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Why would an air screw not have any effect on running?

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Iain.
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Joined: 04 Dec 2014
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PostPosted: 10:59 - 06 Feb 2015    Post subject: Why would an air screw not have any effect on running? Reply with quote
As title suggests really...what'd cause that?

I can run the screw all the way in, and all the way out with no change to tickover on this new carb. It's a 21mm Motoforce...looks like some sort of Dellorto clone & takes 5mm Dellorto jets.

https://www.pedparts.co.uk/c/products/lg/Motoforce_Racing_21MM_Carb_Kit.jpg

Quote:
21mm Motoforce racing carb kit, these are high quality carbs of significantly better quality than most other carbs in their price range.

They include fuel, oil and vacuum connections and 2 overflow nipples. The carb comes as standard with a cable choke adaptor and air filter adaptor allowing you to use different size filters.


Obviously it changes massively when you take the screw completely out of the carburettor due to huge air leak, but it's pretty bizzare & I can't help but wonder what is going on. Should note that it has a Stage 6 manual choke lever fitted to it, and that's operating correctly - however the ped starts without choke, and flicking the choke on kills the engine.

Doesn't really matter cause the ped only sees full throttle or zero thottle, so part throttle fuelling isn't a concern...but I'm interested Thinking
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chris-red
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PostPosted: 11:06 - 06 Feb 2015    Post subject: Reply with quote
is it an air screw or a fuel screw?
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Iain.
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PostPosted: 11:12 - 06 Feb 2015    Post subject: Reply with quote
chris-red wrote:
is it an air screw or a fuel screw?


No idea. Looks to be a clone of a PHBG carb if thats of any help whatsoever?

https://www.mopedarmy.com/wiki/How_To_Tune_PHBG_Carburetors

Reading this, is it possible that my idle jetting is massively out and thats causing the screw to not be able to make enough of a difference to notice?
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Triton Thrasher
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PostPosted: 13:33 - 06 Feb 2015    Post subject: Reply with quote
Check for a blocked pilot jet.
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stirlinggaz
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Joined: 22 Jul 2007
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PostPosted: 14:10 - 06 Feb 2015    Post subject: Reply with quote
Iain. wrote:
chris-red wrote:
is it an air screw or a fuel screw?


No idea. Looks to be a clone of a PHBG carb if thats of any help whatsoever?


hi,
its fairly easy to tell tbf.....
Iain. wrote:
is it possible that my idle jetting is massively out and thats causing the screw to not be able to make enough of a difference to notice?

YES

cheers,
GAZ
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Iain.
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PostPosted: 14:25 - 06 Feb 2015    Post subject: Reply with quote
chris-red wrote:
is it an air screw or a fuel screw?



stirlinggaz wrote:
its fairly easy to tell tbf.....



How so? On every carb I've looked at you have a mixture screw, and an idle screw.

Ordered a load of idle jets in. Thumbs Up
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chris-red
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Joined: 21 Sep 2005
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PostPosted: 14:54 - 06 Feb 2015    Post subject: Reply with quote
Iain. wrote:
chris-red wrote:
is it an air screw or a fuel screw?



stirlinggaz wrote:
its fairly easy to tell tbf.....



How so? On every carb I've looked at you have a mixture screw, and an idle screw.

Ordered a load of idle jets in. Thumbs Up


Mixture screws AFAIK either let in more air or fuel, hence Air Screw or Fuel screw.
____________________
Well, you know what they say. If you want to save the world, you have to push a few old ladies down the stairs.
Skudd:- Perhaps she just thinks you are a window licker and is being nice just in case she becomes another Jill Dando.
WANTED:- Fujinon (Fuji) M42 (Screw on) lenses, let me know if you have anything.
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P.addy
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Joined: 14 Feb 2008
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PostPosted: 15:03 - 06 Feb 2015    Post subject: Reply with quote
Fuel screw to adjust how much fuel goes in? That's jets no?
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chris-red
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PostPosted: 17:38 - 06 Feb 2015    Post subject: Reply with quote
Paddy. wrote:
Fuel screw to adjust how much fuel goes in? That's jets no?


The mixture screw bypasses that to change the mixture, it either lets more air or more fuel in.
____________________
Well, you know what they say. If you want to save the world, you have to push a few old ladies down the stairs.
Skudd:- Perhaps she just thinks you are a window licker and is being nice just in case she becomes another Jill Dando.
WANTED:- Fujinon (Fuji) M42 (Screw on) lenses, let me know if you have anything.
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YBR Ric
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Joined: 16 Jan 2012
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PostPosted: 18:15 - 06 Feb 2015    Post subject: Reply with quote
Unless you have something exotic, then the general rule of thumb is that the fuel screw is found on the engine side and an air screw on the filter side of the carb.

Most probably an incorrect float height is the cause of your problem.
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stirlinggaz
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Joined: 22 Jul 2007
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PostPosted: 19:31 - 06 Feb 2015    Post subject: Reply with quote
YBR Ric wrote:
Unless you have something exotic, then the general rule of thumb is that the fuel screw is found on the engine side and an air screw on the filter side of the carb.


^^^^this^^^^

HTH,

cheers,
GAZ
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Old Thread Alert!

The last post was made 11 years, 107 days ago. Instead of replying here, would creating a new thread be more useful?
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