CBR 125 Cutting out on idle?

Hey all, hoping someone more mechanically minded than I am could help out a bit here. Friend of mine bought himself a CBR125 that developed a bit of a problem on his first ride. For him it cut out when riding, but I can’t account for what he was actually doing at the time and I think there has been some fiddling with the idle speed and mixture screw, I’ve now set these back to what I think is close to default - if not at least it’s within range.

Bike starts up fine and ticks over happily at about 2k. I took it out for a ride, working through the rev range and up to speed and everything seemed good even when idling - I was out for about 5-10 minutes. Returned home, back to idle and the revs start to drop and then cut out. Bike starts up again each time with a bit of throttle, revs slowly drop and die again.

My knowledge of carbs suggests a problem with the idle jet, as it runs okay at higher revs but I would have thought that this problem would be present all the time if that’s the case - not just when warmed up?

Any help on this would be great - thanks!

CBR is fuel injected so maybe he altered the throttle position sensor

CBR 125 (2005 model) :wink: It’s carbed - not FI

they should idle around 1k revs
wind the air mixture screw half way out & set the revs via the throttle stop then adjust the air idle screw so you get a nice even idle without any lumpiness
around 3 & 1/4 turn out from full in should be a factory setting

I have tried setting the mixture at different positions from all the way in to all the way out - it doesn’t seem to make that much different really. No matter where I set it the same thing happens, the idle speed set to 2k shouldn’t be a problem at all. Obviously the mixture setting shouldn’t be set all the way in or out, but I was checking one extreme to another to see how the behaviour changed.

setting the idle @ 2k revs is to high it will be using the main jets
its a case of getting it to idle then playing around with the settings
have you checked the spark plug to see what its condition is ?
if adjust wrong it could be fouled up with carbon & wont allow the bike to run correct

2K surely wouldn’t be on the main jet? Either way - if it’s on the main jet that still doesn’t explain why the revs drop and cut out?

Plug seems to suggest it’s been a tiny bit rich but not to excess - cleaned the plug off anyway.

if it was me I would change the plug unless you used a spark plug cleaner & not just a wire brush (In the past I have used a rothernburg blow torch to burn off any crap)
2k is mains
do the revs rise before dying or does it just die

After riding the revs return to ~ 3k and then about 3-4 seconds later down to 2k. Sits there for about 10 seconds before slowing down to stall. The bike has only done 1300 miles, so the plug is in pretty good condition. I could change it out but the bike idles and rides fine when cold - doesn’t seem to suggest a problem with the plug

sounds like air starvation to me
is the piston in the carb rising & dropping smoothly ?

I’m not sure, I believe so but I haven’t taken the carbs off the bike

you don’t have to remove the carb just remove the airbox pipe & check with your finger & or take the top of the carb off & check the diaphragm for any holes

I’ll take a look, thanks

Sorry to OT but I love reading your posts wise, they make me smarter every day. (y)

I think its the joys of being such a tight arse & breaking something before it needs fixing then fixing it
& I own 3 classic cars

Has anyone checked the air filter yet…

kinked fuel line

I haven’t had chance to take a look at it in the last couple of days, but the fuel lines, air filter, breather hoses etc. are all in good shape.

tank breather blocked?

Finally got round to having a proper look last night, seems I was on it to begin with - carbs just needed cleaning out. The idle jet was pretty gunked up.

Thanks for everyone’s responses, I know it’s not easy troubleshooting these issues remotely!

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