order to have no more than 1 coil being charged at the time & draining the battery (before the engine runs).

But if you now tell me the ETZ has a similar long open time, I cannot understand how that engine can run backwards. It's not dieseling, is it? So you need a spark. You only get a spark when the breaker opens, not when it closes. So for it to run backwards, the breaker has to close some 20° after TDC, when running in the normal direction. Then it opens some 20° before TDC when running backwards, and the cam profile would be approximately symmetric over the TDC-point.

Machining such a cam (or lobe as you call it) is not easy. (And I would not take out the hump at the end; I think you need that to get good contact.) I was only thinking about it in the sense of having a longer open time (to prevent running backwards & also have a cooler coil & less current drain), but if the standard profile has a long open time already, the idea is a non-starter.
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Edit/add:
I just had a look on my ETZ251, which has the original ignition & electrics.
- I verified that you only get a spark when you break the connection, so when the breaker opens. You can check this quite easily: put a plug in the cap & lay the plug body on the earth. Pull the green wire off the connection with the breakers, you can then easily simulate the opening/closing of the breaker: green-to-earth = breaker closed (& coil charging); green-unconnected = breaker open. And you can see (when you switch on the ignition) when it sparks: making the connection gives no spark, breaking it gives a spark.
- With the standard cam, the breaker is indeed open for something like 200°, as you said. It closes when the piston is on the way back up, but still close to BDP. So that's when it sparks if the engine is running backwards: then the breaker opens a little before BDC, and closes 2.5-3.0 mm after TDC. So for the life of me, I can't understand how it can run backwards with that ignition, and what I said earlier about modifying the breaker cam is nonsense. The standard cam is fine (of course).

This cam profile is quite different from that found on 60's MX bikes. Those have no battery (so no coil heating issues), and the breaker is closed for much longer, with a profile more or less symmetrical around TDC.

[Puffs]

Overheating coil: I have low hopes that solves it, but trying to swap a resistor plug for a non-resistor plug (or v/v) is easy.

Do you already have any idea how it can run backwards?

[Blurredman]

As always, life, girlfriend, other projects (3 other bikes.. and 2 cars) gets in the way.. I haven't touched the MZ in question since putting it in my lockup..

I have a Honda CX that i am trying to get on the road for a camping event on the 11th of May, but I had to buy a new master cylinder for it, battery, and then I tried to bleed the brakes and the nipple sheared off!!
Been fixing things on my American Buick like heating controls, Tightening torque converters and replacing (for the millionth time) vaccuum lines etc and using it to get things to the tip. Namely furnature.
My Peugeot 205 needs both front suspension strut mounts welded up as they have rather large holes in them, now that I poked them through...



All I need to do is put a measurable amount of oil down the spark plug hole with the piston at TDC and use that to see how much combustion chamber space is contained. But.. The lockup is at the other side of the city.. I do think this will be a very easy resolution to this problem, just chuck on some extra base gaskets.

And that job can only take a very little amount of time in my hands. But.. you know.. Thank God my daily MZ is okay for getting to work, even though it is 'completely shagged'. Not nice, but usable.

[Blurredman]

** UPDATE **

I finally got from my arse and got the bike home.. It ran terribly as I had 'estimated' a 'get me home' ignition timing, as I had previously removed all ignition items.


First. Everything apart. I opened up the barrel and found that the piston actually overlaps the bottom of the exhaust port by a good 1.5mm-2mm.

I had previously ordered a 2mm gasket set as a hunch and cut it to shape/size. The piston then fit in nice and snug and inline with the bottom of the exhaust port.


As a side note, I measured around 2-2.5mm of head gasket. I reduced this to around 1.25mm. I timed the bike with the 'traditional method' of a tool down the spark plug hole, and measured the 2.7mm BTDC.
I then got it started and noticed how much better it both sounded, and ran from this afternoon's ride getting it back home.

I hooked up the timing lamp with programmable offset, and set it to 22 degrees btdc and it was perfect... The timing lamp and the spark plug hole now correlate..!


I went for a ride. It was good...
But... when i got home it was through coughing and hickuping. I touched the coil... It was boiling...

