Author Topic: Throttle stop screw (Concentric carb)  (Read 874 times)

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Phil C

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Throttle stop screw (Concentric carb)
« 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

JulianS

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Re: Throttle stop screw (Concentric carb)
« Reply #1 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.

Phil C

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Re: Throttle stop screw (Concentric carb)
« Reply #2 on: 07 November, 2021, 18:32:40 »
Okay, it's off with the carb again then! Thanks Julian.  Phil

chaz

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Re: Throttle stop screw (Concentric carb)
« Reply #3 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

Phil C

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Re: Throttle stop screw (Concentric carb)
« Reply #4 on: 07 November, 2021, 19:20:47 »
I'd better have a look at it!

chaz

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Re: Throttle stop screw (Concentric carb)
« Reply #5 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

Mike40M

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Re: Throttle stop screw (Concentric carb)
« Reply #6 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.

Phil C

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Re: Throttle stop screw (Concentric carb)
« Reply #7 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

Phil C

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Re: Throttle stop screw (Concentric carb)
« Reply #8 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

Phil C

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Re: Throttle stop screw (Concentric carb)
« Reply #9 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

chaz

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Re: Throttle stop screw (Concentric carb)
« Reply #10 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

Phil C

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Re: Throttle stop screw (Concentric carb)
« Reply #11 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

chaz

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Re: Throttle stop screw (Concentric carb)
« Reply #12 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.

Phil C

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Re: Throttle stop screw (Concentric carb)
« Reply #13 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

Phil C

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Re: Throttle stop screw (Concentric carb)
« Reply #14 on: 08 November, 2021, 18:26:42 »
Btw, no smoke.