Hey folks, hoping someone can help or has run into similar issues.

My daughter has a 2008 CBR125R (Canada) with only about 4,000kms on it. When it works, it works great. But after riding for awhile (20-30 minutes) it develops an issue where it won't exceed 6 or 7k RPM (I need to take it back out to get the exact RPM).

When it's happening it feels very ignition timing or electrical related, there are no hesitations or misfires, it just WON'T go above that RPM. If you pull over and wait 5 mins it will resume normal operations for a few mins and then it will come back. Next ride the first 20-30mins are fine and then repeat. Occurs in all gears so at low speed you can mitigate it by not trying to pass that RPM range, but if you're riding along above that RPM when it happens it hard drops that RPM suddenly which can be quite jarring.

It seems like something spark plug, ignition coil, R/R or stator related, but I don't know if this issue is commonly known and I'm trying to avoid taking it to Honda at least initially for them to "poke around" when I can do the initial poking.

I'm going to replace the plug and clean the injector and see if that resolves it, but it feels like something is overheating and misbehaving as it only manifests after a bit and then goes away after cooldown.

Anyone having experienced similar and can provide some feedback is greatly appreciated, thanks!

 

Cracked things open on the weekend.

The airbox was spotless, the injector is very hard to get at to remove so I left it for now but one find was that the previous owner used a CR6E plug instead of a CR8E.

Looking at the plug it looked ok against a standard "read your plug" chart although maybe a little bit of extra carbon, you can't really tell unless you put a fresh plug in and ride it and then check it.

But a CR6E is hotter than an CR8E by 2 steps, and with the high RPMS of the little 125 I'm hoping that was the issue and it needed a colder plug. She rode it the other day for 40 minutes without any issues after the new plug, but one time in a row certainly isn't definitive.

I also checked the breather hose for the gas tank and it seemed to be clear but I blew it out with compressed air anyway and there was definitely flow into the tank.

So we'll ride it for a bit an see if it comes back!

 

Good find, well done and fingers crossed.

 

One month later and multiple rides, no more issues!

So it was either the plug OR the tank vent was plugged. But I have a happy kid with an issue free ride now which is the important part.

Thanks!

 

Good job! Might want to pull plug and compare to earlier one.

 

So guess what's back? Same issue.

Need to check the plug and the tank vent again, but we did put a new OEM ignition coil and plug cap in and no difference.

Going to check the plug and maybe try a CR9E (listed as optional in the service manual) and see if it's any different.

 

Plug looked okay but I swapped out to a CR9IEX anyway, I did check the tank breather hose again and it was tough to blow back through but eventually did blow air, I'm wondering now if it was pinched or is just otherwise clogging up. Might be worth putting a fresh length of silicone hose on to the fitting to ensure it's not having issues.

Currently running fine again though without issue, so I'm more apt to think it's the breather hose still, if it acts up again I'll do JUST the breather check and not touch the plug and see if it resolves it.

 

Had it into Honda, they gave it a quick look over and ride and said it’s starving for fuel. Some rust in the tank and fuel pump from previous improper storage.

New fuel pump on backorder ($350CDN) but in the meantime I took the old one out and disassembled it and flushed and back flushed the filter as best I could with CLR, and a few other solvents before getting it back together and it’s working great now.

I’ve ordered a full unit from AliExpress as well as a backup which is $80 instead of $350 so vast improvement.

Continuing to monitor but it’s working great now.

 

Thanks for letting us know, it always helps for next time :-)

 

If you suspect the tank vent, the solution is to simply open the gas cap the next time the bike stalls out and see if it sucks air in / restarts fine...