SU HD6 carb lean on acceleration, rich on deceleration

I have a '61 Mark 9, 3.8 with 9:1 compression. I put in a Air Fuel Ratio gauge (AFR) because I think that is the best way to find out what is happening to combustion during driving. I have been adjusting the jet and the idle mixture on the SU HD6 carbs. I just drove about 30 miles today on all sorts of roads and speeds. I am finding that the carb leans out to almost 16 on acceleration and richens to 9-10 on deceleration. I would think this would be opposite if the dashpots were slow to rise and sink. I read 20-50 oil works best, so I used it.

I have the spark timing right on the money, the valves are
set properly. I did put in an electronic replacement fuel pump, but it was supposed to be a direct replacement. Tomorrow I am putting in an actual fuel pressure regulator and get the FP down to 3.4 PSI.
What am I missing? The acceleration is crappy and I don’t want it to run too lean on a long highway climb.

While the “rich on the overrun” is interesting it’s the leaning off that would concern me.

One option is to fit a stronger spring on the dashpot.

Another would be to change to a needle that is richer where you are leaning off.

Firstly though I’d be interested where you have your mixture set at idle.

I usually use a CO meter and aim for around 5% CO at idle. I’m not sure how that would translate to AFR/lambda but it might be that you simply have the car tuned too lean all across the board. The XK engine prefers a bit of fat rather than lean…

I adjusted the jets to get close to 14.7 AFR cruising at 50 mph, steady load. to gain that, it idles at 10, very rich. At 65 MPH, on the highway, and any steady acceleration, the mix goes into the 15-16 even 17:1 AFR.
I am using non-ethanol 91 octane fuel.
There must be something wrong with the carbs. I followed all the carb tuning sequences in the book and on this forum. I might have the floats set a little low, but I would not think that would be the cause of the problem.

I had this exact issue on my car after installing AFR gauges. I went to richer needles because I also have a freer flowing exhaust and air intake, so it was too much flow for the stock needles. That helped, especially because richer needles are the same at idle but richer in the mid range. I also noted that my rear carb was leaning out under accel more than the front, so I raised the float in the back ever so slightly, and ended up putting a washer inside the dashpot to slow the rise of the piston. That has made both carbs about even and they stay around the 13’s under acceleration while also being around 13 at idle.

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i checked the SU web site The S.U. Carburetter - SU Carburetters for their tech on tuning SU carbs. Lots of great info such as what I have heard before. They say that float levels (within a range) do little to affect the running of the carb. Needle profile and spring tension determine fuel mixture at various speeds, loads.
Seems I have original springs and tapered needles. Odd that it runs such wide ranges beyond stoichiometric. Any advice?

I think you are very smart to use AFR for tuning. A few thoughts:

Is your data good? Where is your O2 sensor located? Ideally just downstream from your manifold.
Are you sure that you have no exhaust leaks between the engine and the sensor? That can give you erroneous readings. I wonder if this could be causing the curious finding of richening on deceleration - my own experience is for high A/F ratios in this situation, not low, which we don’t worry about.

Are your needles inserted to the right depth? Check the location of the shoulder. Make sure they are not worn. Is there a proper fit between your pistons and dashpots? You can find the standard drop times online for hs6s. Are your throttle shafts/bushings worn?

If you’ve ruled out an air leak and worn out parts, you probably need to try some different needles to get you in the range you want. Most important is the wide-open-throttle during acceleration reading which should be 12-13. So you are shooting for around 14 at idle and 14-15 cruise. You will never achieve the ideal 14.7 on a carbureted vehicle so don’t worry about that. I tend to run a tad rich rather than too lean. If you don’t trust your O2 numbers, check your plugs after a good run at speed (start with clean plugs).

So, how to get the right needles? First, you have to know what you are running, so find the number printed on them. While you are at it, note whether there is a color code on the springs. I’ve mainly dealt with this on HD8 and HS2 carbs, so can’t advise you on needle profiles, but call Joe Curto 718-762-7878. He is extremely helpful, and you can buy the needles from him. He can also advise you on springs and damper fluid.

A friend who recently built a '38 MG TA special with a turbocharged engine set up his Go-Pro to view his dashpot with a homemade gauge in place of the damper so that he could watch the height of the piston during various stages of operation. By looking up the needle profile, he was able to customize the needle to his unusual car. With a stock engine and carbs in good condition, you shouldn’t have to go to that extent.

