NS125R won't go - any ideas?

[Supersport]

My son got an NS125R for his 17th birthday a fortnight ago. Yesterday I got a call for help - it had broken down with him. The bike had previously been starting easily and running well. He rolled up to a junction and it had stopped and wouldn't start again. When I got it home and checked it out there were sparks, plenty of petrol, it has compression and the inlet sucks when it is turned over. I tried replacing the plug - no difference. I took the carb off and checked it for dirt/water in the jets - nothing there. I checked the reeds were intact - they were fine. When kicking it over it doesn't fire at all but every 7 or 8 kicks it gives a "Phut" out the exhaust, so it is pumping mixture through and igniting it in the exhaust. This suggests to me that the ignition timing is way out, yet the pickup is firmly attached and the flywheel hasn't slipped on the shaft. Can CDI units fail in this way where the appear to produce adequate sparks but at the wrong time? Any help will be gratefully received, I have a 17 year old new biker who is pretty glum at the moment.

[badlydamaged]

Does sound like the timing is out, this happened to my bike after the previous owner assembled the engine without a thrust washer, allowing the crankshaft to move back and forth when the clutch / gear lever was operated and in turn allowing the timing to slip. I would think it was unlikely this is your problem (It wouldn't have lasted so long before going out of time I dont think) but I thought I would mention it just incase.
I dont know much about CDI igniton systems, but I guess it could very well be possbile that the CDI has packed up, after all dont they control the automatic advance / retard of the spark under different running conditions? By the way, according to Parts Partners your ignition timing should be: 19 deg idle,10 deg @ 10000rpm. Also the correct spark plugs for your bike are BR7ES with a gap of 0.7mm, just in case.

Good luck with with it

Cheers, Jonathan
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[mr jamez]

Have you tried bump starting it? it sounds like it wants to fire, could possibly be the coil or perhaps the kickstart/electric start is knackered.

[Supersport]

Won't start with a bump either and the timing can't be checked unless the engine is actually running. I think I'll lift the head and see if there is anything amiss in there, although it does seem to have adequate compression.

[Robby]

Check the compression too. And the killswitch of course.

[Kickstart]

Hi

Compression should be 156psi, give or take 43psi.

The spark plug should be a BR6ES, DR7ES or a BR8ES.

If it is sparking then the kill switch is probably fine.

Ignition coil primary resistance = 0.16~0.2 ohms. Secondary resistance (with plug cap) 7.35~10.85 k ohms, 3.6~4.6 k ohms without the plug cap. Pulse generator coil resistance = 50~180 ohms. Pulse generator air gap = 0.9mm. Exciter coil resistance = 50~200 ohms.

I would go with the ignition timing being out, possibly due to the flywheel moving slightly. This can happen when the woodruff key that locates it onto the crank shears off.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[theshmoo]

this is a bit of a cowboy trick but heat up the spark plug on your cooker until its roasting then put it in your bike this will help confirm compression and fuel the bike should turn over faster and fire up a bit i have had faulty plugs straight from the shop before
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CBR 1100 blackbird.

[Jammy-G]

Might be a long shot but have you filled up with fuel since you got it? If the bike had been standing around for a while with fuel in the tank the fuel can 'go off'.

Just a thought.
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[Supersport]

Thanks for all the tips guys. I pulled the flywheel at the weekend but it hadn't moved. I then decided to lift the head and the problem was revealed. A bulge on the top of the piston next to the exhaust port. It seems that localized overheating has caused the piston to melt/distort so the rings won't be sealing properly resulting in poor compression which is why I can't get it started.
This has probably been caused by the fact that a few days before, my son had rode it home 20 miles with a cracked exhaust and this was his first outing after I got it welded. He subsequently told me that prior to packing up it had been performing poorly and when he pulled up to a junction it cut out and wouldn't restart.
Looks like a new piston and rings will sort it out.