Tuning triple carbs

Hi all hope u r all well, not been on here for a while but hopefully u guys can help, I got the car out after a few years , drained the fuel etc starts fine but when driving it’s popping from the carbs under load , it’s got 3 carb 2 inch hd8 from memory, Xj6 electric dizzy , I feel it’s running rich but I am not sure , I can see two screws on each carb .i unwound the little one then turned them down two turns the bigger screw I turned them out 1.5 turns .
I don’t think there’s not any other screws that I can adjust ?

You need to make yourself familiar with the HD, they are easy to tune, set the jets flush and then lower them two and a half turns, small screw, throttles are disconnected so they are individually closed, connect the linkages when you are sure. Then set the idle screws (large ones), until they sit and back up three and a half. That is your starting point. Don’t forget the damper oil, many use atf or engine oil will work.

Start engine, should run, let warm up. Set idle speed and listen to the hiss of each with a length of hose to your ear. With the idle screws, get them equal. Try to make them idle at the desired rpm, roughly.
Now lift the pistons just a little. One or two mm. This makes the mixture lean. If the engine gets faster, you have to raise the jet a little. If it slowly dies, jet must be screwed down.
Give it a bit of throttle every now and then so they can “reset” themselves.

If on all carbs the engine shows no reaction or increases just a hair, reset the idle and check again. You now have the idle set.

If after all that the idle is fine but it’s stumbling when you drive, it has the wrong needles for your engine and apparently for today’s fuel. Remove the needle from one carb, code should be on the shank. Go a size up, or two - and while they are in the mile you can drive with a fatter mixture so it doesn’t stumble; the stumble is bad, and who cares about idle.

The HD8 idle on the main jet and if you have the right needle you have good mixture when the idle is set right. In my case the UM needles are way rich at idle when it’s set so it’s driveable and not too lean. Popping from the carbs indicates very lean, unwinding the little screw makes it worse. They are so many documents on tuning the carbs on the Internet.

Is the fuel filter clean?



Hi David thanks again for your help.
I’ve managed to do what you suggested and noticed the back carb nearest the bulkhead had to have the little screw nearly all the way in , I’ve not driven it yet but on hard throttle the carbs still popping but nowhere as bad ,now the filter is off should I buy one of them balances that you push into the carb.
Kind regards

I would try a hose to the ear first as they suggest in the manual. Did you make sure your float levels are okay?

David I disconnected the linkages unscrewed the little screws and put in feeler gauge to make sure the screw wasn’t touching unfortunately I’ve got cataracts so I can’t see great , I undid the underwing panels so I could take the filter casing off so I could lift the piston slightly, the middle and front carb wasn’t to bad but when I lifted the rear one it stalled straight away but was ok after adjusting the little screw.
I will try to get the manual about the hose test.

More accurate way to balance is to use a Unisyn air flow gauge. It has a clear tube with a small red plastic ball inside that raises with air flow.

I have one of these (uni syn) and I didn’t like it much at all. But they look cute.
The hose, just put one end in your ear and the other in the throat (of the carburetors!) so you can listen to the hiss of the airstream. Then compare. I was sceptical but the uni syn didn’t make it scientific.

The pistons don’t have to be lifted much (too much and the engine will always die). I reckon that 1-3 mm should do it, just lift them a little.

This reduces the vacuum, less fuel is sucked out of the jet, and the mixture gets leaner: if the engine dies immediately, you need to lower the jet on that carburetor, that means screwing the mixture screw down. Then just check, repeat, etc. until you have it dialled in. And if you can’t get it right… start again… and again. It’ll run sweet eventually!

I’ve used a Uni-Syn for decades, never having mastered the art of hiss-tube-listening, though that may have something to do with my tinnitus. In order for the Uni-Syn to function properly you must press it firmly onto the carb throat after removing the carb trumpet assembly, which is a bit of a pia but how often do you have to rebalance your tricarbs anyway?

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Me too! I have two, dating from the sixties, for my Corvair. I used them for the Jags for probably twenty years, but (finally closer to the present) I got one I like better about 20 years ago. It reads in absolute units (no adjustment), fits the choke without removing anything, and seems more stable. Got it used; photo is of one on the web.

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Bekause itz German! Looks nice.

I put a donut of foam on the unisyn so the air filter hardware is not in the way and it seals. At idle rpm the engine dies. Maybe it’s my fault. When I have it so it just registers in the lower third, the engine idles okay but my ears do a little better than the unisyn.

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Exactly what I did so I don’t have to take the K&N base plates off. Works like a charm.

Me too. Since High School actually, and I don’t want to think how many decades ago that was….

I also have a pair of rods that press onto the piston damper chambers. They have pointers so you can watch as the pistons rise to ensure they are moving the same amount.

Hi all , just an update the car started first time with the choke ,ran nicely and settled down lovely but unfortunately she’s still holding back &popping when driving , I checked the needles and they are UE
David suggested new needles which I can order but which ones should I try first.

If you go to mintylamb.co.uk/suneedle you can enter your needle (HS8 will do) and compare. UM is standard for the twin and triple 4.2 engines. I don’t know more than that, have zero experience (my needles are in the mail) and can’t help you.

Make sure that your fuel supply, float bowl vents and air filters are all good. It will run lean without the right filter and the needles have to be changed if a different filter is used, etc.

If it used to run like it is I would look into anything but the needle. Easier things first, think of it like a problem tree and cut off the easiest and largest branches first. Air - filter, choke. fuel supply - filter, line, tank, float level, float, jet, then adjustment, etc.


I was curious:

Thanks David ,unfortunately that chart doesn’t mean anything to me I wish it did, carbs rebuilt ,new tank,lines, air filter the original rectangle under wing ,I think I fitted an electric fuel pump (not original)so maybe that’s not supplying enough fuel?
I will do a pressure test on the pump but it feels the float chambers very quickly .

If it fills the chambers quickly I think the pump should be good.

The diagram is a plot of how thick the tapered needles are in mm at different points. Look closely at the numbers and imagine the needle is on its side.

You see the needles are all about the same at low „rpm“/load (idle). That’s where you tune the carbs by lowering and raising the jet. When you drive, the pistons rise to keep a steady vacuum at the jet, and allow a certain amount of fuel past the gap between jet and needle.

The thinner the needle taper gets the more fuel can get past (the gap in the jet gets larger when the needle is thinner).

If you set the idle right, and it goes lean when you drive, you must put in a thinner needle that allows more fuel to be sucked past. The green line is the thickest Needle with the least taper. It would be very lean but as you can see, they are all the same at idle.

How about the air filter and float levels?

David I will check the float levels, I’ve left the air filter off while test driving it’s got 3 individual wire pancake style filters enclosed in the air filter box.


Me too! I’ve never been happy tuning them that way. It restricts air flow and I feel I end up adjusting them endlessly.

With a hose I han hear the pitch of the ”whistle” at each throat, easy to tune if you have a good ear. YMMV.


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If you’re really lucky that’s all what is wrong with it… :slightly_smiling_face:
Try again with the filters, let us know, and good luck!

Good to know I‘m not alone with the unisyn :grinning:

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Bestimmt! :slight_smile: