Rev limiting issue

[danspratt90]

I have a Derbi Mulhacen 125, 15k miles 2012. It has a rev limiter set at 11,500 and will easily do that in neutral or with the clutch pulled in. But under load on the road its as though the limit is reduced to 10,300 - 10,500. Can anyone speculate as to why its doing this?

Thanks

[WD Forte]

Load
____________________
bikers smell of wee

[danspratt90]

I should add, it didn't always do this.

[jaffa90]

Increased load.

[Old Git Racing]

Powerband is worn out, put a new red one in.

OGR
____________________
2022 Tiger Sport 660 2019 Street Twin 2003 K3 gixxer 1000 1998 Srad 600 track hack

[andys675]

someone has stolen the platinum out of your spark plug
____________________
Some people feel the rain, others just get wet.

[Rogerborg]

What happens when you hit that limit in neutral, and while under load?

Chain is slack.
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[danspratt90]

@andys675 its a 4 stroke.

@Rogerborg hitting the limit in neutral happens exactly as you would expect it to, the revs rise linearly up to the red line of 11,500 and stay there. But when in gear the bike will start coughing and spluttering at 9,500 like im rapidly getting on and off the throttle. But will slowly make its away to 10,300 and not go any higher.

Its smooth and totally normal at revs under 9000.

[RhynoCZ]

Dirt/water in fuel? Clogged filter? Stuck choke?
____________________
'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[stevo as b4]

Why do you want to rev it to 11500 through the gears under load anyway? There'll be fuck all power after five figures, so you'll go slower, probably alot slower if you bend a valve or throw a rod.

Also forgetting specific rpm numbers, go and have a think about how an engine works and breathes both under load and with no load on it.

If you think about cylinder filling, dynamic compression ratio, and gas flow through the engine, then it should be pretty obvious to you why the maximum rpm the motor will reach is different between no load revving and full load acceleration.

If you don't understand this, you shouldn't be working on bikes or worrying about how they run IMO.

[danspratt90]

An explanation of why to shift at the redline:
https://www.youtube.com/watch?v=zZBqb0ZJSwU

My bike had power after 10k when I first got it.

Vehicles have red lines for the engine own safety, it would have a lower red line if 11,500 was unsuitable.

The engines power would be terrible all through the rev range if it had a bent valve.

"If you think about cylinder filling, dynamic compression ratio, and gas flow through the engine" I did not make this engine. i would hope the designer of the engine would have thought about that. An engine should reach its designed red line regardless of load.

It has an auto choke, which turns off when warm (signaled by a drop in idle speed).

Fuel is fresh, and flow is fine through the pet cock.

A video would probably be best to show the problem, but I dont have any kind of action cam...

[Pete.]

It might be that with no load the engine revs so quickly that the limiter is not killing the spark/injectors fast enough once it senses the cutoff rpm.

There's no good reason to rev the engine to the limiter with no load on it. It's a good way to break something.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[NJD]

The biggest problem with the bike will be the abuse the owners giving it if you carry on revving the tits of it all the way to redline in neutral, what's the point?

Just get on and ride the bloody thing.
____________________
The do it all, T̶h̶e̶ ̶b̶r̶o̶k̶e̶n̶ ̶o̶n̶e̶,̶ ̶T̶h̶e̶ ̶b̶i̶g̶ ̶l̶u̶m̶p̶,̶ ̶C̶h̶o̶n̶g̶ ̶N̶o̶o̶d̶l̶e̶

[danspratt90]

I dont understand why you think that i just rev it to the red line in neutral on the regular. Its purely for testing purposes.

you all have plenty of things to say about how to treat the bike and no suggestions on what it might actually be.

"Just get on and ride the bloody thing." Yeah i would love to, if it didnt have a very unsettling cutting in and out at high rpm!

[RhynoCZ]

[stevo as b4]

From Google it says your Derbi makes 15bhp at 9250rpm. The power will be dropping to shit by 10300, and 11500rpm is fantasy land, and or rod bending territory. As others have said the tachometer might not be accurate to 500 or even 1000rpm.

Add that that, you can always rev higher in neutral to the point of fuel or spark cut, or on a bike with no limiter higher than the engine will ever run under load. You don't need good cylinder filling and breathing under no load, but a loaded engine needs to be able breath in and expel enough air to keep up with crank speed. When it can't do this the power drops away and the engine won't pull anymore.

Im not 100% saying that a std Derbi engine is not safely capable of 11500rpm, but a 15bhp 125 peaking at 9250rpm, won't breath well enough to do it.

With tuning and a hairy cam profile you might be able to push the useful power that high up the Rev range, but on a std engine forget it and learn how to change gear.

[Kawasaki Jimbo]

He did more or less say it used to rev all the way.

[uberkron]

In my own meandering experience with 125, limiter above 2nd gear is usually not going to happen. I have found 5th gear is an overdrive that keeps the revs down and torque up so you aren't ragging it in 4th.

If it used to have more power check valve clearances. If they close up it won't rev the same and if they open up it won't rev out.

Give it a service.
____________________
i ride a zip 50 2smoke. It overtakes a hundred times a day.

[Teflon-Mike]

[Kickstart]

[danspratt90]

i would seriously like to know why you all think i rev the nuts off it on the side stand? I have done that a couple time just to see what happens for up to 1 second.

@teflon-mike you have missed the point of my question entirely. what a waste of a post.

[Kickstart]

[Pete.]