Carb throttle slider needle wear

Would tend to run rich.

As Tony says — and will hunt and generally be unhappy at moderate throttle openings, symptoms which will disappear as you open the throttle.

Note that if your needles are worn, so will the jets be.

If the slides are knackered they will also do in the needles and their jets.

If you are lucky and the bores for the slides are not too worn, Burlen's hard anodised slides may sort out rattly slides (they did for me).

When Ethanol was introduced by stealth in 206/7 I had only used two of my singles  and both had increased needle wear. New carb in one and new needle in the other + I removed the pet seal. The gunge didn't help either. All well now with E0.  That's my view, your choice.

I tried new needles and jets  and still suffered both weakness off the pilot and then richness . Eventually a 30 year old (or more) rusty and stained needle somehow  solved both issues . Despite having the correct markings it was dimensionally different to the new ones , Perhaps it will last me out !!. 

The needles and needle jets should not make much difference at full throttle where the main jet is in use . Could be you are suffering from dirty plugs that are missfiring . What main jets and what plugs are you using ?.

Chris,

You say you have stuttering at full throttle at 75+; will the bike accelerate? If so, does the stuttering clear up?

Sounds to me like a plug chop is in order.

Julian

It gets to a point where you can't set the carb up or if you, it soon needs doing again. The problem is compounded as indicated above, as a worn carb often allows easy starting, which owners confuse as being OK. 

Yes, worn carb first sign in my experience is the lack of the need for choke.

Robert, stainless needles  are a good, very good idea. Where do I get them from?

NB: My Esso station says they hey have just stared including a small amount of Ethanol.  Why?

Just after I filled up my two seater!

Needle jets being brass do wear. However I have never known a needle to wear in all my years of motorcycling. The first sign of needle jet wear is increasing petrol consumption followed by plugs getting progressively sooty. You can of course compensate by lowering the needle a notch until you run out of notches then it really is time to buy a new needle jet.   

I agree with Gordon,needles hardly wear over years but needle jets can wear in no time , half a thou will impact fuel consumption and the plugs  . But I don’t think that’s the posters problem at all. Sounds more like fuel starvation which could be something as simple as a blocked tank cap or delivery from taps or flow through the float valves. A weak link in the ignition chain will also snuff out the spark at big throttle pressures.

The ones in place when I first acquired my Fastback were in a terrible state — see my earlier post in this thread, and the pictures attached.

Yes Julian, mine looked the same as your top photo after I got the goo off, caused by you know what and wear of course  But where can a but a stainless needle? that has to be the answer.

If the slides are badly worn then they put pressure on the needle in the jet ,then you get accelerated wear on both.If using a slightly worn slide (or body ) a smear of molibdimen sulphide grease on the slide is helpfull in reducing clearance and wear. It will affect the mixture slightly.

Sounds like insufficient fuel delivery, check the bore of taps and tank , a tank coating by a previous owner had half blocked the taps on one of my bikes .Consider replacing fuel valve with alcahol version .

Your symptom is not a usual one , try dropping the needle a notch . How do the plugs look if you cut the ignition when the problem occurs and coast to a halt  then inspect ?. Do the plug chop before dropping needles ,dont risk a too weak mixture seizure.

As Mr Tuck said.

It’s worth checking how much petrol comes through the tap in a minute and then how much comes out of the float bowl plug of each carburettor in a minute.

I think you would want to see over 300 ml/minute total from the carbs.  Anyone disagree with that figure?

Amal jet sizes usually equate to how many cc's of fuel per minute they will pass. 

'72 Commando Combat 230 main jet will pass this figure in one minute at full throttle.

One would assume that the float needle orifice will pass a percentage more fuel than the designated main jet in any given carburettor fitted to it.

Who, other than the racing fraternity will want maximum flow of fuel other than the original jetting arrangement supplied?

Just make sure that the float valve is clear of any crud which also applies to all other jets and galleries within the system.

There isn’t really any need to assume, when measuring is fairly easy.

Fuel supply restricted by tap design, choked gauzes, blocked tank cap vent and/or float valve  issues is a common and sometimes serious problem on big British bikes.

Fuel reaches the float chamber by gravity feed.  Fuel is pushed through the main jet by the difference between atmospheric pressure and venturi pressure.  Those are different forces.