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Carburettor mounting- gasket o-ring B25 #637977
02/01/16 8:16 am
02/01/16 8:16 am
Joined: May 2015
Posts: 43
Surrey
M
Mike73 Offline OP
BritBike Forum member
Mike73  Offline OP
BritBike Forum member
M
Joined: May 2015
Posts: 43
Surrey
Hi All - i am just about to refit my carb to my Fleetstar and I am a little puzzled by the mounting arrangement. The carb page of the parts manual shows an O-ring fitted in the carb mating flange groove; the cylinder head page shows two gaskets fitted to the intake manifold; a paper one next to the head itself and a Tufnol one between that and the carb. My question is really that this seems like an awful lot of sealing, overkill maybe? I recall on my Triumph twin the O ring is designed to be compressed leaving the carb with a little wiggle room and there is no gasket as such... then again its a thicker O ring. I just wondered if all of these are really needed or if they are alternatives? All were fitted when I took the thing apart and so I will refit them as found but it still seems a bit excessive.
Cheers
Mike

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Re: Carburettor mounting- gasket o-ring B25 [Re: Mike73] #637980
02/01/16 8:30 am
02/01/16 8:30 am
Joined: Dec 2004
Posts: 9,375
Scotland
kommando Online content
BritBike Forum member
kommando  Online Content
BritBike Forum member
Joined: Dec 2004
Posts: 9,375
Scotland
Go with the Trimumph method or the smaller O ring and tufnol spacer etc as you need a good seal plus both give a thermal barrier stopping the engine from heating the carb and the fuel for easier hot starting. I use nyloc nuts so you are not tempted not over tighten and warp the carb flange to try and stop them coming undone.

Last edited by kommando; 02/01/16 8:58 am. Reason: Clarified O ring
Re: Carburettor mounting- gasket o-ring B25 [Re: Mike73] #637981
02/01/16 8:35 am
02/01/16 8:35 am
Joined: May 2015
Posts: 43
Surrey
M
Mike73 Offline OP
BritBike Forum member
Mike73  Offline OP
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M
Joined: May 2015
Posts: 43
Surrey
yep so you reckon one or the other? That was my feeling too. I think compressing an O ring between a hard ally surface (the carb flange) and a deformable surface (tufnol spacer) is probably ineffective anyway.

Re: Carburettor mounting- gasket o-ring B25 [Re: Mike73] #637985
02/01/16 8:58 am
02/01/16 8:58 am
Joined: Dec 2004
Posts: 9,375
Scotland
kommando Online content
BritBike Forum member
kommando  Online Content
BritBike Forum member
Joined: Dec 2004
Posts: 9,375
Scotland
To explain better, you need the thinner O ring with the Tufnol spacer and gasket or the thicker Triumph O ring and no spacer. Any air leaks at this joint will mess up your carb settings and the bike will not run properly.

Re: Carburettor mounting- gasket o-ring B25 [Re: Mike73] #638005
02/01/16 12:12 pm
02/01/16 12:12 pm
Joined: Aug 2012
Posts: 309
Hampshire, England
DoubleDiamond Offline
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DoubleDiamond  Offline
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Joined: Aug 2012
Posts: 309
Hampshire, England
Its not so much to do with sealing but as suggested minimising heat transfer from the head to the carb.

My suggestion is to use the paper gasket first, then the Tufnol spacer then the carb with the 'O' ring in the groove.

Don't over tighten the flange nuts - using nyloc as suggested is a good idea - so that you can just move the carb slightly if you grab it with both hands.

Not only do over tight mountings cause air leaks but also distort the carbs sometimes causing the throttle slide to stick - not helpful!

HTH

DD


BSA: '71 B175; '68 B25; '71 A65; '71 A75
Triumph: '87 T140; '72 T150v

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