SU carburettor float valve needle assembly

[storeyn]

Hi all

Continuing problems with my 1500... since I had it periodically the rear (mainly, although the front did once) float chamber on my HS4 carbs keeps over flowing. I have cleaned the bowl out several times removing some debris, have now fitted an extra fuel filter. But it continues to overflow, normally after a cold start with choke, and a sharp tap to the top seems to cure the problem

I have had the valves out several times to clean and check operation and they seem to move freely enough.

I thought then perhaps best to replace, so checking the su carb website they don’t currently have any in stock, but looking at the exploded diagram it shows a needle valve assembly as opposed to the one I have fitted which is a ball bearing type arrangement... so do I have the wrong type fitted do you think? The ball has no spring fitted so relies on gravity for it to return.

I did also wonder if the fuel pump is delivering at a too higher rate? It doesn’t have the ac one fitted but suspect an aftermarket one. I do have two brand new ac pumps I could fit if needed.

Just wondered if anyone had any experience of this.... reluctant to go too far until cured as it drops onto the exhaust being the rear carb...

[storeyn]

Forgot to mention have also replaced all the under bonnet fuel lines from pump to carb

[cleverusername]

Does your valve look like the one in the link:

https://www.moss-europe.co.uk/grose-jets.html

Apparently it is a Grose jet valve and isn't suppose to stick like the old the needle valve.

Assuming it works as advertised, the problem must be either too high fuel pressure, incorrectly setup float or maybe a blocked vent.

However with the old needle valves, I have always found the problem to be a sticking valve.

[cleverusername]

Out of curiosity I did a quick search and there are complaints about the Grose type valves sticking:

https://www.britishcarforum.com/bcf/arc ... 58316.html

Some are suggesting the ball bearings need to be cleaned before installation but maybe switching back to needle valves wouldn't be a bad idea.

[storeyn]

Hi

That is my valve..

Looking at the second post might take them out again and give them another clean.

The fuel pump that is fitted is a Moprod one I’m not sure if the output is greater than a ac pump. I have a couple of those so might try fitting one instead.

Can’t get hold of the needle valves at the moment.. SU carbs are out of stock...

[AlanH]

I had problems with these. In the end I swapped them for the old type (there are plenty on ebay)

[storeyn]

Thank you for the advice

Have taken the plunge an bought an original set, I think I will change the pump to original too.

Will update on progress when I have fitted them....

[soe8m]

Thank you for the advice

Have taken the plunge an bought an original set, I think I will change the pump to original too.

Will update on progress when I have fitted them....

The fuel pressure on su float needles is around 0,15 bar. The max they can handle is around 0,3 bar. Some aftermarket pumps can go up to 0,5 -0,6 bar and the needles don't hold anymore. All pressure above 0,3 bar causes running problems. Too rich at idle because of "pushing" the fuel. When setting the idle mixture correct again then way too lean at driving. Having seen many fantasy mixture needles and settings in SU carbs over the years because of a wrong fuel pump fitted, mechanically or electrical. The best is to use a filterking pressure regulator and have a constant fuel pressure in all conditions so no variable during adjusting the engine.

Jeroen

[storeyn]

Hi Jeroen

I did wonder if it was too much fuel pressure, as listening to the valve when it was running it was clicking furiously...

I will get a fuel pressure regulator too, I have one on my sprint with its electric fuel pump...

Hopefully all here by the weekend and I can get it all fitted...

Cheers

Nigel

[storeyn]

Success....

I renewed the float valves with needle ones and put a filter king pressure regulator in...

No leaks so far and have driven around 100 miles... tick over is much better too consistent and correct.

Still running a bit rich on choke, but luckily doesn’t need a lot of choke. Will have a look at the mixture in the week or so...

Thanks for all the advice....