What is the spring for?

[petersy]

On cleaning and replcacing the carbs on my 1974 CB550 four, one of them is overflowing and luckily it is the number 4 (end) carb, so I can remove the float bowl quite easily to check the float valve.

But I can't figure out the purpose of the small sprung needle in the centre of the needle valve.

Does anyone know please?

[mike the bike]

It's to keep pressure on the needle valve into the seat to stop the petrol flowing.

[petersy]

Wouldn't the floats keep pressure on the valve even if the spring wasn't incorporated in the needle i.e. just a plain fixed pin lodging on the float tab?

[Bryanj]

Its so the pressure stays when the floats are bounced by the supersmooth roads

[petersy]

Never thought of that - thanks for enlightening me.

[petersy]

Its so the pressure stays when the floats are bounced by the supersmooth roads

Just taken the carbs off my CB550 K1 again (which I could have done without).
After taking the float bowls off and turning the carbs upside down, I lifted each float in turn and let it drop back onto the needle valve.
One float sprung back (off the spring in the valve) and the other 3 just dropped flat.
So I'm assuming that the tab on the float spindle is pushing the (weak) spring into the valve 'before' pushing the valve up into it's seat - hence the valve not completely closing and my carbs overflowing.
I'll take acloser look later.

[petersy]

Further to my previous post:

I'm assuming that I might need new float valves but they are not cheap.

Having looked on eBay I found these:

http://www.ebay.co.uk/itm/AFTERMARKET-CARBURETTOR-FLOAT-NEEDLE-VALVE-HONDA-CB550-CB-550-74-78-NEW-/281612951431?pt=UK_Motorcycle_Parts_13&hash=item4191709f87

They definitely do not look like the ones in my carbs.

Has anyone tried these before or is the eBay listing incorrect?

[mike the bike]

They sound better than your existing ones, go for it.
When setting the float heights, adjusting the tab on the floats, remember not to compress that float spring atreat all.  I stand mine up on end to keep the weight of the floats neutral.

[petersy]

Once the bank of carbs were off, I put them on he work bench with inlet ports facing upwards, which leaves the tab lightly touching the sprung needle.

After checking them in this position, all the heights were 22mm.

When blowing down the fuel line and then pushing the floats upward, the airline is blocked on all carbs, so I assume that it is the weakened springs that is letting the fuel through and causing the carbs to overflow.

[petersy]

Ordered new set of four float valves for £35 compared to £64 at David Silver.
Waiting for them to arrive and get them fitted, to see if it solves the overflow problem.

[petersy]

My CB550 float valves arrived from Germany this morning and are now fitted.
When I fired it up it sounded great with the American 4 into 2 exhausts sounding off in stereo unison.
I've hit the sweet spot!
It just shows how weak springs in the needle pushrods can make such a difference.
Will now put the rest back together and post a picture when I've finished.

[mike the bike]

Well done, glad it's sorted 👍

[petersy]

The CB550 is now road ready - but still waiting for the registration to come back from the DVLA

[petersy]

My CB550 is on the road at last...

Video here:
https://youtu.be/PKgI_SKtObU