YBR ESD Power Loss

[jeffyjeff]

12 June 2026

https://live.staticflickr.com/65535/54575078048_e81c6eccd1.jpg

2015 YBR125 ESD purchased new (old dealer stock) in 2019, 23100 km on the odometer. The ESD model has a carbureted engine (Mikuni BS25) sold primarily, dare I say, in the third world. The Pakistani YBRs are ESD models as are YBRs sold in Central and South America, to my knowledge. No electro-mechanical issues other than a TPS replaced at 20800 km. The bike is well-maintained (over-maintained to be honest) and runs better than it did when new…up until yesterday.

I was riding on Moroccan highway N6. It was 35 degrees C, and I was about 60 km into my trip and riding at 80 kph. Several km after turning the fuel tap to "reserve", the bike suddenly lost all power. To wit: instantaneous loss of throttle response and uncontrolled deceleration. After slowing to around 60 kph, the power magically returned. I kept riding, and after an uneventful 10 km, I started to think, "maybe that was just a little hiccup, nothing to be too concerned over." Then it happened again. No hesitation, no stumbling, nothing to indicate that the carburetor was starving for fuel; just an immediate loss of throttle and resulting deceleration. I thought "ignition power loss," but then I noticed my high-beam indicator illuminated even as the bike was in the state of power loss. As I slowed, the power immediately returned when I downshifted from 5th to 4th. Well that's odd. The glitch recurred 3 times over a 12 km stretch, and each time I was able to recover power by downshifting, except the last time required two downshifts to third before power returned. Six uneventful kilometers to the petrol station, where I filled the tank. I rode twenty-eight kilometers to home (after filling up) without any recurrence whatsoever. I had the fuel tap on reserve most of the way to see if that might be a contributing factor.

The instantaneous onset of power loss suggests ignition system failure, but it is odd that downshifting the transmission could bring about such an immediate return of power; coincidence? Maybe, but I have never experienced a repeatable coincidence like that.

I would normally dismiss the fuel system as the cause due to the split-second onset of symptoms, but it is hard to ignore that the problem manifested shortly after turning to "reserve", and only occurred when the engine was drawing fuel on the reserve standpipe.

I'm in Morocco until September. When I return, I'll probably have a spare fuel cock and an extra ignition coil in my bag. In the meantime, I intend to keep the fuel tap in the reserve position to see if that makes a difference. I checked the fuel filter (part of the petcock) and it was unobstructed. Once I run the tank down to less than 5 liters, maybe I'll drain it and remove/inspect the petcock. Specifically, I will be looking for a crack, hole, or damage to the shorter standpipe.

If anyone can suggest actions or observations I might make in the meantime, I'm all ears.
Thanks for reading.
____________________
History shows again and again how nature points out the folly of men - BOC

[stinkwheel]

Well, a fuel tap obstruction seems possible.

Or even vapour locking in that kind of heat with a prolonged ride (fuel in the float bowl boiling). Especially with a small amount of fuel in the tank to get heated up.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[jeffyjeff]

[jeffyjeff]

Yesterday I drained 1.5 litres out of the tank on both fuel tap positions: 78.9 seconds on the run tap, and 78.4 seconds on the reserve tap; so there is no apparent obstruction in the petcock reserve position. Also, no evidence of water in the fuel whatsoever.

I checked the side stand switch circuit for continuity. Seems that a momentary open in that circuit could result in the symptoms I was experiencing without generating a check engine light. Especially considering the downshifts brought the engine back to life. Maybe it wasn't the downshift, but the clutch action (clutch switch closed) that caused the ignition circuit to reactivate. Possible, but does not explain why the engine kept running after the clutch was reengaged. I should keep a paper clip on the bike in order to bypass the side stand switch if need be. I did not trace the wiring further than the side stand switch and pigtail, so if the problem lies further inside the wiring loom the paper clip idea won't work. I do not intend to dissect the wiring harness until I'm more convinced that the problem lies there. In the meantime, I'll keep poking around looking for abnormality.

