2" Jag SU Carbs on L28

Z

zedhed

Club Member
Hello, I'm restoring a 240z with a L28 engine which has a pair of 2" Jag Carbs (see pic).

These are quite good with a very quick response, engine (apparently built by a Mr Wardle whom I've not met yet) revs like hell and is possibly flowed and balanced. I've no idea if the flywheel is lightened or what has actually been done. It has good power though, and a dyno graph I got with the car shows 185hp at the flywheel which feels about right.

Anyway the needles are a bit irregular and slightly bent, with a few flat spots, especially near the bases having been filed down. They're stamped #BAW which is for a 4.2 litre Jag, and they're running a little rich.

I'm thinking about trying a new pair of #BAU needles for the 2.8 litre Jag engine - after all this is a 2.8 at least that's my thinking so far.

My questions are

- I was also thinking about replacing the jets too while I'm at it, any idea what type they are from the picture attached?

- wondering if anyone has advice about trying the #BAU needles or any other needles with these big Jag carbs?

thanks, Nick
 

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morbias

morbias

Well-Known Forum User
BAU needles are already at the weak end of the scale, maybe they've been filed down too much? If that's the case then replacing with new BAW needles might make it too lean.
 
Z

zedhed

Club Member
I assumed whoever tuned the car removed material because it was too lean already (trying to let more fuel in at low revs). Also assumed that because they were filed down unevenly, perhaps the flat spots and file marks would cause an uneven or erratic mix. However I cam across this graphing tool which compares needles and they're similar until station 7 where the BAW gets thinner quicker...

Hoping this setup is a popular mod and someone out here has a similar setup ;)

Here's the link to the graphing tool:

Minty SU Needle Compare-o-rama
 
morbias

morbias

Well-Known Forum User
What I mean is, maybe the BAW needles were a bad choice and as you say they were filed down to make them work. If you replace them with BAU needles which aren't filed down then the carbs might run too lean instead of too rich.

I have a couple of KL needles I can send you if you like? They are supposed to be a good starting point for 2" SU carbs on an L28.

Is it running too rich at idle only? This would suggest to me that the mixture has been turned up to get the right mix at higher revs and is now running rich at idle because the bases of the needles have been filed down. ie. you would actually need needles that are richer at top end and leaner at idle.
 
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Rob Gaskin

Rob Gaskin

Treasurer
Staff member
Site Administrator
Nick, it may be worth asking the questions to SU direct?

SU Carburetters Home Page

I tried to get them interested in supplying 1.75" and 2.00" kits for Datsuns but they weren't convinced it would be worth it. It's a shame because we end up working with old carbs or buying new triples.

Irrespective of what you end up doing a rolling road session is the best way to get the mixture right but who do you go to these days? The guy I used about 12 years ago is no longer with us - he was very skilful. Perhaps SU know someone?
 
A

andrew muir

Club Member
I was under the impression that you could bolt on an HS 8 carb fairly easily.:unsure:
Please let me know what is involved, do you need a new manifold etc??:unsure:
 
morbias

morbias

Well-Known Forum User
Burlen sells all the parts to build your own, including the carb body itself

[EDIT] just checked their new site and it looks like they don't any more. They also used to have sets you could buy but that section is gone unless I missed it.
 
Z

zedhed

Club Member
morbias said:
What I mean is, maybe the BAW needles were a bad choice and as you say they were filed down to make them work. If you replace them with BAU needles which aren't filed down then the carbs might run too lean instead of too rich.

I have a couple of KL needles I can send you if you like? They are supposed to be a good starting point for 2" SU carbs on an L28.

Is it running too rich at idle only? This would suggest to me that the mixture has been turned up to get the right mix at higher revs and is now running rich at idle because the bases of the needles have been filed down. ie. you would actually need needles that are richer at top end and leaner at idle.
Click to expand...

Hi Morbias, yes running too rich at idle but fine at higher revs. For my carbs, I think I need the needles with spring attached to fit, and will gladly take you up on that, will PM.

Spoke at length with a guy at SU who knew about carbs but not about their use outside the set menu they offer. Very helpful though.

Mike & Morbias, I modelled the needles you suggested on that graphing tool (Minty SU Needle Compare-o-rama) and the ones you suggest have a very linear progression compared to the jag ones. the BAR and KL narrows down more abruptly between stages 4 & 6. The KL is a very similar shape but drops off completely at 14mm so perhaps that's the end of the needle...?

Yes it looks to be a special manifold... Still can't figure out what type of jet this is and suspect the springs are 4.2 litre Jag ones. Need a rolling road but any with knowledge of SUs are as rare as hen's teeth.

Maybe time to switch to webers (more knowledge out there) but I wonder if that's going to be just as complicated when it comes to choosing the components ;)
 
toopy

toopy

Club Member
zedhed said:
Mike & Morbias, I modelled the needles you suggested on that graphing tool (Minty SU Needle Compare-o-rama) and the ones you suggest have a very linear progression compared to the jag ones. the BAR and KL narrows down more abruptly between stages 4 & 6. The KL is a very similar shape but drops off completely at 14mm so perhaps that's the end of the needle...?
Click to expand...

Been doing a bit of research recently on needles etc as i have some HS8 carbs from a Jag but unknown model/spec. My needles are BBC so quite different to the BAR and probably what helped my car run like poo!
I have a SU reference catalogue and the BBC needles are listed for a 4.2 XJ6 engine!
I believe the KL needles are the fixed type and slightly shorter, where as the BAR are biased type with the little spring at the top.

Also bare in mind your BAW needle in the SU catalogue uses a AUC 4826 piston spring for a 4.2 Jag engine and has a load of 11.25oz, whereas the BAU is listed with AUC 2107 spring for a 2.8 Jag engine and is a weaker spring at 4.5oz albeit the same size/length. Both engines running on HS8 carbs.

HTH

Anthony
 
Leeski26

Leeski26

Club Member
according to the Haynes SU manual the jet size is determined by the grooves.
No grooves 0.090 jet
One groove 0.10 jet
Two grooves 0.125 jet
 
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