Dazman1360 Posted April 22, 2014 Share Posted April 22, 2014 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 Quote Link to comment Share on other sites More sharing options...
Dazman1360 Posted April 23, 2014 Author Share Posted April 23, 2014 I did swap over the the needle guide & needle between the 2 carbs, the one with the more prominent ridge now being on the rear carb, this didn't change anything so doesn't appear to be the cause of the problem. Quote Link to comment Share on other sites More sharing options...
nang Posted April 23, 2014 Share Posted April 23, 2014 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. Quote Link to comment Share on other sites More sharing options...
Dazman1360 Posted April 23, 2014 Author Share Posted April 23, 2014 Needles both the same AAQ, and fitted flush, so happy with that.D. Quote Link to comment Share on other sites More sharing options...
Nick Jones Posted April 23, 2014 Share Posted April 23, 2014 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.....NickPS, 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. Quote Link to comment Share on other sites More sharing options...
Dazman1360 Posted April 24, 2014 Author Share Posted April 24, 2014 Float height checked and correct, the float chambers do have some movement.Thanks for for explaining the needle angle, at least I can discount that now as a problem. Quote Link to comment Share on other sites More sharing options...
piman Posted April 24, 2014 Share Posted April 24, 2014 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 Quote Link to comment Share on other sites More sharing options...
Dazman1360 Posted April 24, 2014 Author Share Posted April 24, 2014 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.AlecHi 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 Quote Link to comment Share on other sites More sharing options...
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