[DAVID THOMPSON]

It was boiling. dwell not right coil is on to long or needs a dropping resistor they can be very warm on some bikes
but it should not burn you ( more heat sink on coil) clamp a pop can around with a couple hose clamps to get rid of the heat

[Blurredman]

It could be extreme coincidence that it has decided to break down within the last year. I have previously mentioned that this particular bike had symtoms of coil issue. And I really wouldn't be surprised considering I have changed two original coils on two other Mz's within the last few years as well...

[Puffs]

So you're telling us that at BDC the top of the piston was some 1.5-2mm above the bottom of the exhaust port, initially, and you corrected that with a 2mm thick base gasket.

The penny has finally dropped. In the very first post you said: "I'm using a 300cc re-bored 250 barrel and head on my ETZ 250.". So it's a barrel with an original bore of 69.00mm, which you drilled out to 75.50mm.

With the shape of the exhaust port (pointing downwards), taking 3.25mm off the cylinder wall will explain the exhaust port being too low. The top of the exhaust port will then also be too deep, but less so than the bottom. Both exhaust issues will be corrected by the extra base gasket, but that will not be ideal for the timings of the overflow ports & the inlet. Particularly your inlet will be a bit too high, resulting in reduced high RPM performance, and better low RPM performance. The overflow ports will also have come down a little bit, as a result of the new bore, but less so than the exhaust, so they'll be over-corrected a little bit by the extra base gasket. But that's likely not a big issue.

The deeper top of the exhaust (before you put the extra base gasket) will result in the exhaust closing earlier, so that you're compressing more mixture, which enhances the tendency to kick back. So you had 2 things enhancing the kick-back: the higher stroke volume being compressed into the same volume of compression chamber; and the deeper exhaust further enhancing the pressure. (And once the crankshaft rotates backwards, it can continue to do so, as explained elsewhere by our German friend.)

Can't help you on the coil. As David says, probably too long dwell time, being charged for too long. Maybe set your points wider. Or mentioned heat sink.

[Blurredman]

Yes

[Puffs]

OK, but then you'll be better off by working the top of the exhaust up a bit (milling, filing), and a very little bit for the overflow ports, while using a normal base gasket (& your 1.25mm head gasket). It's only the top of those 3 ports that count, their bottom is not so important.

I'll see if I can find the standard port timing somewhere, and at any rate I'll be measuring mine shortly (bore = 70.50mm, so that's not standard either).

[Blurredman]

If the bottom of the ports don't matter so much, but the top of them do. With that barrel bored out to accomodate a 76mm piston (Yes- it was the dreaded 'RAM' made piston, btw), how offset are the ports!?

[parrbd]

Maybe try the old time method of setting spark advance. Set it up well advanced so that it pings at full throttle at high speed. Then progressively retard the timing until the pinging stops. Do not let it ping for more than a few seconds at a time or you will hole the piston. At high speed on an MZ, pinging will sound like tinny rattling in the muffler.

[Blurredman]

Well, thanks for that Parrbd. I've just come from a ride on the machine.

I switched the coil to a known working one. And once again it starts to 'act up' upon returning home, strangely the exact spot it usually does in the past times i've tested it in the recent days.


I let it 'misfire' for want of a better word, getting out a replacement spark plug (because it can also be the result of a failing plug) until i realised the tacho was no longer working (just put on a new cable today) and realised what this meant. Sure enough, I put the machine in first, and the rear wheel spins backwards... It was not the coil causing the mis-fire. I can only presume that once the machine warms up sufficiently enough it gets more advanced, as it hasn't started backwards when cold, after putting a thicker base gasket on.


Other than switching it back to the original 250 piston and barrel, I'm a little stuck as what I should do other than retarding the timing, putting aside the ports breifly.. To be sure, it is currently set to the 22 degrees firing. The TDC markings I put on the lobe and stator would obviously not change... Though I think I might try it again with new markings.


I was wondering actually, that perhaps the barrel is not really thick enough any more to produce even expansion

[DAVID THOMPSON]

2 strokes so simple but can be a big pain in the A