BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: Phil C on 07 November, 2021, 17:22:37

Title: Throttle stop screw (Concentric carb)
Post by: Phil C on 07 November, 2021, 17:22:37
Evening all, I seem to be going through the various parts of the carburettor one by one, asking advice on every single part as I go along. This time it's the throttle stop screw. I've put the carb back on the bike, and am looking to set the screw at a reasonable starting position from which I can tweak it later as necessary. I would have thought that having released the twistgrip and heard the throttle slide bottom out, I'd just be able to wind the stop screw in and feel it butt up against the bottom of the slide. But I can keep winding a long way feeling nothing. Could some take pity on me and explain this no doubt very basic thing? Thanks in advance.  Phil
Title: Re: Throttle stop screw (Concentric carb)
Post by: JulianS on 07 November, 2021, 17:37:34
You need to screw it up so that there is an air gap under the engine side of the throttle slide, easiest with carb off. I would allow about 2 or 3 mm. Then with it back on the bike adjust the cable back lash. With engine running then adjust the screw to give a fast tick over and when warmed up you can set it to give a reliable tick over. The Amal Hints and Tips leaflet quite uesful.
Title: Re: Throttle stop screw (Concentric carb)
Post by: Phil C on 07 November, 2021, 18:32:40
Okay, it's off with the carb again then! Thanks Julian.  Phil
Title: Re: Throttle stop screw (Concentric carb)
Post by: chaz on 07 November, 2021, 18:35:32
wouldnt be the first screw Ive seen with the tapered un threaded end broken off or shortened.
is yours complete?
http://amalcarb.co.uk/generic-600-series-mk1.html
Product No: 622/169
Title: Re: Throttle stop screw (Concentric carb)
Post by: Phil C on 07 November, 2021, 19:20:47
I'd better have a look at it!
Title: Re: Throttle stop screw (Concentric carb)
Post by: chaz on 07 November, 2021, 21:57:47
one Ive had in had the end plain bit missing so wound full in,, wouldnt touch bottom of slide.
likewise Ive had them bent so that they would not come out
Title: Re: Throttle stop screw (Concentric carb)
Post by: Mike40M on 08 November, 2021, 07:14:20
I've done initial adjustment as Julian said without taking carb off by using a thin welding rod under the throttle slide. Turning the adjusting screw in until the rod can move free. Not certain if it works on a B40 where it's not much space to get a bent rod in.
Title: Re: Throttle stop screw (Concentric carb)
Post by: Phil C on 08 November, 2021, 10:54:20
Mine's not quite the same as the one on the components list (thanks for that, Chaz) at the screwdriver end, but looks pretty similar at the pointy end. See photo. I think it's okay? I'll take the carb off again today and hopefully set it as advised by Julian.   Phil
Title: Re: Throttle stop screw (Concentric carb)
Post by: Phil C on 08 November, 2021, 12:44:03
This morning I've removed the carb and adjusted the throttle stop screw to a preliminary position, as advised by Julian (I  could see the benefit of Mike's suggested in-situ method but thought I'd take the carb off so that I could see exactly what was happening.) Tickled it, kicked it over and HOORAY! it started and kept going. But it was at very high revs, and very smokey (rich, I guess. Btw, I took a punt yesterday and set the needle on its middle groove.) Whether it will keep going when the revs are lower, I don't know. So anyway, two things to address: high revs, and smoke. I'm now wondering in which order, and how. Any advice please?  Phil
Title: Re: Throttle stop screw (Concentric carb)
Post by: Phil C on 08 November, 2021, 12:51:03
I'm thinking that to begin with I need to screw out both the throttle stop screw and the pilot air screw?  Phil
Title: Re: Throttle stop screw (Concentric carb)
Post by: chaz on 08 November, 2021, 15:23:49
dont worry about the smoke at the moment Phil.
as you have been having problems, it may have run overfuelled up to now with unburnt fuel sucked in when opening the throttle but killing the spark.
go for lowering the throttle screw first then when happy you can check the smoke/plug colour
Title: Re: Throttle stop screw (Concentric carb)
Post by: Phil C on 08 November, 2021, 15:34:31
Just seen your post, Chaz, having come in from the garage, where I've been adjusting the two screws. Now running quite  nicely (in my opinion!) and just the faintest bit of grey smoke coming out of the exhaust. Pretty chuffed really! I guess I'll now look at the plug. How long do you need to run an engine on a particular carb setting before the plug will give an indication of richness/weakness on the new setting?   Phil
Title: Re: Throttle stop screw (Concentric carb)
Post by: chaz on 08 November, 2021, 16:40:21
difficult to say, at higher revs you should get the fuel burnt off better, at lower revs the mixture needs to be right or plug fouling occurs.
when we were racing two strokes the norm was to do a plug chop before you got back to the paddock to avoid overfuelling. This resulted in many riders paddling the bikes back.
generally, ride as normal then check , its different if on the bench/ramp as no load on the engine.
Title: Re: Throttle stop screw (Concentric carb)
Post by: Phil C on 08 November, 2021, 17:19:01
I started with the throttle stop set as shown in the photo but, as I said, the revs were very high and there was a lot of smoke. By the time I'd finished adjusting the screws I'd unscrewed them both about a full turn. I don't know how much clearance that will have left below the slide. The bike seems to run okay in the garage, I keep the revs up for a while to begin with, then it idles okay
but after a while will conk out. The plug is black and oily.   

