KX 125 over-revving?

[liverpoolphil2130]

Hope you can help me with this

My 87 KX is currently running the best it has since I have had it (new cylinder head, powervalves, carb service, air filter etc) BUT it seems to rev past itself at 3/4 to full throttle. Its great going through the gears (in terms of power) but its seems to be most noticable in 5th and 6th when, if you give it too much throttle, it revs much higher than it should

This problem can be over come by simply going easy on the throttle and being carefull not to give it too much through each gear but I am wondering if this is 1) normal? and 2) if anything can be done to sort it?

I have tried the timing both advanced and retarded (and inbetween) and this DOES have an effect, but only on power delivery and not the over-revving problem

Any advice appreciated

 

[CaptainObvious]

Two-stroke engines can not over-rev. Short of down shifting from 5th to 1st, RPM is limited to the engines ability to scavenge expended gas. If your engine continues to rev beyond where you think it is normal, you old top-end was more worn-out than you thought.

Have you examined your spark plug? It is possible that your main jet is too lean, giving you a lean surge. If this is the case, don't allow this surge to happen, you could seize your engine.

How is your throttle response? It is possible that your pilot jet and needle are to fat, making the main seem lean.

Of course, now that you have replaced all of those worn-out parts, you may not remember what it feels like when the bike gets on the pipe.

 

[steve.emma]

i have had simaler problems where by you get too much revs for the amount of throttle that you give it. often this is caused by an air leak into the cylinder, the air bypasses the throttle slide which raises the revs and makes it run lean and bog. if its sitting and you rev it it can take ages for the revs to come down to idle.when you ride it you get the bog and flat spot.
the most common cause of this is a crack or hole in the rubber manifold boot between the carb and the head,either this or the gasket for the reed cage may be leaking.
good luck.

 

[liverpoolphil2130]

thanks Steve.emma

Thinking about it, I know that the gasket holding the left KIPS cover is not the best (due to the amount of times this has been fastened and removed fromt he cylinder), could it be this?

Going to check it over this afternoon, a guy on another forum suggested the clutch may be slipping, whats your view on this?

Maybe a new gasket set is in order

 

[steve.emma]

its hard to diagnose stuff from peoples descriptions of their problems... i should have thought of a slipping clutch that really could be your over revving problem. in a bike a slipping clutch ie pretty much the same symptoms as in a car.
ride in a straight line try being in 3rd or 4th gear just at the begining of the powerband and then open it right up.
if its a slipping clutch the revs will start to skyrocket but the bike wont accelerate. in this case check clutch adjustment then clutch assembly (springs,steels,friction plates...) often in old bikes the clutch springs have lost tension and this causes it.
steve.

 

[liverpoolphil2130]

thanks steve - after a couple of weekends riding I can elaborate on this a little more

It seems to only over rev when cold - i.e once it warms up, it runs great. An air leak is a possibility but would this disappear once the bike was at operating temp? I am suspecting the clutch as this is the only part of the bike I have not had to repair yet, is this a difficult/expensive job?

Also I have notice lately that when cold the bike finds it difficult to start with the choke on, close the choke and after a couple of kicks it fires up, I know this probably isnt right but I am I doing any damage by not using the choke?

 

[dirtbikr99]

I have actually owned a 87 kx 125.
If your clutch is messed up i have a whole clutch if u need one perfect working condition....some knotches have started on the basket but still works good

How many other bikes have u owned? I have a feeling nothing is wrong with the bike, i belive that bike has very close gear ratios which if making you feel as if it is revving out to far.

It could be a combonation of the clutch slipping and not very much low end on that bike so it bogs a little then hits powerband and the revs go way up making you think its over revving

There is also a chance you have an air leak

could it be that your silencer packing is all blown out cause the bike to seem very lound making you think its over revving
now that i think of it, doe sthat bike even have a repackable muffler, the one on my bike did not seem rebuildable. there is a chnace that maybe someone just put on a diffrent silencer from another modle vehical (on my bike that is) tho


I just want to say I am not a mechanic, i may be totaly wrong. Those r just a few sugjections because i have owned that model bike.
If you ever need any part for that bike i have all for free if u want them

 

[liverpoolphil2130]

thanks a lot for your post dirtbikr99, appreciate it.

The suggestions you have made are valid and I feel you are about spot on with your comments.

I dont think there is anything majorly wrong with the bike and after further inspection I noticed that once it is warmed up, the over revving problem goes. I did find a minor air leak in the left KIPS cover so this could well have sorted it. I have also re-packed the muffler recently so know its not related to this.

I would be very interested in the clutch you have (even if its just a spare for my own) are you serious when you say I can have it for free?? What about postage and packing (it will be going to Ormskirk UK) I would feel like giving you some money at least for the parts, what do you have??

Thanks again for the post

Speak soon

Phil