SU H6 idling speed adjustment

I’ve just rebuilt my H6 carbs, which have also had the throttle spindles re-bushed with new shafts and throttle discs. I’ve set the mixture screws to 2 1/2 turns out from fully in as a starting point, and am wondering how far the throttle discs should be open for initial starting. Is there a recommended number of turns on the idle adjustment screws from where they just make contact with the pad?

Here is one procedure for getting going. Disconnect the linkage between the two carbs and close the throttles all the way on each. Then put the adjustment screws to be just free enough of the stops that a dollar bill pulls easily through the gap. That way you know the starting position of each screw and can keep them all similar. Then put the connecting linkages tight again. Recheck the dollar bill clearance stayed the same and the throttles all are completely closed. Then screw all adjustment screws the same amount of turns down to be able to start and keep the engine running cold, low enough turns to keep the engine moderately low, say 1000 rpm. Let engine warm and then proceed with adjustment screw reduction and idle jet adjustment. Others may have different methods or perhaps a correction to this.

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Thanks, Roger. I think I’ll start with the idling screws screwed in one turn from just making contact with the stop pads (I guess a £5 note would work as well as a dollar bill…) I’ll start it with the button on the solenoid and then screw the idle screws in enough to get about 1000rpm idle speed and wait for it to warm up. I have a balance tool so I can have each carb sucking the same.

Hi Chris.
I recently checked and tuned the carbs on mine. After confirming timing, distributor points gap, spark plug gap, float levels I followed the procedure in the workshop manual in regards to adjusting the mixtures (engine at operating temp) by lifting the dash-pot dampener of one carb and with a tacho meter connected I could observe it performing just as the manual said it would and making adjustments to the other carb I got it running nicely.

Refitted the carbs yesterday and got the car running. All wearing parts were new - needle valves, floats (unsinkable), jets, needles, all sealing washers of various sorts. Set the float levels, set jets screwed out 2 1/2 turns from fully in, and after looking at a YouTube video on setting up SU carbs on an Austin Healey, screwed the idle screws in 1 1/2 turns from just touching the pads. Once I’d got the 3 wires correctly connected on the starting carb, it fired straight up, but it was revving way too high - what’s correct for an Austin Healey is obviously not so for an XK120! After the engine had reached operating temperature I got the idle down to about 750rpm. I can already sense a real improvement since the throttle spindles were re-bushed. When I receive my Crypton Synchro-Check carb balancer this week I will balance the carbs and play around with mixture adjustment. So, all is good so far! Thanks to all for your help.

Did the final carb tuning yesterday - very pleased.

The Crypton Synchro-Check tool worked very well, though checking the air flow on the front carb was tricky, due to lack of clearance with the inner wing. Using the rubber cone attachment, I had to hold it at a very funny angle which meant reading the scale somewhat awkward. However, I managed it in the end.
With a nice smooth idle at 500rpm, thanks to the re-bushed spindles(!) I then set about fine tuning the mixture, the jets having been already set to the recommended 2½ turns out from fully screwed in. First off, I tried doing it by the service manual, ie: lifting the piston a small amount to in order to adjust the jet on the other carb… No matter how often I tried this it made absolutely no difference! The engine just speeded up a bit. Finally, I tried adjusting the carb whose piston was being raised, and hey presto! immediate results! Now, I remember reading a post here some time ago which discussed this very situation, but I haven’t been able to find it. I can only conclude that either the factory procedure is wrong, or there is some difference between an A Type and B Type heads regarding inlet port design. My 120 currently has an XK150 B Type head.

Another thing - I was surprised that there seemed to be quite a bit of fuel leakage down each jet adjusting screw. I can only think that the small seal on top of the jet is allowing fuel past. I’m using the black greasy type of seal as supplied by Burlen as opposed to the original cork type. I think this is to protect better against modern fuels? I’m starting to wonder if maybe I’ve left some remnants of the original cork seals in there…? However, there are no leaks once the dome jet cover caps are screwed back on, as they have a small fibre sealing washer in them.

Pretty much everything that can wear has now been replaced - floats, needle valves, jets, needles, all sealing washers and glands, etc.

By the way, the factory procedure for adjusting the thermo starting carb seemed to work well.

Anyway, she’s now purring like a kitten, with rock steady 500rpm idle. Once the weather clears up I’ll see how the driveability has improved.

If you raise one piston a small amount, you ARE evaluating that carb…raising the piston a large amount makes THAT carb inoperative, thereby evaluating the OTHER carb…so…you got there nonetheless!

Hi Chris.
The tuning you are referring to is in this post above your latest.
It’s funny you mention that as I’m sure the tuning procedure for the Mark V is as you describe it regarding tuning the carb you are lifting and after reading the opposite in the 120 / Mk VII manual I thought it important to point this out.
I realise I am talking about two completely different engines but for the procedure to do a complete turn around has me surprised.
Anyone with greater knowledge able to clear this up?
Regards, Graham

So, what is a small amount and what is a large amount, Lee? I found it’s very hard to judge how much you are lifting the piston. Would the full lift on the lifting pin equal a “large amount”? I was lifting the piston directly with a flat-bladed screwdriver on its edge only a very few mm.
Thanks, Chris

Thanks, Jordy. I wasn’t sure which car it was you were tuning - might have been a 140 or 150, which may have a different procedure, for all I know. I was referring to a past thred where the whole subject was gone into in some detail, but I can’t find it or remember the conclusions that were drawn!
Thanks, Chris