Thumper Club Forum
Technical => Bike Problems/Questions => Topic started by: richardeblack on July 11, 2026, 10:14:59 AM
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Hello All,
I'm having problems with my poor old XBR. Started when I was preparing for Stafford in April (the reason I didn't get it to Stafford). It was ticking over warming up and after about 5 minutes it just died. I first thought that it was bad fuel (I had just topped u p and wondered if I'd used wrong fuel) but after a carb clean out I still wouldn't run. Had the carb off again and got some horrible rubbish out so drained the tank, cleaned it and refilled with fresh fuel. It started! Ran it up the road and as soon as it got hot started playing up again. Basically it won't fire and then after a while of trying, it fires and I get a huge backfire in the silencer.
When it cools down it starts so I wondered if it was the alternator exciter coils again (had them fail 3 times of various XBRs) so with it hot I checked the resistance of the exciter coils and the pickup, both of which were about right. I didn't check the voltage output though. The ignition coil resistance readings are ok as well when hot. There is a spark but seems very weak and might not be there under compression.
Basically I don't think I have a good alternator stator left to swap over and a new one is £130 which might not fix it and I'd have yet another stator which charges ok but doesn't run the ignition.
Does anyone know of any crank end self generating ignition systems that will fit and aren't too expensive. The ones i've seen are £300-£500.
I guess the alternative is to find an ignition cdi system that runs of the battery instead of the exciter coils just to rule out one possible cause.
Any suggestions or advice welcome.
Thank you.
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I spent months trying to track down what I thought was a carb problem on my XBR, which sounded quite similar to yours. I broke down a few miles from home and had to be recovered. When I told the recovery driver about the problem, he said "ECU". And he was right. I tried 2 different ones: an expensive one from Holland which turned out to need rewiring and whose supplier was very rude when I told them that, and a cheapo from ebay that worked perfectly.
The expensive one was from Carmo Electronics (https://www.carmoelectronics.co.uk/ (https://www.carmoelectronics.co.uk/)) - I attach below the email I sent to them. The cheapo one was from ebay: https://www.ebay.co.uk/itm/404753268868
"I have to say I’ve been disappointed with my experience with your company. First of all, the unit arrived with no instructions whatever and was described as “plug and play” on your website. It was far from this. When connected via the small harness you supplied I had no spark. This turned out to be because pins 1 and 4 were swapped in the link harness. I did try connecting without the harness and this immediately blew a fuse on the bike when I operated the starter – I now realise bacause the XBR CDI has a +12 feed on pin 5 which you have broken in the link harness. When I cut this wire and instead fed a direct earth to the CDI, connecting without the link harness, I had success.
I understand that this positive feed may have a function in retarding the ignition when the starter is operated, however cannot find any documentation to this effect. I would have hoped that you would incorporate this feature, or at least point out to the user that it existed.
I have in no way been helped by you other than your providing a link to an information sheet which you should have given me at the outset. You should also have provided details of the advance curve and other characteristics. I have spent hours in my garage checking other components (entirely unnecessarily) but have been greatly helped by members of an online forum who have made many helpful suggestions. I should not have needed to do any of this. The only positive aspect is that I have now learned a great deal about CDI systems.
It has become plain to me that the unit which you sell as specific to the XBR500 is instead a generic unit. This raises questions in my mind as to what the advance curve is and whether this has been tailored to the motorcycle or – as I suspect – you have merely used a standard curve which may or may not suit the engine characteristics. I cannot believe that you have actually supplied any of these units to an XBR500 owner and that they have fitted it without problems.
It may interest you to know that in desperaion I bought a generic “pit bike” CDI unit complete with coil from ebay at a cost of less than £12. This was delivered to me today and I find that it replaces your unit directly and seems to give similar performance. It’s interesting that the pinouts on this unit are identical to those on your unit WITHOUT the link harness ie with B/W to pin 1 and red/black to pin 6. I accept that its internal components may be inferior but am left with the feeling that I could have saved myself £100 or so by purchasing this in the first place."
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Thank you Ian
I just wrote a reply with some figures on which promptly disappeared into the ether. I'm too tired to rewrite it but have dug out a cdi unit to try out. I'll let you know how I get on.
I haven't tried the spark plug as I think it might break in the head as has happened twice before and I can't face dealing with that right now
The plug is the one that was in it when I got it 60,000 miles ago though so it may be that!
Thank you
Richard
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Those pickup and exciter coil readings:
Exciter coil cold: 208? 65V AC Hot: 220? 70V AC
Pickup- cold: 466? up to 250mv ac Hot: 483? 120 to 200mv ac
The pickup voltage readings fluctuated quite a lot both hot and cold. Not sure if that is indicative of any problem.
I fitted a spare cdi last night so will start it up later this morning and see if it behaves itself. Next thing to try is another carburettor although I have cleaned this one very thoroughly and can't see any problems with it. I do have the fuel shut off valve and the vacuum take off blocked off although I can't see why that would start to cause problems now.
If all those fail, I will summon up the courage to take the spark plug out although I'm sure it will break off. Then I will have to have the head off and can sort out that cam chain that's been rattling for the past 60,000 miles. Problem is that I will probably find the barrel and piston are completely worn out and will have to decide how far to go.
What joy. The pleasures of a high milage bike.
Thanks again.
Richard.
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I don't know what the figures indicate, but for my money I'd try one of the pit bike ECUs as I mentioned. For less than a tenner what can go wrong? Yes I know.......
I seem to remember you've had a lot of problems with this bike dating back some years Richard.
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My SRX would stop every so often changed the ECU and it was fine after. Also have you checked the choke isn't stuck on
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Thanks again Ian and Steve
tried again with another cdi with exactly the same result. I've just rebuilt another carb to see if that makes any difference. Next is to try and remove that very old spark plug which will almost certainly break off so head off which will mean that I can finally fix that rattling timing chain.
Thanks again.
Richard.
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Rebuilt another carb just in case although I don't hold out much hope. I will try and find time to fit it tomorrow and see what happens.
I'll let you know how I get on.
Thanks again.
Richard
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Another possibility is to check if the tank breather is blocked.
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I've just had a tank breather problem on my latest project which resulted in similar problems.
In all the many years I've been riding I can't recall this happening before so it's definitely worth checking.
On my bike , a 1989 Honda 400/4, not the classic twin shock SOHC model, there are 2 breather in the tank under the filler which I cleared by running through some electrical wire from top to bottom and another in the cap itself.