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SU Carbs - Erratic Burn


Dazman1360

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Hi All,

Last June I fitted a new engine along with a pair of HS2 SU Carbs to my Herald. Ever since then I have been living with a rough idle, but once you get the revs up the engine runs well and seems nice and smooth. The carbs were off ebay and I reconditioned them before fitting. Today I thought I'd have one more attempt at smoothing out the idle so it was back to basics and set them up again from scratch. The good news is I'm pretty sure I now know what the problem is, bad news I'm not sure what's the solution.

With the aid of a colortune I can see that the front carb is giving a really erratic burn, the colourtune is showing it move around from blue to red constantly. As you richen the mixture the red becomes constant as you'd expect, but as you lean it off you cannot get a consistent blue burn, no matter how hard you try. The rear carb behaves exactly as you'd expect and I can get a consistent blue burn with that one.

One thing I did notice is that on the top of the needle guide there is a ridge along one side as shown in this image:



Which means the needle will always be off centre when fitted, as seen in this image:



This ridge is on both guides, but more prevalent on the misbehaving carb. It doesn't cause an issue with the pistons, they drop with the required metallic clunk quite happily.

I have ruled out air leaks, float & float height, the only serviceable parts not changed on the carbs were the floats and butterflies. The spindles were changed but not rebushed, but doesn't appear to be pulling in air here.

So my questions are should this ridge be on the needle guides?

Are there any other potential causes for erratic burn?

Cheers



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Suggest you check the numbers on the needles and see if they're the same. That ridge doesn't look right to me but others on here may know better. Make sure the bush that holds the needle is flush with the piston body. Apart from checking float levels to see if they're the same I know no more from a distance !
Tony.

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Check float height and needle valve condition.  Also make sure that the float chamber is still flexibly mounted to the main body of the carb.

Some SU pairs also have different depth float chambers fitted to front and rear carbs to allow for engine installation angle affecting the fuel level as seen at the main jet.  Don't think the Spit carbs were like this but still worth checking as you never know what could have happened over 40 years of POs.....

Nick

PS, the lump on the jet holder is intentional.  These sprung needles are intended to lightly touch the side of the jet and the lump is there to give a positive bias in a particular direction.

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Hello Dazman,

this is a personal observation so may not be correct. I think that any oil being burnt in the chamber will give an erroneous reading with a colour tune, i.e. make it look rich, could that be the case in your engine? At idle you'll have a fairly high vacuum so may be drawing oil down the inlet valve stem?
As you say the car runs well, double check the mixture by checking the plug colour after a good run and adjust accordingly.

Alec

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piman wrote:
Hello Dazman,

this is a personal observation so may not be correct. I think that any oil being burnt in the chamber will give an erroneous reading with a colour tune, i.e. make it look rich, could that be the case in your engine? At idle you'll have a fairly high vacuum so may be drawing oil down the inlet valve stem?
As you say the car runs well, double check the mixture by checking the plug colour after a good run and adjust accordingly.

Alec


Hi Alec,

This is a recently rebuilt engine, new valves & guides 3000 miles ago, plugs on 1 & 2 are dry, and are a biscuity brown with hits of black which kind of backs up what can be seen through the Colourtune. No sign of burning oil.

Darren


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