Piston

[stebec]

Help needed, I have just rebuilt my twin HS4 carbs, on one of them if I manually lift the piston, it wont drop down back onto the bridge, the other piston will, any ideas what i have done wrong?

Steve

[JohnD]

Recenter the needle.

https://www.theminiforum.co.uk/forums/topic/353535-su-hs4-jet-recentering/

Edited January 19 by JohnD

[stebec]

My needles have springs on them, so shouldn’t they centre themselves?

[Pete Lewis]

you need to slacken the big jet retaining nut  this allows the needle to self centre 

when the piston is fully dropped , then re nip the nut   

this is more for fixed needles  , do also check the needle is not bent .

retest it drops ok ,

being a 1500  many  HS4 have biased needles these only fit one way round on the locating screw this makes sticking far less likely   and the lack of drop may just be the slide is sticking

give it a good clean or very light abrasion .

Pete

Edited January 20 by Pete Lewis

[stebec]

I have just checked the new needle code number and the are ABT the old needle code is AAR will this cause a problem? also is this collar for the needle supposed to have this lip on the top?

Sorry about all the questions but I'm new to the classic car world

[Steve P]

You can`t have 2 different needles as the mixture will be affected between the 2 banks of cylinders.

S

[Pete Lewis]

yes the lip is what the needle flange pivots against  there should be a small spring that holds on the top of the needle head

as far as i see ABT is the std  needle  from 1974 to 1979

are you running pancake air filters or any non std stuff  like exhaust etc.

 

Pete

 

Edited January 20 by Pete Lewis

[stebec]

The car came with K&N air filters on it, I have just changed my standard exhaust to a bell twin pipe exhaust which has a branch manifold.

[Pete Lewis]

well thats the rub  with KN and a 4-2-1  you need possibly richer needles and maybe stronger piston springs 

or its will run far too lean .

many buy these must haves and that generates the headaches once you leave the triumph 

design spec  you are into guesswork .

??? why did you buy a replacement needle ???

 

Pete

Edited January 20 by Pete Lewis

[stebec]

I had a fuel leak from a split in the jet tube sleeve, so bought a complete rebuild kit

[Iain T]

1 hour ago, stebec said:

have just checked the new needle code number and the are ABT the old needle code is AAR will this cause a problem?

I think the AAR needles are slightly thinner hence will give a richer mixture than ABT. Perhaps the previous owner used the AARs to compensate for the pancakes and your tubular manifold will also require a richer mixture. Playing around with needles, damper oils and damper spring rates has aged and frustrated me over the last 2 years! 

Iain 

[stebec]

Thanks for all your replies