Bike stalling when downshifting

jetblast787 · Derestricted Danger Joined: 22 Jul 2015 Karma :

Hello all!

I'm a new biker (just passed the 100 mile mark today!) on a 2009 CBF 125. While slowing down to a stop when downshifting, my engine occasionally stalls before stopping. It doesn't seem like time of day or cold or hot engine makes a difference to the stall, except for when downshifting.

I am a new rider, but I always hold the clutch in when downshifting, but it seems like when pulling off the throttle and holding the clutch, I feel the engines rev getting dangerously low all the time when riding, occasionally leading to a stall.

I've adjusted the throttle idle, but after initial tinkering, it doesn't seem that the idle stays high, but goes to its pre-tinkering state.

Anyone know how I can stop my bike from stalling when coming to a stop? It feels a little dangerous because I'm forced to focus on keeping the throttle slightly open while negotiating the road and working the gears.

Thanks!

Ariel Badger · Super Spammer Joined: 02 Dec 2006 Karma :

I think you are being over cautious in your riding and knocking down the gears too early.
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jetblast787 · Derestricted Danger Joined: 22 Jul 2015 Karma :

Ariel Badger · Super Spammer Joined: 02 Dec 2006 Karma :

Keep on down shifting and keep the revs up.
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Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

Hobby-Horse time; but very common Newbie habit, especially with riders coming from cars, is to short-shift, and not use the engine revs. Result is, you deny yourself engine response to change speed easily, and make more work, and trouble for yourself, making too many unnecessary up-shifts, that then beg too many unnecessary down-shifts, making your whole ride, harder, less 'smooth' and less finesses... while chucking up little anomalies like this, or jerky changes or un-charged batteries, that pop up as symptoms here in work-shop, when they come looking for something to do to the bike, to fix the problem, rather than the rider...

Your bike, has a rev range that extends from about 1,200rpm tick-over, to about 11,000 rpm, just beyond the red-line. Peak torque, the point n the rev range that the engine is working at it's most 'optimal' is probably about 9,000 rpm, with peak power, probably around 10,000rpm.....

BUT, it's a small, air-cooled single, in an 'unfaired' commuter style bike; so, there's water-jacket, or body-work to shield you from the noises it makes, and being a single, it only makes one bang' every other crank rev, it's not very 'smooth' so sounds more strained than it really is, and in consequence, (and compared t car engines) 'thrashed' if you use more than about half the rev's t has... so many newbies don't... and short shift before 3Krevs, and rarely exceed 6K....

But, first gear, will take you from rest, to perhaps 30mph, 2nd. from 20, to about 40, third from abut 25 to 50.... forth, will take you from about 30mph, to around 60, and you only ever really need 5th, when you are on a 50+mph road, with little likelihood of needing to change speed, so a more open A-Road or duel-carriageway, when you use t to knock the revs back for more relaxed cruising and maybe eek a couple of extra points of speed out of the top end, if you are really determined to max it out....

What are my odds, f I bet a quid you are using 5th, quite a lot more often, and at speeds as low as 30?

This would put you at least three gears higher than you need be, when you start slowing for junctions.. most likely on 20/30mph limit roads.. where, there's no real need for you to be in a gear higher than third, and IF you were in 3rd, slowing down, you would only have to make ONE down-shift, after rolling off the throttle, and loosing 10mph of that as the revs drop.. WITHOUT them dropping so low it 'feels' like it's going to stall.. like it would if you were in 4th or 5th, at bugger all above tick-over before you roll off the throttle, where yeah! Apply the brakes, to try slowing down, with the engine driving the back wheel, clutch engaged, it WILL try and make the motor go lower than its 'stall-speed', wont it! Which is why you are haling in the clutch... and more than likely, trying to crash down three gears in one go, without letting the clutch out between...

Don't shift up, you don't need to shift down! Use the revs, you can slow down On the throttle, NOT the gears, and you can do it without risking the motor stalling, AND you can do it all 'under drive' rather than "coasting", where you DON'T have the same degree of control over the bike.. and with the clutch 'out', and making only one distnict shift at a time, clutch loading and turning the shafts n the box between shifts, givig the cogs a change to line themselves up, you'll make smoother, slicker, more possitive chages on those fewer occassions you DO need to make a shift.

Try it - I'll lay the winnings of that quid I bet earlier, your 'tickover' problem goes away without a single screwdriver being twiddled"!
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pompousporcup... · World Chat Champion Joined: 15 Apr 2015 Karma :

what ^^ he said. I used 4th and 5th whilst doing 30/40 mph for a month or so thinking it would be more ecomonic but in fact it just labours the engine and means more shifting with left foot which becomes tiresome on a commute.. also means you have less to think about if you're not shifting so many gears coming up to junctions/negotiating idiots etc etc. only one click and you're back in decent revs to get you up to speed Thumbs Up

pompousporcup... · World Chat Champion Joined: 15 Apr 2015 Karma :

one thought.. if you turn the idle screw.. does the idle speed actually change? and stay changed? ie, turn the idle speed up, do the revs stay up or do they settle back down to what they were before?

Rogerborg · nimbA Joined: 26 Oct 2010 Karma :

Reading what OP has actually written, I'd also check the clutch cable tension.

There should be just the merest sliver of slack at the lever before the tension comes on.

Too tight and you'll be slipping the clutch inadvertently while riding.

Too slack (which is what I'd suspect) and it might not be disengaging fully.
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jetblast787 · Derestricted Danger Joined: 22 Jul 2015 Karma :

Thanks for the replies guys; I've ridden it for another 2 weeks and during this time I've noticed that this only happens when the engine is cold, so after about 3 stop starts at lights it doesn't stall anymore. Anyone know what this may be?

chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

jetblast787 · Derestricted Danger Joined: 22 Jul 2015 Karma :

chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

Try raising it slightly see if it still cuts out.
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jetblast787 · Derestricted Danger Joined: 22 Jul 2015 Karma :

Robby · Dirty Old Man Joined: 16 May 2002 Karma :

How many miles on the bike?

From memory, CBF125s from around that time had an issue with fuel pumps. The best explanation that I heard was fuel dissolving some kind of protective coating on the inside of the tank (or maybe just paint) and fucking the pump. A replacement pump fixed it - the stuff that killed the pump was long gone by the time the pump failed - but Honda never issued a recall.

If your bike is low mileage, it is possible that the fuel pump is the original and knackered. It could also just be the way you're riding it. It's a 125, more revs, less mechanical sympathy.

jetblast787 · Derestricted Danger Joined: 22 Jul 2015 Karma :

Ariel Badger · Super Spammer Joined: 02 Dec 2006 Karma :

Do not fiddle with the mixture screw.

It only affects running at tick over and your revs do not drop to tick over when down shifting.
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chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

jetblast787 · Derestricted Danger Joined: 22 Jul 2015 Karma :

[quote="chris-red"]

chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

Say you are coming to a stop, you shouldn't just hold the clutch in and go down all the gears in one go, firstly it's good practice to use engine breaking. Second it can stall the bike.
____________________
Well, you know what they say. If you want to save the world, you have to push a few old ladies down the stairs.
Skudd:- Perhaps she just thinks you are a window licker and is being nice just in case she becomes another Jill Dando.
WANTED:- Fujinon (Fuji) M42 (Screw on) lenses, let me know if you have anything.