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Random massive deceleration at high revs, please help! (YBR 125)


aj-1
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Hope I'm not breaking any rules by posting here i've just got a mysterious issue with my bike that I need to fix urgently and thought it would be worth posting here to ask for advice. 


In short my YBR 125 Custom (2016?) is having an issue where randomly (always over 7,000 rpm in 5th after 2 minutes or so) a massive amount of engine braking will kick in and the engine light will continuously flash (No code just constant quick flashes till the breaking stops). This will deceleration will happen for a seemingly random amount of time.

This issue has resurfaced multiple times over the past year of me owning the motorcycle and it has me and my mechanic step-farther stumped. The urgency is due to me starting work 50 miles away at a covid testing center on the 21st with no other means of transport there.

For a brief timeline this first happened in January before replacing the ignition barrel, spark plug ad neutral switch (in order to fix other issues). This seemed to solve the issue for a month before it happened again, potentially solved by returning the original spark plug yet I cannot remember if other things were also done that may have made a difference. Move forward to last month or so, the issue happens again. I order new spark plugs and replacing seemed to have fixed the problem entirely however 4 days ago it has returned, the new spark plugs are NGK/CR6HSA (as mentioned as the proper plugs in the manual) however the one that came in the bike when purchased is a NGK/CR7HSK. Today we replaced the ignition coil as the resistance on the original was a little high to that stated in the haynes manual, no difference was made.

The bike has done 14,000 miles, oil levels are good and the bike is started every two days minimum. It is also regularly ridden at 5th at full throttle for long periods of time. 

Is this a common problem with these bikes? Is there anything I might be able to check or common causes of issues like these ?


Any help and advice is greatly appreciated.

Edited by aj-1
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If anyone's reading this in the future it was fixed by using a brand new NGKCR7HSA from Halford's instead of a NGKCR6HSA.

No clue why as both the Haynes manual and owners manual reccomend a CR6 spark plug. Parhaps all 3 CR6 plugs off Amazon were Chinese counterfeits or the CR7 is better for when it gets colder in the UK.

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Yeah... Don't buy plugs off Amazon... If they look like a great deal, be suspicious..!

 

I'd be looking at what the actual difference is between those two plugs.

It may be that you just bought dodgy fake plugs, but it may be that the other plug is higher rated, which leads you to the question: WHY did someone put uprated plugs in the bike?
Better spark?
To solve a problem?
 

What I'm saying is: Just because the issue appears to be 'fixed', don't stop looking for the problem!

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  • 2 weeks later...

Snakebite was certainly correct in advising me not to stop looking for the issue.

About 500 miles later the issue returned. As it happened only at 45mph+ I was able to get home safely and along the way test a theory. Kicking out the side stand whilst moving seems to replicate the rapid deceleration exactly, flashing engine light and all.

Following this I checked the sidestand cut off wires which hung low under the motorcycle. Although covered in dirt I realised that one of the wires to the cut of switch were damaged (half torn through) and corroded. Bypassing the side stand switch (soldering the cables together) seemed to fix the issue immediately. I replaced the side stand switch with another following this which also seemed to work flawlessly.

Most of that information is probably useless though because the issue has yet again resurfaced roughly 200 miles later.


It might be worth noting that the first appearance of the issue the last two occasions were on the same road briefly after filling the tank from low at the same pertol station with premium unleaded.

Anyone have any ideas for next steps to take ?

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Hmmmm...

Since you mention the petrol station, might be time to take the tank off and empty it to check the internal condition.
Check your inline fuel filter.
 

I'm assuming it's fuel injected? I don't know much about injectors, but make sure they're all working properly and not blocked...

 

Although, now I've said that, I'm looking back to the whole 'sidestand' issue...
You've changed the switch and wires, but, did you take a good luck at the part of the harness it plugs into? I'm wondering if it's been arcing as it shorts out and maybe damaged the harness further up?

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It may be the side stand switch itself. Mine failed on a trackday,  randomly cutting out, then powering back up. Prove this by bypassing the switch. Disconnet the plug where the switch joins the loom, fit a jumper between the plug on the the loom to simulate the switch.

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I'm not massively confident that I won't be posting here again however the issue has once again subsided.

I had previously checked the side stand switch in the method mentioned by neversaydie however the input and advice is still greatly appreciated.

Per snakebites advice I checked the fuel filter, examining the quality of fuel pumped out of the tank (all seemed fine). I also checked the wires mentioned and there seemed no issue there also. We admittedly did not check the fuel injectors.

What we did do is remove the battery overnight, scrubbing the contacts with a wire brush to clean them up a little. This supposedly resets a computing element in the motorcycle, I'm not sure the correct name. A sort of motorcycle mentor also suggested removing a piece of rubber leading into the air box (an air duct). This supposedly increases airflow to the engine at higher revs.

Regardless the ybr has performed perfectly over the past 2 days, I'll be sure to update in the likely event that the issue resurfaces.

If it does not resurface thank you greatly to all who provided advice (especially snakebite) and I hope this forum serves as a useful tool for anyone else who may encounter this problem.

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I should just point out that at best, I'm an amateur mechanic. Things I say should be taken with a pinch of common sense! :D

I just work through stuff in a progression (usually cheap to expensive! lol).

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Same as i do,,  its a learning curve   , my mate  has a garage, 90% of running problems are fuel related,,  the other 10  is ECU"s  or duff wiring -  low battery,

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All my problems seem to be stupid stuff, like failing an MOT for a left grip that twists a bit on the outer end (but can't slip off because of huge bar weights) and dodgy bearings lol.

The only time the Thunderace hasn't started, it was because I failed to turn the fuel tap back on! :D

  • Haha 1
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