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SU Needle Advice


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I have a MK4 1300 Spitfire, the camshaft is from a MK3 but the cylinder head is MK4, twin SU HS2's with AAN needles, standard air filter set-up with 20/50 oil in the dashpots. The car has a wideband controller fitted to aid tuning, at idle it's around 13.8~14.0:1 and at cruising speeds it's about the same. However under light to moderate acceleration and under load the AFR readings lean right off to something like 17.0:1, which I'm guessing is way too lean? Was hoping someone might be able to give some advice on the best approach to try and correct this.

If new needles, then which would be a good place to start, I have looked at http://www.mintylamb.co.uk/suneedle/ but tbh whilst I can see the changes on the graph I wouldn't know which is the best option.

Would just fitting stronger springs be worthwhile before trying richer needles?

Any advice appreciated! 

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Hi Darren,

Your engine sounds like the one in our Spit - Mk3 but with a Mk4 big valve head.  It's on injection now but did run on 1.25" SUs for the first couple of years.  We also had a 4-2-1 manifold and wheel barrow exhausts.  IIRC the AAN is the standard Mk4 needle - it's pretty lean.

We tried ABT (std 1500 needle) AAT and AAQ IIRC.  Can't remember which one was favourite check these out (and others) at

http://www.mintylamb.co.uk/suneedle/

 

Assuming you still have the original Spitty 1300 springs fitted you are probably best staying with them as they are selected to set the carb airflow range and without a big cam and head mods you are not going to change that enough to matter.

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Hi Nick. 

I have a Bell 4 branch manifold & semi sport exhaust and I do have double springs on the valves. 

Thanks for the needle suggestions, will give one of them a try and see what it gives me. 

Cheers 

Darren

 

 

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14 hours ago, Darren Groves said:

Would just fitting stronger springs be worthwhile before trying richer needles?

Hi Darren. I'm no expert on desirable AFR but not sure either of those options is the solution to transient lean mixture, happy to be corrected though. 

What's your crankcase breathing setup, are you using the mk4 inlet manifold or have you got a mk3 one with the Smiths PCV? If the latter, that opens when manifold vacuum drops so could be a source of leaning off under acceleration or load.

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3 minutes ago, PeteStupps said:

Hi Darren. I'm no expert on desirable AFR but not sure either of those options is the solution to transient lean mixture, happy to be corrected though. 

What's your crankcase breathing setup, are you using the mk4 inlet manifold or have you got a mk3 one with the Smiths PCV? If the latter, that opens when manifold vacuum drops so could be a source of leaning off under acceleration or load.

No PCV, crankcase connected direct to carbs. 

 

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I Nick said don't change thedashpot springs, but I am tempted to suggest trying some stronger ones. If you have access to some HS6 carbs, I "think" they have the next springs up, check Burlen. But ideally you want to borrow some. 

Likewise it may be worth trying some stub stacks, they may weaken the mixture at cruise, and allow you to use a richer needle. Lots of variables involved. Makes EFI look simple....

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I've not had any time to check this issue further, but did decide to order some stronger springs (yellow), the old ones I assume were the standard red, but as there were no colour markings left on them I'm not sure.

I don't suppose the uneven lengths on the old springs were helping much though.... 

 

20220811_182609~3.jpg

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