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Are all HS2 AAN biased needles equal? Mk3 Spitfire


Graham Ness
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I've been having problems with the fixed needle SU HS2 carbs fitted to my Spitfire Mk3 and rather than investing more time trying to solve the problems there, decided to fit a pair of biased needle HS2 carbs that I inherited sometime in the past.  I've fitted the carbs and briefly had the engine running.. but it has since failed to start.  Well, it starts but misfires and then seems to run on only 3 cylinders.  I've checked timing, spark and fuel delivery, and all seemed OK.  So I thought I should next check carb float levels.  On removing the piston from each carb I noticed that the needles appeared to look different.  I removed the needles and can confirm that each needle clearly has AAN stamped on the neck near the top.  But the two needles are clearly different.. see the photograph.  The overall length is different by about 4-5mm and the longer needle has a longer thin part of the neck.

So, to my question.  Are all AAN needles the same?  Well, clearly not, since I have two different ones here.  Should they match in all ways?  Has a previous owner changed one needle and not the other?  The shorter needle was in the rear carb. if that makes any difference.   I've got to get the car going in the next week for an MOT and don't really want to swap the carbs back again.  Should I simply order a pair of new needles, check they are identical, fit and continue?

Any constructive feedback appreciated.. and any input on the difference in these two needles..

Graham

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AAN is supposed to be the profile but that doesnt explain the length variety you seem to have acquired  almost as if the needle is misplaced in the top bush /collar 

why not fit the earlier pistons and fixed needles in the later bodies

do they have Wax Stat jets fitted 
Pete

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6 minutes ago, Pete Lewis said:

 almost as if the needle is misplaced in the top bush /collar

I think Pete may have hit on something there. The needle on the right has a discolouration around the top. If it could be pushed into the collar by that far it would match the one on the left.

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Thanks for the prompt feedback.. don't you guys have better things to do on a Bank Holiday afternoon than wait for questions from the likes of me?  (believe me, I do appreciate your inputs)

Good point about fitting in the collar, since the needle length from tip to start of the first shoulder looks the same.  I only noticed it because one needle was sticking out of the bottom of the piston more than the other.  And now that I check more closely, I note the shorter AAN needle has a curve in it.. someone has (accidentally?) bent it slightly.

The biased needle carbs are not Waxstat, so I could try swapping over the piston and needles.  That would be a quicker (temporary?) fix than ordering and fitting replacement needles.  I'll try that and report back..

Thanks, Graham

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The original reason for removing the pistons from the carbs was to check the fuel levels in the main jets.  Having almost got over the needle differences.. I now noticed that the fuel level in the rear carb appeared much (much) lower in the jet.  Opening up the rear float chamber.. I discovered it was empty.

If I'd simply hit the carb with a spanner to make sure the float valve wasn't stuck (as it was), then I would probably have not noticed the needle length differences.

I've fitted the fixed needle pistons from my other carb set and checked the rear carb has some fuel and - hey presto - an engine that runs on all four cylinders.. albeit a bit lumpily.  Time to set up the carbs a bit and re-check the timing with a strobe light.

I remain intrigued by the difference between those two needles.  Incidentally, both needles sit fully in their respective black collars, but the longer thin part of the collar on the longer needle protrudes from the bottom of the black collar.  Give than one is bent, I may purchase a new pair, check they're the same and fit them anyway.

Thanks again,
Graham

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