36 replies to this topic

[mini_man96]

I am having this exact same problem accept with twin hs4's and like monteemini am completely stumped!

[carbon]

Fixed or swing needles?

[mini_man96]

swing

[carbon]

Then you should not have any problems with needles sticking in the main jet as long as the original HS4 carb parts have not been mixed with other HS4 parts of different vintage.

 

The swing needle HS4s should all have a non-centering main jet bearing (AUC3414 coarse, JZX1183 fine). This is a close fit in the carb body, and position cannot be adjusted from side to side. The swing needle moves around, so this all works fine.

 

If you have a small annular gap of about 20 thou between the main jet bearing then you may have a fixed jet bearing. In this case you will need to centre it by eye, not ideal, or alternatively find a HS4 piston and dashpot with fixed needle and use this to centre and then see if the swing needle assembly works.

[carbon]

Quick guide to identifying type of HS jet bearing you have fitted:

- remove jet bearing from carb body (Note: the large nut is a Whitworth size, and normally very tight)

- measure the smooth end of the jet bearing where is goes through the carb body

- then take off the adjuster nut compare thread pitch on jet bearing with a 1/4 inch UNF bolt

 

Jet bearing top diameter = 9.55mm, thread coarser than 1/4 UNF = AUC8460, coarse for fixed needles (common)

Jet bearing top diameter = 9.55mm, thread finer than 1/4 UNF = JZX2075, fine for fixed needles

 

Jet bearing top diameter = 10.42mm, thread coarser than 1/4 UNF = AUD3414, coarse for swing needles

Jet bearing top diameter = 10.42mm, thread finer than 1/4 UNF = JZX1182, fine for swing needles (common)

 

Most fixed needle HS carbs use the coarse thread adjuster, use 12 flats down from fully up for initial setting

 

Most swing needle HS carbs with Waxstat use fine thread adjuster, use 18 flats down from fully up for initial setting

[carbon]

I am having this exact same problem accept with twin hs4's and like monteemini am completely stumped!

Have you also tried taking both needles completely out, refitting the dashpot and piston assemblies, and then checking the pistons are both moving up and down fully throughout their travel with no sign of binding?

[mini_man96]

Really sorry carbon I should have been more clear! What I meant was that I am having the same high idle problem even with the idle and fast idle screws backed fully out. That information is really helpful though, I had no idea there were two different thread pitches and jet bearing sizes!

[carbon]

If you replaced the butterfly spindle and / or the butterfly itself, or disturbed the old ones, the butterfly will need to be seated properly in the closed position.

 

You'll need to take the carb off, back the Idle Screw right out, loosen the butterfly screws, wiggle the spindle about with gently pressing on the butterfly until you see no gaps at the top or bottom of it. If you hold it up to the light, that can help, though, you'll usually see a tiny bit of light coming through, but not much.

 

If it is seated, it could be the the SU Body has worn from the Butterfly around the Spindle, if worn, it tends to only do it on one side.

For fast idle with idle screws fully out it's probably worth rechecking Moke Spider's first suggestion

[Monteemini]

 

 

 

If you replaced the butterfly spindle and / or the butterfly itself, or disturbed the old ones, the butterfly will need to be seated properly in the closed position.

 

You'll need to take the carb off, back the Idle Screw right out, loosen the butterfly screws, wiggle the spindle about with gently pressing on the butterfly until you see no gaps at the top or bottom of it. If you hold it up to the light, that can help, though, you'll usually see a tiny bit of light coming through, but not much.

 

If it is seated, it could be the the SU Body has worn from the Butterfly around the Spindle, if worn, it tends to only do it on one side.

 

Okay thanks I will check this, i did indeed replace the butterfly spindle and the valve itself.

 

 

Ive stripped the carb down again and checked the positioning of the butterfly valve. It was slightly out of place (i could see a slither of light through it when closed)

Now when i reassembled it i really struggled to centre the jet in the jet housing, the piston would not drop all the way to the bottom, even with the jet bearing nut loose

 

So i figured the needle must be slightly bent out of shape, the first 80% of travel on the piston was fine but is always got stuck on the final 20%.

 

I can't see any damage/bending of the needle but i assume its only a little off centre, could this be causing my problem?

 

Strange, would have expected your carb to be fitted with swing type needle.

 

When you have the piston out of the carb does the needle have any side-to-side movement at its end?

 

There is no side to side movement when the needle is secured in the piston, good solid fit.

 

I have got another needle for testing and have tried to again to centre the needle. its not happy at all...

 

At both high and low jet position with the bearing nut loose the piston is silky smooth, however tighten the bearing nut the smallest amount at any position and the piston locks up.

Lossen it a little and it moves but gets suck with the needle fully in the jet. This is with a brand new AAU needle.

 

What am i doing wrong?

[carbon]

Monteemini,

 

You say that there is no side to side movement when the needle is secured in the piston, good solid fit.

 

If you take the piston out with needle all attached, how much side-to side (swing) movement is there at the end of the needle?

[Monteemini]

Monteemini,

 

You say that there is no side to side movement when the needle is secured in the piston, good solid fit.

 

If you take the piston out with needle all attached, how much side-to side (swing) movement is there at the end of the needle?

Literally no movement at all.

[Monteemini]

Monteemini,

 

You say that there is no side to side movement when the needle is secured in the piston, good solid fit.

 

If you take the piston out with needle all attached, how much side-to side (swing) movement is there at the end of the needle?

 

After looking at pictures of other HS4 pistons, looks like i have a non standard needle collet?

 

[carbon]

Monteemini,

 

AAU is a swing needle, there should be movement at the end of the needle.

 

Is there a brass collar with small spring attached to the blunt end of your AAU needle?

 

Without any movement the chances are the needle will foul the main jet, which is the problem you appear to be having.

[Monteemini]

Monteemini,

 

AAU is a swing needle, there should be movement at the end of the needle.

 

Is there a brass collar with small spring attached to the blunt end of your AAU needle?

 

Without any movement the chances are the needle will foul the main jet, which is the problem you appear to be having.

 

Thanks for the info Carbon

 

There is no brass collar, it looks like it as been replaced wit a DIY turned collar that is a very snug fit to the needle.

 

is it possible to just get the brass collar? there wasn't one in any of the rebuild kits from SU.

[Dusky]

Burlen SU sells the collars. Don't worry about the jet. You can use either with fixed or swinging needles. Only your dashpot piston is important and the jet centering if you have a fixed needle.