February 4, 2019 at 12:20 pm #11075
This has actually been bugging me for a couple of years now, and I’ve lost track of what avenues I can now focus on to getting this machine to run reliably.
Back in… 2017(? I forget!), the big end bearing went on me.. Eventually I got around to re-building the engine a year later. I had been on holiday and had sourced a reconditioned crank and that got me all keyed up to build it. So I did. Completely re-built, new bearings (obviously) and it all went together smoothly.
At the time the crank went, I had been using a 300cc barrel/piston that worked for a few years. This had to be bored out a tad because the piston was all scored because of the maceration of the bearings. Anyway. For the time being, after the re-build, the original 250 barrel /piston/head that i had been using for a couple of years before the 300cc kit, went on the bike. I checked the squish band was okay (1mm) the piston was a little worn being an original size 69 with 40k miles on it, but perfectly usable.
The bike has been running backwards every now and then since. I’ve checked the stroke to make sure the conrod replacement was correct, and accoring to the Haynes, 65mm is correct. The ports line up, the timing is on point. I even went out of my way to purchase an electronic ignition, which unfortunately results in the same. It is quite capable for the bike to switch between running forwards and backwards on it’s own several times if left idling.
The bike starts okay when cold, but takes a little while to warm up. It eventually does and continues to ‘feel’ fine. But most of the time the bike will kick back and run backwards ‘When warm’. It rarely fires backwards cold, but still does occasionally. When it does- I turn the ignition off, and the next kick it always runs forwards.
I’m really lost here and hoping some of the members might have proper proof of the cause of this, perhaps it is something I am missing. Ignition is set at 22 degrees BTDC / roughly 2.5mm.
February 4, 2019 at 8:48 pm #11077Membership ManModerator
Don’t have an immediate solution but before going any further can you confirm what type of electronic ignition you are using. Does it still use the points and the points cam?
February 5, 2019 at 10:18 am #11078
I have tested on both the original points system, the polish ‘Zaplon’ and currently on a hungarian ignition. All three gives the same result. All three use different cams and triggers.
February 7, 2019 at 10:14 am #11083Membership ManModerator
Hmm, not familiar with either of the electronic systems you have tried but if they don’t use the standard cam to trigger then that blows one theory away. There have been instances of badly machined cams which can trigger at more than one point especially if the points gap is a tad small.
I had a similar problem with a TS150. I am not convinced I ever got to the bottom of it and it seemed to become more prevalent after I fitted a Velleman transitorised gizmo – which reatins the mechanical points.
two things I can suggest but try them one at time. Firstly retard the ignition to 2mm btdc and see how the engine behaves. It should reduce the kickback and probably will not have a huge effect on performance though there could of course be a risk of running hotter. If that works then at least you know a cause and may be able to reach a compromise on timing. The other thing to try is a faster idle speed. Slow steady idles are great but they would tend to facilitate engine reversal if the motor is prone to it. My TS150 would reverse whilst ticking over at traffic lights – keeping the speed up helped.
Best I can suggest. If you do ever solve the problem let everyone know how as it will add to MZ folklore.
February 7, 2019 at 10:39 am #11084Alan JarmanParticipant
The famous thing to have a T/S running backwards is a to far advanced
ignition and to that end I believe the German Messerschmitte 3 wheeler had 2 sets of points the second set advanced so when you needed reverse you switched to that set and then go through the gears up to it’s max if you were so inclined. I recon that’s why it had a bubble canopy so you had a good view while you are going a blast in reverse.
Safe riding Matt take care Alan.
February 7, 2019 at 11:47 am #11085
Thanks, but as already mentioned the timint is on point. 🙁
February 8, 2019 at 7:07 am #11086Derek ReynoldsParticipant
That’s as maybe Matt, but try retarding the timing just a little. The running backwards (a way of reversing in some single cylinder hot bulb diesel two strokes) is due to the timing being advanced enough to promote sending the piston down the ‘wrong’ way. Back it off a bit and see what happens.
The Swedish Bolinder hot bulb engine (single pot about six litres in capacity, often called semi-diesels) with no reversing gearbox, could achieve reverse by allowing the engine to almost stall, then operating a separate oil lever to fire just before the piston reached it normal (running forward) firing point. This would then fire, and send the piston back down the bore reversing the direction of the propellor. If missed, lots of blue air and a crash!
February 9, 2019 at 6:38 pm #11088
Have run at 1.5m btc but still been an issue.
I have however measured the stroke to be 66mm on the ‘suspect’ crank.
I’ll be tearing down the engine tomorrow and removing the suspect crank and replacing with one I’ve recently had reconditioned by Burwins.
February 27, 2019 at 11:17 am #11151
Just an update.
After re-assembling the engine, the putting the contactless ignition on i found it still ran backwards.
Then I realised that it seems to run backwards at a perfect 22 degrees just as forwards. I find that the rotor trigger has two magnets at around 45 degrees! Obviously when kickback occurs, the other magnet allows it to run backwards. For now I replaced with contact breakers and it has fixed the problem seemingly. It does kick back, but it doesn’t ‘support itself’ and continue to run in reverse.
I guess case closed.
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