Engine reving high with throttle closed
 

On a R80G/S of a friend it is near impossible to start and once running it keeps cutting out when idling, and when opening the throttle, or sometimes on its own, it pics up revs to about 3000 to 4000 rpm and will not return to idle after closing the throttle. Sometimes the only way to stop the high revving is to cut the ignition.
Does someone know about this problem?

Wild stab in dark - stuck choke cable?

Simon

I dont exatly know type of this bike. But i think that it is little bit older model with carburettors. I think that there is a membran (diaphragm) on top of the carburettor. Check those from both sides.

I have the same problem with my NX250.Starting is ok.But it doesnt turn idle sometimes,it stuck on 2500-3000 rpm.When i hit the throttle instanly and release it;it turns to normal idle speed.Bike has a piston carburettor.

Throttle fix
 

I would suggest the following:

Check both throttle cables are not hooked up somewhere, especially at the carby.
Take the carbies off and at least partially diassemble them. You're looking for the slides sticking for some reason.
While they're off, clean them (get yourself a maintenance manual).
Set the cable lengths up
That will probably do it - they're really not that complex.

Diaphragm replaced recently, one with really big hole but the symptoms was rather a loss of power at higher revs and the carbs were running really out of balance.

Choke cables stuck can give the same symptoms but I have checked that and even replaced the gaskets on the enrichiner (Choke) housings as well.

When the carbs were off I went through them to check that everything was OK and I always push the piston up to see if they return to their normal position without getting stuck, I do this with the carb assembled by sticking my finger in from the air box side. You can also check that the small tabs on the diagram has been correctly located with assembly as the half moon shape at the bottom of the piston will not be central in the throat of the carb.
I also make sure the cables is not stuck but everything OK here when the bike start to pick up revs on its own or keep revving when you close the throttle.
Impossible to balance carbs because it either dies or start to rev.

Thanks for the ideas but I think that this problem can also be electrically related by not having a regular spark????

I would have checked valves and timing.
Try to retard the ignition, might be bad springs or dirt in the ignition sensor.
http://www.realoem.com/bmw/diagrams/g/b/1.png

Checked the timing and replaced the timing trigger or advance unit with other one.
You hit the Nial on the head with the valves though AliBaba.
By the time I suggested to recheck the valves the motor was already too hot and my friend did not believe it could have been the valves because we adjusted them only 1000km before, so we moved on to check other stuff.
This morning I decided it will be the first thing I do, and surely the exhaust valves were both not closing properly and even the inlets were less than the normal 0.1mm setting.

I checked the choke and throttle cables,but there is no problem with them.Will any tear on the accerelator pump's diaphram cause it ?

A torn diaphragm will more likely make the engine run out of balance as the rpm increases and at higher revs you will feel a loss of power or sluggishness.

I would have plonked for the previously mentionted auto advance mechanism, probably sticky or weak/broken spring(s)

Back to basics
 

When having a problem it is so easy to let yours, and others minds go into overdrive, A mechanic friend of mine gave me a good bit of advice once and that was "go back to basics" Everyone has an opinion, from Choke to Valves, before long it will be your make of tyres!. Everyone means well, but go back to basics and be 100% sure all the cables run smooth before getting into large expense trying to check/repair/replace costly items, do you have any friends with identical bikes that you could swap carbs with if the cables are not the problem? all the best.

We are not into overdrive here :biggrin3:

Case is closed, read reply #6……

I have 3 bikes being converted to HPN models at the moment so I have tons of parts that I could use to check with, a proses of elimination if you like.
Problem was that I did not start with the basics first, or shall I say not all the basics. I checked the carbs, throttle and choke cables, coil leads and wiring to the coil and the spark and spark plug gap. But one crucial element was the valve clearance or valve gap, this you have to check first - always - because the moment you started the motor you can't go back or else you have to wait a hour or two for the motor to cool down as the vale clearance can only be checked with the engine cooled down.

It gives you room for a lunch at Mr Pickwicks (Long Street). They used to have nice chocolate cake. :thumbup1: