Timing a modified bike

[Puffs]

Removed

[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]

Removed

[Puffs]

Removed

[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]

Removed

[Blurredman]

Yes

[Puffs]

Removed

[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