SU Carb jet locking nut

Technical advice Q&A

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Robbiee
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#1 SU Carb jet locking nut

Post by Robbiee » Sun Nov 04, 2018 11:21 am

Hi all
I’m rebuilding my triple HD8s and am trying to tighten the jet housing with the locking nut on the middle carb body.
I find that whilst i am able to fully tighten the nut it leaves the jet housing wobbly
On the other two carbs the nuts tighten the jethousings easily
I then noticed that the problem is that the nut doesn’t screw far enough in to grip the the jet housing

Checking the threads in the carb body I noticed that the problem body has not got its thread cut as deep as the others therefore not allowing the nut to tighten any further
I am attaching photos to compare
I’ve tried every combination just in case I’ve swapped the jet housings/nuts.
I didn’t notice a problem when I stripped them but they had not been used for 30 years and were stuck anyway!

Is this a common problem?
Should I cut a little more thread into the body to match its neighbours?

Advice appreciated
Robbie
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Robbie 1962 3.8 Coupe OSG 1962 OTS ODG

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mgcjag
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#2 Re: SU Carb jet locking nut

Post by mgcjag » Sun Nov 04, 2018 12:51 pm

Hi Robbie.....iv rebuilt a few sets and not seen this problem...get in touch with David Lonsdale Classic Carburetters Cirencester, UK 0 7786 912011 he is well known on the forum, a one man band and very helpefull......may be able to provide a replacement body or retap yours.......Steve
Steve
1969 S2 2+2 & Building a C type replica

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Robbiee
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#3 Re: SU Carb jet locking nut

Post by Robbiee » Sun Nov 04, 2018 3:22 pm

Thanks Steve
I’ve done just that, although he’s retired from rebuilds he still has the correct taps and, following receipt of my photos, he will advise on the best plan...probably sending him the offending body for cutting of a slightly deeper thread!
Robbie
Robbie 1962 3.8 Coupe OSG 1962 OTS ODG

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Series1 Stu
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#4 Re: SU Carb jet locking nut

Post by Series1 Stu » Sun Nov 04, 2018 3:51 pm

Yes. It looks like the thread is stripped in one of the holes.

It should be an easy repair using a Helicoil.

Regards
Stuart

If you can't make it work, make it complicated!

'62 FHC - Work In (slow) Progress
'69 Daimler 420 Sovereign
'94 X300 XJR basket case

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Robbiee
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#5 Re: SU Carb jet locking nut

Post by Robbiee » Sun Nov 04, 2018 5:19 pm

Thanks Stuart
Actually the thread is fine. It’s just not deep enough at 7mm
It should go to 8.5 mm at least ...as do the threads in the other two
Robbie
Robbie 1962 3.8 Coupe OSG 1962 OTS ODG

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JerryL770
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#6 Re: SU Carb jet locking nut

Post by JerryL770 » Sun Nov 04, 2018 5:38 pm

I think Stuart is referring to one (the nearest) of the 4 screw threads for the bottom cover.
Jerome Lunt
1970 E-Type S2 FHC - Fully restored

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Robbiee
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#7 Re: SU Carb jet locking nut

Post by Robbiee » Sun Nov 04, 2018 6:43 pm

Hi Jerry
My mistake..sorry
Actually the threads are all fine it’s probably the focus on the photo
Robbie
Robbie 1962 3.8 Coupe OSG 1962 OTS ODG

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Robbiee
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#8 Re: SU Carb jet locking nut

Post by Robbiee » Sun Nov 11, 2018 1:51 pm

Hi Friends
David Lonsdale kindly cut another couple of thread circumferences into my throttle body and it’s now perfect
He’s no longer working at rebuilding carbs commercially but he very kindly helped me out
He will give advice over the phone
Robbie
Robbie 1962 3.8 Coupe OSG 1962 OTS ODG

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ALAN COCHRANE
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#9 Re: SU Carb jet locking nut

Post by ALAN COCHRANE » Sun Nov 11, 2018 10:04 pm

Robbie

I had this problem while rebuilding my carbs last winter. The only ditfference being that it was on the slow running screw of the rear carb. I was lucky that the thread turned out to be 1/4 UNF so I was able to cut the extra myself. I can't understand why none of the POs or garages they used hadn't spotted this in the last 50 odd years.

Alan
1962 S1 OTS-850389,1968 Triumph TR250, 1971 Triumph GT6 Mk3, 2008 Porsche Boxster RS60 Spyder

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