Any thoughts/comments anyone?
Phil
Title: Re: Throttle stop screw (Concentric carb)
Post by: Phil C on 08 November, 2021, 18:26:42
Btw, no smoke.
Title: Re: Throttle stop screw (Concentric carb)
Post by: chaz on 08 November, 2021, 19:43:42
plug fouling, would be interesting to try another dry plug to see if it runs straight away.
http://amalcarb.co.uk/downloadfiles/amal/amal_tuning_guide.pdf
http://amalcarb.co.uk/downloadfiles/amal/Mk1_Hints_and_Tips.pdf

obvious question, black plug, is it fuel or oil? over fuelling or rings?
Title: Re: Throttle stop screw (Concentric carb)
Post by: Phil C on 08 November, 2021, 20:51:55
Might be obvious to you Chaz! 😁

Before I had the conking out problem which brought all this about, I didn't have a black, wet plug. So I guess it's something to do with the carb. Maybe tomorrow I'll wind up the throttle stop screw a bit (that might help prevent the eventual stalling that's happening) and unscrew the pilot air screw a bit (which might weaken the mix a bit.)

Phil
Title: Re: Throttle stop screw (Concentric carb)
Post by: royblackburn1@btinternet. on 09 November, 2021, 00:41:20
I have always been told to take my bike for a run then check the plug condition when the bike has been under load not sat on the drive and just revving it hope this helps
Title: Re: Throttle stop screw (Concentric carb)
Post by: Phil C on 09 November, 2021, 06:32:09
I'll do that - thanks Roy.  Phil
Title: Re: Throttle stop screw (Concentric carb)
Post by: Phil C on 09 November, 2021, 15:57:12
Okay well I've been doing a bit of tweaking with the two adjusting screws on the carb and it sounds to be - in the garage anyway - running okay. I'll take it for a little run out tomorrow (if it's not raining) and see how it goes and then what the plug is showing.  I confess I've lost track of how far in or out the two screws are. Does that matter if it seems to be running okay?   Phil
Title: Re: Throttle stop screw (Concentric carb)
Post by: AdrianS on 09 November, 2021, 22:00:13
You adjust the screws to make the engine run ok. If the engine is running ok, the screws must be adjusted correctly!

When the engine is running and warmed up, screw in the throttle stop screw to give a fast tick over. Then adjust the air screw to get the engine to run as fast and cleanly as you can. Then back off the throttle stop screw and carefully adjust the mixture screw to give the smoothest running of the engine.

The mixture screw only affects tick over and low revs. The mixture at higher revs is changed by needle position, main jet size and slide cut away.