yorkshire_spam Posted March 20, 2020 Share Posted March 20, 2020 I've rebuilt a pair of HS4 carbs for my Spitfire 1500. I had the carb bodies faced and new spindle bushes fitted and then did the rest of the work myself. I'm trying to set-up and tune now, it doesn't seem to matter what I do, I get a reasonable idle but it stutters if you apply the throttle and will pop back through the carb if you push it. The only thing I've changed on the engine is the carbs - so timing is exactly as it was (running nicely) on the old carbs. Any ideas what I'm missing? I've tried a couple of different needles, at the moment AATs Link to comment Share on other sites More sharing options...
thebrookster Posted March 21, 2020 Share Posted March 21, 2020 I would be considering the dashpots maybe? Starting with the really simple - you did remember to add oil to them? Springs the same? Dashpots are moving freely as well? Also, fuel level is correct? My thoughts anyway, for what they are worth! Phil Link to comment Share on other sites More sharing options...
PeteStupps Posted March 21, 2020 Share Posted March 21, 2020 Spitting back usually indicates over-lean mixture, if I recall correctly. Sure you've already done this but have you had a thorough check over for potential air leaks? Does it behave any better with a hint of choke? Link to comment Share on other sites More sharing options...
yorkshire_spam Posted March 21, 2020 Author Share Posted March 21, 2020 Thanks Phil/Peter, appreciate the sense check and ideas. Can't find any air leaks, but I'll check again Choke doesn't seem to help Pistons move freely Springs are the same Plenty of oil in the dampers (engine oil? Should I try something else? I'm inclined to look at the fuel levels in the jets and the float settings. Link to comment Share on other sites More sharing options...
Nick Jones Posted March 21, 2020 Share Posted March 21, 2020 Fuel level check is sensible. Did you keep the pistons and dash pots matched? The SU book (maybe one of the Vizard books) has a dashpot/piston drop test description in it. Engine oil in the dash pots should be ok. Spitting back on throttle opening implies weak mix(as said above) /piston rising too rapidly Link to comment Share on other sites More sharing options...
zetecspit Posted March 21, 2020 Share Posted March 21, 2020 Timing unchanged? Only the carbs been worked on? I would set up as the initial settings once fuel levels checked. I think 2 full turns down from level jet? That will be (or should) close enough to run well if all else is OK. But you know that! Have had odd issues over the years where the base setting won't run correctly, Usually takes a bit of digging but it is a good indicator. Link to comment Share on other sites More sharing options...
yorkshire_spam Posted March 21, 2020 Author Share Posted March 21, 2020 Thanks Clive/Nick - jet height doesn't seem to make much difference. My base setting is 1mm to 1.5mm below the "bridge" because of the slight change in jet due to "knobbling" the waxstats. That generally seems ok, not this time! I'll look up the rise/fall test for the pistons - they seem to move nicely and have obvious damping when moved by hand. I did everything I could to make sure the pistons/dashpots stayed together and matched, I guess there's an outside chance that the set I rebuilt were already mis-matched? Only the carbs have changed. No timing or owt like that. Most annoyed, I've probably missed something totally obvious/stupid. Link to comment Share on other sites More sharing options...
zetecspit Posted March 21, 2020 Share Posted March 21, 2020 If both pistons fall at the same rate, that should be a reasonable test. If jet height is making little difference fuel level could well be the thing to check. And an aside, there are a few types of fuel bowls with different heights. Was this a pair of carbs you had previously used?? Link to comment Share on other sites More sharing options...
yorkshire_spam Posted March 21, 2020 Author Share Posted March 21, 2020 5 minutes ago, zetecspit said: And an aside, there are a few types of fuel bowls with different heights. Was this a pair of carbs you had previously used?? Ahhhhh! I think you might have it, but need to get out to the garage to check. I didn't check that the bowls were the same on the "donor" carbs vs the originals..... time for a trip out to the garage. If that's it then I feel very stupid. Link to comment Share on other sites More sharing options...
yorkshire_spam Posted March 21, 2020 Author Share Posted March 21, 2020 Bugger, that's not it. Correct float bowls front and back. Link to comment Share on other sites More sharing options...
zetecspit Posted March 21, 2020 Share Posted March 21, 2020 Worth a try... Hmm, fresh out of dumb ideas. Link to comment Share on other sites More sharing options...
mpbarrett Posted March 21, 2020 Share Posted March 21, 2020 Are the needles new, are they the correct one and are the needles fitted in the piston correctly? Mike Link to comment Share on other sites More sharing options...
yorkshire_spam Posted March 21, 2020 Author Share Posted March 21, 2020 Hi Mike, I've tried 2 sets of used needles that I had from the previous carbs. After a bit of furtling around in the garage: Float bowls have the same part numbers as the old carbs. If I yank the jet down using the choke cable the fuel level in the jet looks way too low to me... it only just come over the top of the jet when it's at it's lowest. I checked the float level VS the gap given on burlen's website and they are in range (just over 3.1mm) and about the same Stuck... how do I have fuel level in BOTH jets too low, but float level looks ok? I'm starting to wonder if the fuel pump has developed an issue during the lay-up or something. Superstition is starting to overtake logic! Arrrrgh I really must have done something totally dumb building these carbs. Link to comment Share on other sites More sharing options...
2.5piman Posted March 21, 2020 Share Posted March 21, 2020 Hello Mr Spam, while I normally go by setting S.U.s by the book, and if you need move far from the base setting something is wrong elsewhere. However the fuel should be about 1\8" below the top of the jet tube, and HS4s do not have adjustable floats like many other S.U.s.. Sometimes people lower the level by adding washers to the needle valve, I would have a look there in case? (As you say they are donor cars). I doubt that it is a fuel pump problem as at the moment you are only filling the bowls so not really putting it under much load. Another thing is the attachment adapters to set the chamber vertical irrespective of the angle of the carburettors. Could your donor floats have the wrong adapters for your car? Alec Link to comment Share on other sites More sharing options...
PeterC Posted March 21, 2020 Share Posted March 21, 2020 How long has the fuel been laid up? Link to comment Share on other sites More sharing options...
PeterC Posted March 21, 2020 Share Posted March 21, 2020 Just now, PeterC said: How long has the fuel been laid up? if from last summer, its less volatile than winter grade, and may well have gone off too. Peter Link to comment Share on other sites More sharing options...
yorkshire_spam Posted March 21, 2020 Author Share Posted March 21, 2020 Thanks Alec/PeterC. Fuel is about 3 months old and was 98ron when it went in. I'm pretty sure the level in the jets is way too low, I just haven't figured out why. I'll try and get back out to the garage this evening and have another look at the float bowl mountings, floats, valves etc Link to comment Share on other sites More sharing options...
PeterC Posted March 21, 2020 Share Posted March 21, 2020 The depression at the jet in normal operation is enough to lift fuel 8 inches. https://supertrarged.files.wordpress.com/2017/07/final-pdf-how-does-an-su-carburettor-work-iwe-2017.pdf if the engine starts and runs even after afashion I doubt its fuel level I would check the points gap and timing Peter Link to comment Share on other sites More sharing options...
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