Unless your carburetors are just worn out, you should be able to tune them for good service.

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What needle did you use? It looks like the original on these AUC 956 carbs was a TU. Moss sells a replacement that is a TS. I have read about UM, UE, UO, and UB, but I think all those are for the H8 carbs on the E-type.

What air filter are you using

I am generally pretty ignorant about ideal AFR numbers, but I thought that you wanted to be richer than stoich (14.7ish) at highway running and I would be quite uncomfortable running 16 to 17 at all as I would be concerned about burning a hole in a piston. I could be wrong though.

My putative 5%CO at idle looks to be equivalent to around 12.5. So if you’ve got 10 at idle you’re way richer; I can’t find the equivalent in %CO but it’s more than 10%.

If you have that, BUT you’re at 14.7 at light throttle cruising and 16 at open throttle then I agree those numbers are screwy.

I only use 98 octane fuel in any of my XK engines… I’ve never tried 91 but it might be worth trying a higher octane fuel.

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“……………There must be something wrong with the carbs. I followed all the carb tuning sequences in the book and on this forum. I might have the floats set a little low, but I would not think that would be the cause of the problem………”

“………….While the “rich on the overrun” is interesting…………”

I can offer a particular scenario that may be at the root of your problem, it has certainly been the case in the past. I think you will also find it is not in any book, or this forum.

I believe your overrun enrichment has been caused by your attempt to tune the carburetters for acceleration, while the float chambers are under depression.

This can be because the overflow pipes are blocked or undersize. However it is usually because the internal serrated fibre washer S,U Part AUC1928 (Jaguar Part 1298) positioned between the float chamber lid and overflow pipe has been squashed, not fitted or there is only one washer on each carburetter.

That is to say that there should be TWO AUC 1928 stacked together on each carburetter. This arrangement was always stressed at the S.U service school. Unfortunately, as far as I know the Jaguar parts manuals were never modified, other operators like R-R, Bentley and Aston made sure that detail was contained in their parts manuals.

Those internal serrations on the AUC1928 are the only path for air to vent the float chambers, if they are crushed by overtightening that air path is further reduced resulting in fuel starvation. In some instances, particularly in North America dollar area, the special serrated washers have been replaced with normal fibre washers, blocking off the air supply. Remember, any owner, or mechanic will receive rebuilt carburetters in part disassembled form and that owner or mechanic will be the one to make the usual fatal error when they attach the overflow pipes. Unfortunately, they remain under the impression that their carburetters are beyond fault……after all they have had specialist attention.

On larger engines like the Jaguar XK the air restriction demands an extra serrated washer is fitted, or even a third washer. The advice at S.U, in the days when technicians knew how these carburetters worked, was to file a “flat” from the underside of the hexagon head on the float chamber cap nut, to form a “D” section. The “flat” section extending down the S.U Part AUC1867 (Jaguar Part 5832) cap nut to the inside of the float chamber lid. This provides a permanent air vent and I believe was a standard feature of Rally cars.

You are frankly wasting your time trying to tune any S.U carburetter without two of these washers on each carburetter and most certainly trying to obtain 14.7 ratios with any carburetter. Those units were never specified using that ratio when new. As someone previously stated they have to run rich on acceleration. Obtaining needle sizes and power output was not done by trying to adhere to 14.7 ratios but by forming power loops to obtain the best MEP and WMMP performance.

I hope that my explanation of a particular scenario may prove helpful, if not, it may explain to some other owner why their car is down on power.


Thank you for your input fellow Jag lovers.

Phil, I had the stock air cleaner off as it is nigh impossible to reach the screws with it on. Perhaps that is part of the lean condition at high load. I will test again with it all assembled.

NWG, that is an interesting tidbit I hadn’t considered. I thought the venting was through the overflow or from the little hole in the top of the bowl cover (which I see now is not really a hole). I will get some new, non-squished washers, and perhaps file a “flat” for proper venting.

Ron Smith, the O2 sensors are located where Innovate requires, and have heaters if they are too cool. I might have a little bit of a leak I will attend to. In that I have two sensors reading about the same, I am pretty sure of what is going on.
I will be replacing needles and jets with a SU rebuild kit. It will also have shaft bushings. The drop time is good. I am getting new red piston return springs.

I called Joe Curto, the expert, and he is sending me some WO3 needles to replace the stock TU needles. He says gas is not what it used to be and is having good results with the WO3 in the SU HD6.

