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Engine very snatchy on over run.
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nickjaxe
Runcorn Cheshire UK
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September 26, 2019 - 9:17 am
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My speedo used to do that James....till I discovered the knurled nut had a long way to go before the cable was fully snugged in the speedo head.

Nick.

My Bantam video              https://www.you.....jpOFmzRZRI

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b175er
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September 26, 2019 - 9:26 am
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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. The only improvement I've been able make is to shut the air screw almost to nothing on the side of the carburettor. This problem's been discussed before and nobody seems to have come up with an answer, unfortunately.

two B175s and a CB360

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SpacedMarine
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September 26, 2019 - 10:17 am
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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. The only improvement I've been able make is to shut the air screw almost to nothing on the side of the carburettor. This problem's been discussed before and nobody seems to have come up with an answer, unfortunately.  

Correct me if I'm wrong but the "air" screw controls the amount of fuel rather than air on a bantam?If you're having to turn it all the way in it means that its therefore rich?Maybe a smaller pilot jet or larger slide cutaway to lean it out is the answer if that's the case?The jerking maybe extra unburnt fuel burning in the exhaust causing an "afterfire"?

What does your plug look like?I'd ask if the choke was working correctly but assuming it's on a closed throttle the choke will be out of the way anyway?

Its possibly a symptom of modern fuels I know mine only likes about 1/4-1/2 turn on the pilot screw.

What's 7/16 in mm again?

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b175er
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September 26, 2019 - 10:41 am
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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 !

two B175s and a CB360

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SpacedMarine
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September 26, 2019 - 10:59 am
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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 !  

It would be if there was no screw.The amount of air passing over the jet is constant at any given throttle opening but the amount of fuel is metered by the screw.I reckon!

What's 7/16 in mm again?

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Piquet
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September 26, 2019 - 11:08 am
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Just looked at the Tuning secrets document in the General Tech download page and it says

The pilot (or the slow‐speed, idle) circuit has two parts:
1. an air passage that is adjustable by the screw on the side
2. an internal gas passage with the fixed pilot jet. This jet is a small brass bushing with a tiny 0.016" (16 thou) orifice
that is a press fit opposite the air adjusting screw. Gas is brought up from the the float bowl and travels toward the
front and is metered before it mixes with the air in the pilot mixing chamber.

Another document refers to the 'Air adjusting screw'.

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

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b175er
saltfleetby
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September 26, 2019 - 11:09 am
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O.K. That's food for thought for me. I told you I wasn't mechanically minded !

two B175s and a CB360

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SpacedMarine
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September 26, 2019 - 11:15 am
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Piquet said
Just looked at the Tuning secrets document in the General Tech download page and it says

The pilot (or the slow‐speed, idle) circuit has two parts:
1. an air passage that is adjustable by the screw on the side
2. an internal gas passage with the fixed pilot jet. This jet is a small brass bushing with a tiny 0.016" (16 thou) orifice
that is a press fit opposite the air adjusting screw. Gas is brought up from the the float bowl and travels toward the
front and is metered before it mixes with the air in the pilot mixing chamber.

Another document there refers to the 'Air adjusting screw'.  

I'd say the air adjusting screw was the throttle slide as it's the throttle slide that controls the airflow?

What's 7/16 in mm again?

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Piquet
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September 26, 2019 - 11:20 am
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List of parts for the Concentric says

20 Throttle adjusting screw (# 622/077)
21 Pilot air adjusting screw (# 622/076)

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

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SpacedMarine
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September 26, 2019 - 11:34 am
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Colour me confused 🤣If the plug is sooty I'm betting it confirms my theory!

What's 7/16 in mm again?

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SpacedMarine
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September 26, 2019 - 11:38 am
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Piquet said
Just looked at the Tuning secrets document in the General Tech download page and it says

The pilot (or the slow‐speed, idle) circuit has two parts:
1. an air passage that is adjustable by the screw on the side
2. an internal gas passage with the fixed pilot jet. This jet is a small brass bushing with a tiny 0.016" (16 thou) orifice
that is a press fit opposite the air adjusting screw. Gas is brought up from the the float bowl and travels toward the
front and is metered before it mixes with the air in the pilot mixing chamber.

Another document there refers to the 'Air adjusting screw'.  

So gas is brought up from the float bowl and travels forward where it is metered?metered by what?the pilot screw!The Bush is in the float bowl.Can you post a picture of the parts piquet?

