Sticking Throttle & Float needle

[Arthur Farrow]

My bike has developed a habit whilst stationary of throttle sticking and the revs go wild! This is a relatively small opening. Only way to stop is to cut the ignition. Has not yet done it in motion, but then I've only manged about 50 miles before the next lot of trouble shooting

The carb is in good order and the slide clean. I've read about the issue on other posts but as if often and annoying the tread fizzles out with no report of cure. One fine fellow said use as stronger spring ( mk 1 concentric) and another reported the chrome slides don't stick . Mine is the anodised alloy version.

What is the latest knowledge on this issue please.

BTW : I do have a cracked inlet rubber ( new one on order)

Also had a problem with that wretched carb flooding into the engine when stopped with petrol on. Discovered my Carb (Mk2 34mm 2900 series) had the very small brass needle as fitted to monoblocs and Mk 1 concentric whereas normally it looks like the needle is a big Ally one . I bought a new float bowl to match the big needle in the stay up kit I bought ( RGM Norton was much cheaper then Burlen) as I read somewhere needles can stick and the ally one is the cure. Maybe mine being an early Silk it had one of the first made Mk2s (engine is no 4) Anybody any thoughts here. So far so good.

Naturally i asked Burlen by e mail several times and left message, but as usual their post sale customer service is dreadful so no reply. Would not mind but in the last year have bought 5 new carbs from them . Matchless, Black Knight and a Commando.

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[hendred]

Yes, they do that!  I believe it is cause by there being very like gap between the induction strokes on a 2 stroke twin breathing through a single carb.  This causes the slide to stick at small throttle openings when the "pull" on the slide is greatest.  My Silk did this and on one occasion got up to 10,000 rpm before switching it off!  My bike hadn't run for 20 years and the throttle bore and slide had a very light film of corrosion on them.  I polished the carb body and the slide (original alloy slide) and put a MKI concentric spring inside the existing spring.  Never stuck since.  |I believe the brass or chrome plated slides help, they are heavier too which will help with closing.  I also think drilling a hole in the top of the carb cap is a waste of time, I don't see how a vacuum can build up above the slide.  Too loose a fit in the bore and there are holes for the cable in the middle of it.  Out of interest, some time ago somebody fitted an SU carb to a Silk and reportedly it ran well.

I put the flooding of the carb down to the angle of the carb when on the side stand. The float and needle will tend to fall to one side and there is just too much friction for the float to push the needle firmly into the seat. Doesn't happen on the move because the forces acting on the float essentially go through the vertical plane (at rest) on the centre line of the bike (centrifugal and gravitational forces balancing out).  I suspect it wouldn't happen on a paddock or centre stand (there's a project for you).  

Al the joys of owning and riding a Silk.

[Arthur Farrow]

Feedback !

I fitted a new spring and a new Chromed Brass slide which is a heavy thing, but seems to have made no difference to throttle feel. So far it has not stuck and the interesting thing is there are 2 very small holes near the orifice for the needle. There can be no reason other than to relive pressure ( - or +) above the slide. I note Kev said a while ago drill a tiny hole to relieve pressure. Sounds daft, but maybe he was onto something.

These slides are crazy expensive and as yet still no response from Burlen to my questions. Not surprised and disappointed as before.

So far with the new float bowl and large diameter needle no fuel flooding, but I am switching fuel off and trying to drain fuel before parking up. Not helped by fact on my early bike tap is behind locked side panel.