RT125 Starting issues

[starfield181]

Had the RT125 on appro for about 3 weeks now and it is driving me up the wall. Every so often it loses sparks. Not as you might expect when it gets hot on a run but when it been left for a while or overnight in the garage. Replaced the plug cap and lead with a new one from Grahams in Taunton (at an eye watering price) and it did start running again. Then it stopped and for 3 days would not go. Yet lo on the 4th day it started again without me doing anything and continued to start for a few days. I even took it for a 25 mile run (with a friend riding shotgun) and it went really well. It was starting and running fine on Friday yet today (Monday) it would not start again - no spark.

I have previously checked the coil which is fine on low tension resistance but down on HT resistance 1.77kohms instead of 7.62kohms. However, I somehow doubt this is the cause as duff coils mostly fail when they get hot and this bike has problems when it is cold, or at least has been left for a period.

When it would not start this time I checked the power supply to the ECU (red/white wire) and that was showing well over 12v so I don't think it is the various cutout relays are causing the problem either. Anyway, the starter motor always spins and it shares the same feed wire via the starter button.

Pushing and pulling on the wires that go to the coil from the ECU (Blue and Brown) did get it running again though I don't know if that was coincidence. I subsequently tried provoking a failure by pushing/pulling these wires quite hard into the ECU but it is (or was about 10 minutes ago) still starting fine.

If it was a 2-stroke MZ I would be able to swap out virtually any part of the ignition system but the RT125 electrics are a mystery to me and I have no access to parts to swap over. Run out of ideas and patience and I am about to return it to its owner for him to sort out. However, he is a good friend and any ideas which might help him to solve the problem would be appreciated. The workshop manual does not give any clues on how to test the ECU or the pick-up/sender unit on the stator

Pity because when its running it is a cracking little bike and I was looking forward to owning it. Now a bit nervous about having one as parts and information are clearly in short supply.

Peter

[DAVID THOMPSON]

screw on left front bottom side of carb is the drain for float bowl
lean bike on its right side against a pole or wall and drain bowl
if these bikes set for a few days the carb drys out and there a beast to start
open drain for a bit fuel runs out of hose on bottom of carb on right side
good quality fuel helps no ethanol
after a few days they just refuse to start with out some fresh gas in the cylinder
to make the carb prime its self
NOTE this carb is used on a Kawasaki 200 or 220 bayou 98 to 2003 body of that carb has no flange
like this one
Dave

screw is opened junk comes out the nub

rt125carb drain.jpg (19.2 KiB) Viewed 24041 times

[starfield181]

Firstly belated thankyou to Dave for his reply but it is lack of sparks remains the problem.

12 months on and the bike is still sitting dead in my friends garage. The issue is still one of diagnosis as we have no access to known working parts to try and swap things around. The ignition bits can be bought but are expensive in relation to the value of the bike and annoying if they turn out not to be needed.

I am investigating replacing the Cdi box and coil with cheap (I assume chinese) replacement DC Cdi units readily available on the web. Several issues arise with this though which are summarised below:

1. I don't know where the auto advance is provided in the standard setup. Is it within the pickup wiring (which would be favourite) or within the Cdi box itself. Very few cheapo Cdi boxes mention auto advance (actually none so far found). Whilts the MZ 2-strokes run happily on fixed ignition, I imagine a the RT125 unit would be less than happy and at the least likely to damage the starter mechanism if it kicks back.

2. The MZ Cdi has two wires to the pickup whereas the cheapo Cdi boxes only require a single trigger input. Earthing one of the MZ pickup wires might provide a circuit, equally it might damage the circuit.

3. The MZ Cdi provides two wires to the coil whereas the cheapo Cdis only provide one. I assume the ground wire provides the earth return. This is the most easily resolved problem as we could use another single wire type coil.

I realise that this seems like a cheapskate approach to resolving the problem, But the bikes are not getting any younger and spares any easier to find at affordable prices. Eventually we are going to need lateral thinking to keep them on the road or they will get scrapped 'for want of a nail'.

Suggestions welcome. Of course if anyone has a spare set of known working RT125 ign parts they are prepared to loan that might at least help to isolate the problem.

[starfield181]

Thanks to the assistance of a young friend who better understands electronics (something not invented when I went to school), we now have the RT125 running again. He was able to set up a test rig which confirmed the coil and pickup were both ok and it was the cdi which was faulty.

As an experiment we bought a Chinese DC cdi from eBay costing a whopping £9. If we had more patience we could have bought the same thing from China for about £6 - postage paid in both cases. We also had to buy the special connectors to the cdi to make a neater job another £3. And it works!. The cdi is adjustable for duration of advance and we had to reduce this to the minimum. Bike now starts easily and a short test run was done without issue. The only downside to this conversion is that the cdi does not provide a driver circuit for the tacho but this is a minor issue. No changes were needed to the bike's wiring and a genuine cdi could be plugged straight in. When I have sorted out a minor and unrelated electrical problem with the rear brake light switch, I will ride it to the MoT station as a longer road test. The bike itself is now being offered for sale if anyone is interested - see picture below.

I have attached a picture of the cdi unit we fitted and another which shows the pin-outs. The MZ pickup has two wires and the Chinese cdi only requires one from the pickup so the black cable was connected to ground.

[Blurredman]

I recall reading about this issue on your blog/diary. Which I love reading by the way...

They're funny looking bikes those new RT's...

I'm glad you found a potential fix! Could you use the ingition release wire to the coil for the tacho?

[starfield181]

Hello Blurredman,

You would have to ask the clever young thing who sorted out the original fix about finding an input trigger for the tacho. Electronics not invented when I went to school. The bike did pass its MoT, having got there and back under its own steam. Lovely little thing to ride. It was never mine, I was just helping a friend and it went to its new home last Saturday. Had an email from the new owner saying its going well on his daily commute. I fixed the rear brake light by using an.d old fashioned mechanical switch from a Japanese bike. Cheap (nothing) and probably work forever.

We are working on a more detailed writeup of the cdi conversion. If not now it will surely be needed some time.

Peter

[DAVID THOMPSON]

detailed write up of the cdi conversion.

i am all EARS
have several engines i would like to use it on
and a version for a flat twin i have several old BMW /2 bikes i want to convert
to 12volt and do away with the magneto

Dave

[Puffs]

Yes that sounds very interesting Peter - looking forward to it!