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Another SU HS4 Question


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Good Evening All,

I have finally got my Spitfire 1500 back on the road (perhaps the only good thing to come out of lockdown for me!) However, the SU HS4s are causing me a minor headache.

When I press on the accelerator the rear carb's butterfly does not seem to close properly. I either have to blip the throttle or I have to manually push it closed. The amount it is open is small, but it raises the idle speed to about 2000 - 2200 revs unless closed.

I have been experimenting over the last week to try and pinpoint the exact source of the issue and have identified the following:

- The butterfly will only remain open if the engine has warmed up to temp. At first I thought that this was the linkage binding, but I have checked this and verified that it is free to move with enough of a gap in all directions not to expand and cause any issues.

- I then thought the butterfly itself was at fault as in the past I have changed the butterflies to solid plates (opposed to the poppet type). I even went so far to remove the poppets from the previous plates, solder up the holes and re-install the old butterflies incase the new butterflies were not made correctly. I also checked, double checked and triple checked the butterflies were the right way around.

- I checked that the throttle cable, choke cable and linkage were all functioning correctly and not holding the throttle open slightly.

What I have now identified is that this phenomenon ONLY occurs when the engine is actually running. When it is off and hot I can exercise the rear carbs butterfly shaft freely and it always closes fully. It is almost as if the amount of airflow pouring into the carb creates just enough resistance to overpower the spring and not allow it to gently close. However, when you 'blip' the throttle it creates a more aggressive closing force that sometimes will shut the butterfly fully. The springs are new and I don't think they are the issue... I think the 'easy' option would be to just install stiffer springs.

Maybe this issue will disappear if I re-install the air filters? I have had them off for my several experiments. I am not entirely sure which avenue to pursue now so any tips would be greatly appreciated! 

Other than this small issue the carbs and engine run great! 


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When you changed the butterflies, did you re-centre them properly into the shaft with full movement? That's a common cause of stickiness.

The other thing that seems likely is that the shafts are worn. That's the only way I can think that air flow (or manifold depression) would influence them. When closing from near idle, the pressure difference across the throttle pushes the shaft against one side of the bore. If the shaft and/or bore are worn, that might make it stiff (or even catch on some roughness).

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Hi Rob,

Thanks for the response.

I did re-centre the butterflies when installing. I hold up the body to a light and check that there is a uniform slither of light around the butterfly. I then exercise the throttle shaft  to check it is seated properly once I have tightened down the split screws.

I would agree with you regarding the throttle shafts. This was my first assumption. However, these are new-ish shafts (less than 200 miles) and I have also tried using some throttle shafts with o-rings in them. I found that the ones with o-rings added too much friction to the whole setup. 

I understand that the bushings can wear in the throttle body too. However, I have done the standard tests of spraying something at the throttle linkage whilst it is running to check for an air leak etc. I have even tried to check for play and movement with more 'scientific' means (using a dial gauge indicator) but cannot detect anything that has raised alarm bells so far.

Maybe, as you suggest, there is some roughness somewhere that I cannot detect but is adding some friction to they shafts. Is there any 'at-home' solution to this? Or do I need to send these carbs off for a re-bushing? Anyone have a good contact for who can do this in the UK? In the past I have been reluctant to send the SUs off for a complete restoration due to the cost (and I like doing these things myself).

Thanks again for the thoughts. I will continue my investigations today! 

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It is also worth noting that your symptoms are very similar to a problem I had on my 1500-engined Mk3 when I first got it running again. That turned out to be my own silly fault - when I rebuilt the carbs I managed to fit one of the return springs in the wrong position.

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Hi Rob,

Interesting. Do you mind expanding on what spring you had put in the wrong position?

Regardless, I will be sure to check mine.

The only thing I have noticed is that I only have the two throttle return springs - the ones that run parallel to each other and attach to the funny U-shaped bracket that also holds the throttle cable. However, in some diagrams I have also seen a third spring added that attaches to the exhaust. My setup has never had this spring. 

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If you look in this photo there is a coiled spring on the carb body, to shut the throttle even when there's no linkage fitted. That was the one I got wrong - hooked round the wrong part of the casting. The two springs to the cable bracket, or the one spring from the linkage to the exhaust, are the secondary springs - the pair on the late linkage, single on the early one.


Although now that you've queried it, my forgettery is hinting that it may have been the choke return spring I fitted wrong. They're very similar.

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Thanks for the picture. I see now that your carb setup is different to mine. My understanding is that there are two types of HS4 throttle shaft setup. Yours, and then mine has large 'arm' which acts as a pivot to shut the butterfly using the spring on the cable bracket.

I have added a picture to try and demonstrate what my setup is like.

Since this issue only comes into play when the engine is hot and I cannot replicate it unless the engine is running and air is being drawn through the venturi I reckon it must be throttle bush wear. Even if it isn't enough for me to noticably measure it must be causing some friction when things expand.

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Hi Paul,

Thanks for that. I hadn't noticed that before. I have some new screws as the two fast idle screws were somewhat 'forced' in my the previous owner and are a bit bent.

That being said, although the picture does not really show it the fast idle screws are backed off enough to clear the cam. As such they shouldn't be the cause of my sticking butterfly issue.



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