Upon removal of the side cover, I did notice that a wire for the carburetor heater circuit had come loose from the thermo sensor, but that is totally independent of the ecu and the ignition circuit.
____________________
History shows again and again how nature points out the folly of men - BOC

[WD Forte]

Might be fuel might not.
Member the YBR that went from Bristol to Stornoway and back in a wet cold winter and got with 24 miles from home and suddenly stopped?

it started again then stopped but had to be shipped the last few miles home just in case
It was the EFI model and the fault code said 'bad coil' but that was bollocks
It was oxidised contacts on the regulator. Once they were
cleaned it ran just fine again.

the point?
check/clean electrical connections and grounds from time to time
but especially if experiencing intermittent problems
Also the side stand switches on YBRs have been known to cause problems and can easily be by passed if suspect
____________________
bikers smell of wee

[jeffyjeff]

So I took the bike out on my local test route last week, and the sudden no-power glitch manifested 4 times in 40 km under very similar environmental conditions. It happened with the fuel tap in both run and reserve positions, and in each case the power returned before I could even react; so there are two more data points. I decided to dive deeper into the ignition system, and was surprised at the apparent condition of the wiring upon removal of the fuel tank. Much of the harness was spotted with a kind of furry discoloration; resembling mold growth. Some of the exposed wire insulation also had dark spots. I cleaned up the harness with a damp rag and compressed air, and inspected the terminals at the accessible connectors, including the ECU and ignition coil. I removed the coil and cleaned up the spade terminal and mounting points with 400 grit sandpaper, then reinstalled and checked the spark.

I was surprised to observe a really weak spark. It was indeed a blue spark, but it was really small, a micro-spark that appeared to dance around the spark plug center electrode. I removed the coil once again and checked primary and secondary winding resistance against the manual specs.

My coil measured 0.9 ohms on the primary, and 7.84 k ohms on the secondary.

https://live.staticflickr.com/65535/55371031618_cd48e2f3c2_c.jpg

My coil has a single spade, like those on the 2005-06 YBRs and all XT models. As stated earlier, mine is a 2015 YBR ESD model, an orphan (at least in terms of available technical information), so I'm really not sure if my readings are out of spec or not.

On a hunch, I went down to my local hardware guy, who sold me a Chinese coil for a Honda GB125 (less than 5 quid!) that matched my OE coil dimensions exactly. It also matched my OE coil ohm readings exactly. I mounted it and found, unsurprisingly, no change in my weak spark condition.

Then I remembered that last year I had replaced the spark plug. My YBR takes an NGK CR6HSA spark plug. Last year, I found a set of four at an attractive price off Ebay. Oddly, they came in a Honda branded box (probably for a Honda inline four of some sort), but I really wasn't concerned about that much at the time.

https://live.staticflickr.com/65535/55370983643_3482b79398.jpg

I checked the spark on a used OE NGK plug and voila! The spark strength returned to about 80% of what I expected. I reassembled the motorcycle, and today I took it for a 60 km test ride with no recurrence of the issue.

So now my suspicions are focused on the spark plug. Perhaps I unintentionally installed a resistor type spark plug into a resistor type spark plug cap. Or perhaps I unknowingly obtained counterfeit NGK spark plugs that do not perform up to standard. Maybe both.

At any rate, that is where things stand today. Time will tell if the abnormal operation has been repaired permanently or temporarily.
____________________
History shows again and again how nature points out the folly of men - BOC

[WD Forte]

Hopefully its fixed
the coil specs on your bike especially the primary suggest CDI ignition to me so you could probably swap out all manner of cheap GY6 coils an stuff to keep it running
Re the plug cap
The early CDI type CXs used resistor plug caps and they were notorious for rotting out internally so it if occurs again it's maybe worth checking
____________________
bikers smell of wee

[jeffyjeff]