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Engine very snatchy on over run.
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mike p5xbx
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September 26, 2019 - 4:00 pm
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SpacedMarine said
So we all have blocked pilot circuits and it's nothing to do with fuel the reason we all run at 1/4-1/2 a turn?

If the pilot jet always runs at max flow if you turned it in it would make it weaker,
 

The idle mixture/pilot screw controls the amount of Air entering the mixing chamber
I have 4 bikes with the Amal concentric the 2strokes are particularly sensitive to the mixture setting re 4stroking/jerking on small throttle at 35/40mph even 1/16 of a turn has an effect
you need to get the engine hot to fine tune it and mine are between 1/8 to ¼ turn out, without any 4stroking/snatchy
I also think a lot has to do with the amount of wear in the carb needle jet and throttle slide

it wont be the pilot jet itself that has blocked but the mixing chamber when this happens you cant get a really slow tickover and you have to raise the throttle slide to keep it running

Amal600pilot.jpg

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D? - D10- D14 Bantams 350 AJS -500 Triumph http://bsanotru.....lfire.com/

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SpacedMarine
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September 26, 2019 - 4:37 pm
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So why does everyone end up using 1/4 turn instead of the 1 1/2 or whatever recommended in the manual.I can understand if the chambers a bit furrred up but what about on new ones?

There must be some fuel element to it?

The amal page says use a number 78 drill bit to clean the passage out I wonder if that would make any difference though as the mixing area looks like it's part of the casting and no amount of drilling would clean it out.

What's 7/16 in mm again?

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Stoo63
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September 26, 2019 - 4:44 pm
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Brilliant post, Mike. ! Clear and informative. So much easier to see it illustrated rather than reams of fractions and twiddling tip-my-hat

'52 D1 direct lighting plunger, '58 Square Four (project), '59 D1 direct lighting plunger,  '59 Tiger Cub, '60 5TA,  '76 FS1-E

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mike p5xbx
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September 26, 2019 - 5:19 pm
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SpacedMarine said

The amal page says use a number 78 drill bit to clean the passage out I wonder if that would make any difference though as the mixing area looks like it's part of the casting and no amount of drilling would clean it out.  

the diagram about shows the idle jet in the carb casing which is only on 4sroke carbs and the B175 bantam
all the earlier D14 etc have the jet screwed into the bottom of the carb case, Red colour hole top left, bottom photo

D? - D10- D14 Bantams 350 AJS -500 Triumph http://bsanotru.....lfire.com/

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Piquet
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September 26, 2019 - 5:38 pm
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SpacedMarine said

The amal page says use a number 78 drill bit to clean the passage out I wonder if that would make any difference though as the mixing area looks like it's part of the casting and no amount of drilling would clean it out.  

Yes, that is why one of the Documents details removal of the blanking plug to help clean it out. Personally I have never resorted to that but I have had to use some flexible copper wire to manoeuvre around the right angle bend/s in the gallery to clean it out along with various solvents etc.

Unfortunately, because you are working blind you can never be sure it is completely clean.

I'm not a complete idiot ............................................ some parts are missing.

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SpacedMarine
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September 26, 2019 - 6:18 pm
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Just out of curiosity.Looking at the middle picture the mixing chamber has holes either side of the throttle slide.Does this mean that provided the passages and holes are clear the bike should idle with the slide completely bottomed out in the carb.ie. with the slide screw all the way out?

Or does the slide need to be raised slightly in any case?

What's 7/16 in mm again?

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cocorico
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September 26, 2019 - 8:15 pm
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b175er said
I think that you may be misunderstanding the problem Cocorico. The "snatching" (on both of my bikes - and Nick's, I reckon) happens in all but top gear when the throttle is closed off and the bike feels as though it's being shunted forward in a very jerky and uncomfortable manner. ...

Very possibly - I'll leave you all to sort it out then. mesmerised

1956 D3 running, lights to sort. 7 other bikes in the Barn, 6 runners (when I get time!) and a still in progress Morini 250

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SpacedMarine
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September 26, 2019 - 8:44 pm
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TAXI!

What's 7/16 in mm again?

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cocorico
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September 27, 2019 - 8:14 am
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Call me a taxi if you like - I've been called worse things...

1956 D3 running, lights to sort. 7 other bikes in the Barn, 6 runners (when I get time!) and a still in progress Morini 250

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SpacedMarine
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September 27, 2019 - 9:55 am
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I don't know what I was thinking but when you see how it works its obvious.Like Stoo said a picture paints a thousand words!

What's 7/16 in mm again?

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carpetralph
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September 27, 2019 - 12:32 pm
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b175er said
I've always been under the impression that the amount of petrol/oil going through the pilot jet is constant and that the air screw  controls the amount of air passing over it and taking it into the cylinder. However, I'm not the most mechanically minded person so maybe I've got it wrong !  

Yes, according to the BSA Workshop manual the air screw controls the vacuum over the pilot jet, letting in more air by screwing the adjuster out bleeds more atmospheric air in and reduces the vacuum over the jet, so weakening the mixture. In for richer, out for weaker.

Sorry, posted this while halfway down page 2. Reading on it has been admirably explained. Shame someone had to saw a carburettor in half to resolve it but at least its clear now.

I`ll get my coat...…...

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carpetralph
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September 27, 2019 - 5:57 pm
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Took the carb off again this aft, just to make sure the pilot air and fuel passages were clear having seen the cutaway and knowing better what I needed to do.

I blocked the air passage by simply screwing home the adjusting screw and squirted carb cleaner through the pilot jet and made sure it was coming out of both tiny holes in venturi. Did the same again with the air screw backed off and the pilot jet covered with a finger, squirting this time down the air inlet tract, and again got cleaner coming out of both venturi holes, so I`m as certain as I can be that there are no blockages.

Made sure the slide was shutting fully, then cracked it open just a fraction with screw before re fitting carb. I closed the air screw fully to give richest possible mixture.

Bike started straight away, and half a turn on the throttle stop screw had it ticking over like a dream, but seemed a bit rich as it was smoking a bit, so cracked the pilot air screw about 1/8 of a turn, still ticking over ok and responding to the throttle with no hesitation.

Peeing it down again so have not taken it down the road, but hoping its ok now. Before when it was revved it took a while to slow down to idle, and it was four stroking as it slowed, but now it seems to slow faster with no nonsense to idle.

Will report after a run. 

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