Size of SU idle mixture screw

Technical advice Q&A

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Annivar
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#1 Size of SU idle mixture screw

Post by Annivar » Sun Jan 13, 2019 5:32 pm

Hello. Does anyone know the size/pitch of the idle mixture (jet adjusting) screw on SU carbs. It appears to be a #6 but has an unusual thread count and I don't know if it's SAE or BSW. Burlen part number is WZX 981. Thanks.

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PeterCrespin
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#2 Re: Size of SU idle mixture screw

Post by PeterCrespin » Tue Jan 15, 2019 12:34 pm

Check the specs for BA (British Association) screw threads. They look like 1 BA or 2BA.

BA threads are used in electrical parts but maybe fuel too? There is also a thread family called BSF (British Standard Fine, which was used on British cars and bikes until the mid Sixties and there are the Cycle threads that are different again. With SU being such an old brand they could have chosen any of them for some historical reason.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 79 S2 XJ12L; 97 XJ6L

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abowie
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#3 Re: Size of SU idle mixture screw

Post by abowie » Wed Jan 16, 2019 12:34 am

PeterCrespin wrote:
Tue Jan 15, 2019 12:34 pm
Check the specs for BA (British Association) screw threads. They look like 1 BA or 2BA.

I just ran a screw through a 3BA tap; perfect fit. If you want to check yours try the little circular thumb nuts that secure the instruments; they are 3BA as well.

Best thing to do is email or call Burlen and ask them. They will know and they will tell you.
Andrew.
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christopher storey
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#4 Re: Size of SU idle mixture screw

Post by christopher storey » Wed Jan 16, 2019 10:13 am

abowie wrote:
Wed Jan 16, 2019 12:34 am


I just ran a screw through a 3BA tap; perfect fit.
Being a real nit-picker , don't you mean a 3BA die ?? :bigrin: :bigrin:

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Annivar
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#5 Re: Size of SU idle mixture screw

Post by Annivar » Wed Jan 16, 2019 11:44 am

Thanks. I will try the instrument nuts. I did email Burlen but no reply yet. What an odd size screw but as Peter said, it is old technology.

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