biased vs fixed needles

Technical MGB discussion
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Hazza1190
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biased vs fixed needles

Post by Hazza1190 » Tue Apr 02, 2019 5:30 pm

Hi,

Whats the difference between biased vs fixed needles? I understand the fact that the biased needles can pivot/sway but why would you need one or the other? Im asking as I picked up a pair of HS6 carbs for my B GT and it has the fixed type installed.

Thanks!
Harry.

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Peter Cresswell
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Re: biased vs fixed needles

Post by Peter Cresswell » Tue Apr 02, 2019 8:28 pm

The biased (or spring loaded) needles don't require time consuming adjustments to centre the needle in the jet each time they are fitted both during production and during maintenance. To some extent they self centring and all you have to do is fit them so the shoulder is flush with the bottom of the piston in the dashpot. The fixed needles have to be dead centre in the hole of the jet so take some time to fiddle around with them to get this right. If it is not correct the fixed needles wont fall completely onto the bridge in throat of the venturi of the carb resulting in poor light throttle running.

You have to remember that these days the skills to adjust carburettors has largely been lost along with the adjustment of points and plugs and tappets. A downside is the spring bias of the needle means it rubs on the side of the jet causing wear and eventually the carb cannot be adjusted to give the correct mixture - they usually runs too rich. The only cure then is to replace the jet and the needle with new ones. This doesn't happen with the fixed type of needle providing they are correctly adjusted in the first place.
Also you cannot fit biased needles in a carb that has fixed needles and vice versa (at least I don't think you can!).
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

Vic Butler
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Re: biased vs fixed needles

Post by Vic Butler » Wed Apr 03, 2019 9:44 am

Weren't the biased needles used to comply with emissions regulations in the 1970''s?
I'd rather fiddle around centering the needle than replace the jets. Fortunately my HS6's have fixed needles.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Hazza1190
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Re: biased vs fixed needles

Post by Hazza1190 » Wed Apr 03, 2019 11:34 am

Hi Both,

Okay cool in that case i will keep the needles as fixed as they are new needles. They have already been centred however i need some new jets as the ones that were fitted were too worse for wear.
I believe you can fit biased needles or non biased needles, i think you just need to use slightly different black collar(?) where the needle mounts to the piston.

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Peter Cresswell
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Re: biased vs fixed needles

Post by Peter Cresswell » Thu Apr 04, 2019 2:44 pm

Hi all,
I've just spoken to Burlen about this and Vic is right that the biased needles were introduced as an emissions 'tweek' to the SU carbs. The reason is that the needle always sits in the same part of the jet so offers the same control of the mixture and is independent of adjustments during service.
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

Ian Fozzard
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Re: biased vs fixed needles

Post by Ian Fozzard » Thu Apr 04, 2019 9:27 pm

Hi Peter,
I wouldn't want to disagree with Burlen (or you for that matter!), but I can't really see how a biased needle would improve emissions or be any different to a fixed needle set-up when making adjustments?
What am I missing?

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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Peter Cresswell
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Re: biased vs fixed needles

Post by Peter Cresswell » Thu Apr 04, 2019 10:19 pm

Having thought about this some more, and as I said in my posts above, the needle always sits in the same part of the jet - that's why it wears out the needle and the jet. This means the needle for a particular engine type can be designed to provide a very efficient mixture control at all throttle openings because the position of the needle does not vary from car to car. Therefore the vehicle manufacturer can guarantee the emissions will stay consistent and within legislation limits - but probably only for the vehicles warrantee period! If the biased needles wear the needle and jet then they are relative simple to change and you don't have to spend some considerable time fiddling with the jet to get the needle in the centre.
Additionally, the biased needles are longer (as on the MGB if you compare a fixed and biased needle side by side) and have 3 more points (16 for the biased and 13 for the fixed) where the needle is measured. This means that at all throttle settings throughout the engine rev range the needle is more in control of the mixture and even at full throttle under load a portion of the needle is still in the jet.
Also since the same needles were specified for several years, only the few engines originally bench tested by the manufacturer will have needles that provide the correct fuelling at every throttle opening. This maybe why when you have a rolling road session even with a completely standard engine they can find you a little more power.
In the early days of emissions legislation the reduction in emissions was very small, and for some time SUs could meet them, but eventually the MGBs exported to the US had to use a single Stromberg carb and had lots of other power robbing fitments to meet the California legislation.
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

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