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Setting & balancing a pair of SU HS2s


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I've searched through several pages of results but can't find a guide to setting and balancing twin SU HS2 carbs, I'd hence appreciate advice from the forum.

My new-to-me car is running fairly well but the mixture is probably rich.  It doesn't like foot-to-floor acceleration in the lower gears, this gives a bit of misfire/rough running above, say, 3000 rpm.  (I haven't taken the engine over 4000 rpm as yet).  In fourth gear, using the throttle pedal more gently, it does seem to pull quite smoothly. The exhaust tail pipe has a layer of soot, as does No4 spark plug.  So I'm thinking that the first thing to do is to lean the carbs a bit. It's been many years since I last tuned a pair of these carbs.

I understand that, with the engine at temp, I first need to sync idle air flows (with air filter removed).  I plan to use a Carb Balancer for this.

Then, a combination of a Colourtune and listening to the engine speed/tone after using the lifting pin to set the mixture. My question should I put the Colourtune in cyl 1 and adjust the front carb, then in cyl 4 for the rear? Or is there sufficient mixing in the manifold to leave the Colourtune in one cylinder?

Car is Mk3 Spitfire.

Appreciate any help, thanks.


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you can set the air flow with cleaners off but you need it in its running state to adjust mixtures 

there are many manuals on setting up the carbs , 

eg http://vitessesteve.co.uk/Other-manuals

not a fan of colour tune and you realy need two to set up twins, as for lifting pins thats a very touchy feely thing you just lightly lift to just make contact with the air piston then raise 1 - 2 mm and listen for a hint of change , not a dramatic change like + or - 50 rpm 

+ is on the rich side 

- is weak

0 is about right 

but if you remove the air piston, set the jet flush with the bridge and then back it down 3 turns is a good base start setting , and both carbs wil be set the same 

( which is what balancing is all about ,) in the factory the carbs would just be set to pre determined mechanical setting ,no gauges or faffing about  , bolt em on and off she goes . 

see also http://sucarb.co.uk/technical-carburetter-maintenance-settings-hs




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Thanks, Pete, very helpful.

This morning, off came the airfilters and on went the Carbalancer, last used in 1976.

I put a Colourtune, also last used in 1976, in cyl 4 and it showed blue/orange combustion.  I also used the lifting pin method to assess the mixture.

I leaned the rear carb; the lifting pin still increases speed but seems better.  The Colourtune in cyl 1 shows more orange than blue.  I tried to lean the mixture of the front carb but it didn't seem to make a great deal of difference so I took the air pistons off.  The rear carb jet to bridge measurement was 0.15 inch and the front, which would not tighten any more, was at 0.095 inch.

The front carb seems to be giving a very rich mixture.  The lifting pin caused an increase in engine speed.  Not happy with the differences in the bridge gap between the two carbs, I've readjusted the front to the same measurement as the rear, even though that should make it more rich.  Something doesn't seem right.

The plugs were very sooty.  I'd quite forgotten the joy of removing the plug from cyl 3.

I've also noticed a very small amount of fuel weeping from the bottom/adjuster mechanism on the front car; there's a bit of staining on the chassis crossmember as you may be able to see in the pictures.

I'll take the car for a drive later today.

Any ideas as to why the front carb doesn't seem to be responding to mixture adjustment?






petrol stain.jpg

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Check the float level in the front carb. It's probably over-full, leading to both the rich mixture and the fuel puddling. It may even be dribbling out of the vent hole in the float camber top. I can't see clearly in your photos but the fuel hose looks lower than I'd expect - a photo of the plumbing might help.

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i have found on the plastic SU floats the platform the needle sit on wears a very small ridge, this locks the float needle and some random flooding persists

any minor flatting just upsets the float height . a new float solves the ongoing mystery

you need a good magnifier to see the wear pattern , as to the naked eye all seems well ...

and its worth fitting a new viton tipped needle while youre in there 

Burlen offer a Stay up float to solve any  ethanol issues 


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Thanks to you both.

I've attached a picture to show the layout, hope this helps.

Took the car for a drive yesterday.  It has done less than 50 miles since rebuild.  It wasn't pulling smoothly. I reached the dizzy speed of 50 mph (fastest so far!) and after a short while the engine felt dreadful and I was sure that it was about to stop.  Bizarrely, it seemed to recover and then accelerate quite smoothly in all gears.  I ran out of road as I don't yet want to venture too far from home (for the first few miles, I had my wife follow me in another car).  Max subsequent speed was about 40 mph and I did another 5 miles or so.  Idle speed is inconsistent, too.  I set it to around 900 rpm , the lowest, smooth running speed.  Sometimes it will idle at 11-1200, other times its less than 1000.

After the drive I removed No4 plug and it was still sooty but less than before.  Likewise, the tail pipe had a layer of soot but not as bad as before.

The gland nut was not tight on the front carb, hopefully the reason for petrol weeping.  I checked this morning and there was a tiny, tiny amount of fuel around the bottom of the jet/float flexible pipe. I'll check again later.

Any thoughts? All input gratefully received.


Carb general view.jpg

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Worth checking the needles are the same in both carbs.

Somewhere, they will have a 2 or 3 letter code stamped on them and they should be the same letters on both needles.  I think maybe right at the top so you need to remove the needle from the piston to read it.

Standard needles for a Mk3 would be "BO".

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