What's 7/16 in mm again?

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johnsullivan
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September 26, 2019 - 12:23 pm
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** Please log in to view **    Burlen's site is invaluable.

67 D10. and a D7    2007 Honda Hornet FA. Honda CD200 81 93 Yamaha TTR 250 Raid, Sinnis SC 125.   75 Montesa Cota 247 an electric scooter of Famous make.

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Piquet
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September 26, 2019 - 12:38 pm
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There are two versions of the Concentric, one has a Pilot Jet the other has the Bush. One or other is fitted in the gallery from the float chamber so they serve the same purpose. In the version with the Bush it is fitted directly opposite the 'Air Screw' (aka 'Mixture Screw') so that the Fuel (=gas) is drawn through it and is then mixed with the air metered by the screw.

The carb documents are available to all Logged in Site & Club Members under General Tech, here

** Please log in to view **

The diagrams and working descriptions can be seen in those documents.

hope-that-helps

ps.

My D14 & B175 'snatch' to some extent too! It seems to be a trait of the later models and all you can do is to minimise it as much as possible.

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

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SpacedMarine
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September 26, 2019 - 12:43 pm
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Wierd they refer to it as the pilot air screw.So does turning it in make it richer or weaker is the question!If its metering air youd think turning it in would make it richer but the lower airflow would mean less fuel is drawn through which would make it weaker 🤯

What's 7/16 in mm again?

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b175er
saltfleetby
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September 26, 2019 - 12:51 pm
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So, the further the screw's in the less air's going in and the richer the mixture !

two B175s and a CB360

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SpacedMarine
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September 26, 2019 - 12:53 pm
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b175er said
So, the further the screw's in the less air's going in and the richer the mixture !  

No because the jet/Bush can flow a maximum of 20cc so with no airscrew it would theoretically flow 20 cc.If you turn the screw in it would reduce airflow and therefore less fuel so you are making it weaker by turning it in.venturi effect?

It is a bit confusing 🤣

What's 7/16 in mm again?

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Piquet
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September 26, 2019 - 12:55 pm
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b175er said
So, the further the screw's in the less air's going in and the richer the mixture !

Correct, screwing it in richens the mixture.

I presume that is why, when the pilot gallery is clogged, if you can get the bike to run at all, the screw invariably has to be screwed right in.

EDIT:-

I think that if the gallery/bush is clear, fuel gets pulled through regardless of where you set the air screw. It's not the air flow past the Air Screw that pulls the fuel through, it's the vacuum in the venturi. The Pilot circuit is not completely separate from the rest of the system.

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

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SpacedMarine
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September 26, 2019 - 1:14 pm
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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?

I'm not being funny I'd just like a definitive answer one way or another as most people say they use about 1/2 turn on the pilot screw even people who have new carbs so the common denominator is blocked pilots and not the fuel we are all using?

My bike was rich everywhere until I used a smaller main jet,put the needle on the lowest setting,turned the mixture screw in and shaved some off the throttle slide all in an effort to make it leaner.If the pilot jet always runs at max flow if you turned it in it would make it weaker, there has to be a certain amount of air velocity to pull it through surely?Are you not altering the pressure difference above and below the orifice in the carb mouth by adjusting the screw?

Just re-read that and I'm confusing myself 🤯

What I'm trying to say is it's a balancing act as once you've screwed the pilot out past a certain point it makes no odds as the jet can only flow a certain amount anyway so if modern fuel likes a weaker mixture you would have to make the mixture leaner to compensate which is why everyone runs 1/2 a turn out on the pilot jet?which is less than 1 1/4 or whatever the manual say?which by my logic means you screw it in to make it weaker?

I could be wrong but I don't think I am dunno

What's 7/16 in mm again?

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b175er
saltfleetby
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September 26, 2019 - 1:35 pm
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You've confused me, too; this is getting too complicated so I'm out of this thread !

two B175s and a CB360

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SpacedMarine
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September 26, 2019 - 1:41 pm
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b175er said
You've confused me, too; this is getting too complicated so I'm out of this thread !  

I don't blame you 🤣🤣🤣 like I say I could be wrong.Wheres coco when you need him he'd know!

20190926_134710-1.jpg

My apologies!sorry

I'm guessing my pilot jet is blocked 🤣🤣🤣

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What's 7/16 in mm again?

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