Su float bowl needle

Post Reply
trevair
moderator
Posts: 27
Joined: Sat Apr 17, 2021 9:27 am
Location: rawtenstall
Contact:

Su float bowl needle

Post by trevair »

Hi all
Just need to veryfy the float needle turn carb upside down and the middle lowest point of the float sould line up with the carb outer body
I get this but is that with or without the neddle spring compressed ie its level as in the drawing but you can still ppush down onthe spring in the needle
Regards trev
User avatar
Interpol2471
moderator
Posts: 1254
Joined: Mon Oct 20, 2008 1:02 am

Re: Su float bowl needle

Post by Interpol2471 »

Trevor you are looking for the alignment as the spring is taken up just seating the needle to stop fuel flow. A good test is to connect a pipe and blow gently feeling where the needle seats nicely and stops fuel flow. On these float height isn't critical as long as pretty close most importantly is the jet height setting.
Various rubbish in various states of decay.....
trevair
moderator
Posts: 27
Joined: Sat Apr 17, 2021 9:27 am
Location: rawtenstall
Contact:

Re: Su float bowl needle

Post by trevair »

Hi interpol i have phoned su and they cant help when you do as they say straight edge across carb body fine but if you do that you can then depress the float bowl further down as the spring goes in the float needle
Now if the spring can go in can the needle not be pressed down when in situ full of fuel by it depressing the spring
Wish i would have stuck to triumphs or matchless
Regards trev
Clive603
moderator
Posts: 33
Joined: Mon Oct 15, 2018 9:42 pm

Re: Su float bowl needle

Post by Clive603 »

Trevor

You are overthinking this.

When the carburettor is inverted the float needle rests on its seat and the spring compresses just enough to support the weight of the float. So long as the float sits at the right height the fuel level will be correct when installed.

This is just a set-up test.

When the engine is running the uplift forces on the float from the fuel in the float chamber act via the spring pressure on the needle to stop fuel flow when the fuel level in the float chamber is at the maximum acceptable. Which may or may not mean that the position of the float is the same as you observe when checking the setting with the carburettor inverted. As far as we are concerned the real, in service, float position doesn't matter. What does matter is that, assuming none of the components are defective, a correct position when tested translates to a correct position when running so the fuel level will be right and the carburettors can be properly tuned.

Clive
trevair
moderator
Posts: 27
Joined: Sat Apr 17, 2021 9:27 am
Location: rawtenstall
Contact:

Re: Su float bowl needle

Post by trevair »

I just eish there was no spring in the needle like other bike iwhich is open or shut simples right height in seconds
Ok this is last post as im losing the will to live
If you imagine the carb is on the bike with fuel in the carbwhen the needle is in the shut position ie the float rising is closing it
Would the spring in the needle be shoved in or extended

Really sorry to be a pain all but dont want to have to remove carbs again
Best regards trev
Clive603
moderator
Posts: 33
Joined: Mon Oct 15, 2018 9:42 pm

Re: Su float bowl needle

Post by Clive603 »

When the float is lifted by more fuel flowing into the float chamber it compresses the spring which pushes the float needle up onto the seat shutting off flow when the maximum level is reached.

So the spring actually operates the valve but the float has to rise high enough to compress the spring enough to shut off fuel flow.

SU carburettors are designed to operate with pump fed fuel systems. Specifically the SU electric pump which has a pulsating flow. Spring operated float needles are pretty much essential with pump fed systems to damp out flow pulsations and generally make things behave better at the higher fuel line pressures.

Simple solid float needles, as per most Amals, work well enough on gravity fed systems with only a few inches of head between tank and carburettor but don't do so well when pump fed. Sprung needles cope with both types just fine.

Clive
trevair
moderator
Posts: 27
Joined: Sat Apr 17, 2021 9:27 am
Location: rawtenstall
Contact:

Re: Su float bowl needle

Post by trevair »

Cjeers clive think ive got it now
Cheers trev
Post Reply