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Spitfire 1500 twin SU HS4 - needle choice.

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As I have a slightly modified Spitfire 1500 I've been using AAT needles in my twin HS4s (K&N filters, nonstandard exhaust, but standard manifolds). AAT or AAQ seemed to be the consensus choice amongst the majority of people for that configuration.

I've always found the tuning problematic on these needles, either too rich at tick-over with little chance of passing the required emissions test (1978 car) OR fine at tick over, but too lean and running out of "ooomph" at higher rpm (basically gutless above 4K)

This week I swapped the needles to the more extreme ABZ profile and re-tuned the carbs (usual method, balance them, then check mixture by lifting the piston etc.)

This morning at the MOT the car passed the emission test without issue AND it pulls very well all the way up to about 6K rpm.

If anyone else is having the sort of problems I was (too rich at tick over OR too lean at high RPM) on AAT or AAQ needles I'd strongly recommend giving ABZ a try and tuning accordingly. (If you can't afford rolling road tuning and custom profile needles that is)

I just wish I had a wideband sensor fitted OR a gas analyser so I could measure all this... I hate guess work!

With thanks to Minty Lamb for the excellent needle comparison! (http://www.mintylamb.co.uk/suneedle/)

Cheers,
Sam

 

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That is very interesting, My wife's Spit. 1500 has a very similar set-up and has run on AAQ needles for years. If I remember correctly the "Triumphtune" manual and also Moss recommend AAQ needles when fitting the sports exhaust system and K&N air filters to the Spit. 1500.

Thinking about it you are probably correct regarding the pattern of performance although it has not been an issue for my wife.

It is something I may have a go at sometime.

Regarding the wideband sensor and gauge, I have fitted one to my TR2 after converting the SU H6s to HIF6s, and found it very helpful in setting them up. They tend to become redundant after that exercise is complete. Also I found that the sensor in the exhaust pipe is very easily damaged by thermal shock caused by droplets of condensation on start-up. I now always wait till the engine has completely warm up before switching it on.

Regards, Colin 

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When i had SU's i swore by AAQs, though every car is different, another thing that is often overlooked is the dashpot oil, most people run too thin an oil in the dashpot, often a thicket oil can solve a lot of things.

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When running a spitfire 1500 as an everyday car, I tried several needles.

The only performance mods where K&N filters and tubular exhaust manifolds + twin rear box exhaust.

None were the correct mixture everywhere.

In the end I used a wideband lambda kit and modified the standard needles.

Not easy as it's easy to overdo it and take too much off and the two needles have to be exactly the same.

It took several months to get them close enough. Testing after each "polish" of the needles.

It went quicker after I found out that upto a certain rpm the mixture while driving corresponds quite closely to the mixture when "revving"  the engine with the car stationary.

Stiffer spring and thicker oil have the effect of enriching the mixture on acceleration, much like an accelerator pump on other carbs.

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