Thank you for thinking of all the possible factors that might go into a lean under load running / rich at idle. I think I should be able to get this cat purring again as originally intended. My desire is to run mostly original equipment, add A/C and heater unit, change to alternator, put in a good sound system, and eventually change out the 60 year old leather and repaint as a two-tone. My wood is great, and there is little rust. I would like to rally with the car, so would like to make it reliable and comfortable so my sweetie will enjoy the ride with me.

BTW, my sweetie, Lois, and I took a 1965 travel trailer (caravan) on a 4 week, 1500 mile tour of midwest USA. I spent a month renovating and retrofitting it. We must’ve shown it 100 times. It was funny to see how after a while, Lois was taking up the chatting and showing. Having an unusual vehicle opens up doors of conversation and loads of smiles. This Jag is doing that already. On to the renovating and updating. Gotta make her run well first. (the Jag, not the wife)


What a fabulous looking motor car

I have a 4.2 XK and original to the car HD8 in good condition. The car ran fine after rebuilding the engine and cleaning the carbs completely, after decades in a barn. I did not dare touch the needles so I rule that out, and the rest I put together exactly as it was as good as I could, and on the first run I adjusted them so it ran even and idled a bit high. Now it has a few hundred miles.

As the exhaust didn’t smell great, the idle eventually got a little uneven and it fluctuated between 4-10% CO, I decided to synchronize them, lower the idle and lean them out a little. This lead to a two hour exercise in cold sweat, the sun had set and I, alone, had to get home since I was in a remote place as to have my peace and not disturb anyone! First time tuning SU.

Now, when I lift the pistons per the books, the idle rises slightly. If I move the jet up one either carb, the idle drops when I lift its piston, and if I lower the jet, lifting the piston results in a significant rise. So far everything seems to adhere closely to the tuning instructions.

It also idles slightly better now, at 600 rpm per the book, standard air filter.

When I accelerate with half - ⅔ throttle and up to 2500 through the gears, it stumbles sometimes, as if a piston is not firing 100% all the time. I don’t know if it’s rich on deceleration, no backfires, nothing pronounced, but I think we might have a somewhat similar issue.

I then went back to the jet procedure but still think that they are just too lean and tomorrow (hopefully) I will report on what lowering the jet by a little has resulted in, look at the plugs etc. According to much that can be found on the internet, they’re just not rich enough.

I’ll follow what your needles will do for you and good luck. I read that “new gas idles richer and is not what it used to be, get new needles” before.

And I wouldn’t look at the idle lambda unless necessary due to legislation.

If it had not happened to me while adjusting, I would have thought of the ASC. High vacuum sucking fuel, then richness at idle and deceleration, but no such ‘enrichment’ at cruise and especially not when accelerating. Some similar principle could be in the carburetor itself but I don’t know how it would be caused other than by the needle, maybe a restriction somewhere… I’m guessing. Simple things first.

@abowie, 91 octane probably refers to the MOZ, so, good USA pump gas and more like 98 ROZ. A 9:1 wouldn’t run so great on 91 ROZ, 95 can work but it depends.

Thank you for that information, sometimes these seemingly unimportant things get overlooked. How many times has one of those washers fallen into the engine bay and been lost, to be thought of as of no consequence and never retrieved? If it’s not leaking, no problem. Still learning stuff every day thanks to this ( and other ) forums


Many of us have been down this road. Especially those of us who have driven a triple SU HD8 E-type.

The factory method and standard needles worked fine until ca 2001-2003, then the fuel was considerably changed.

Just like Joe Curto was quoted, it’s true: the fuel ain’t what it used to be!

I had to move to richer needles, otherwise it was just as you described: drove really great but idle was stinkin’ rich!

About the same story on our 1975 XJ6C with twin HS8’s.

EFI can manage as it can adjust mixture on the fly (mostly) especially with a lambda and closed loop etc.

The carbs however have no idea what’s going on, they just move according to vacuum and throttle opening and the spring and the needle affect the mixture and create the fuel and air mixture.

The fluid dynamics of today’s 98E5 are different than old school ”Super” or ”Super Plus” (we used to have 99 and obviously 0% ethanol) the surface tension, the vaporisation temperatures etc they are all different than what they used to be.

That’s about the only reason why I have been to a dyno with most of my cars: to find out the AFR throughout the range, so I don’t blow a piston in Germany at 5.000rpm or don’t fowl the plugs and put out black soot at idle in traffic lights.

I don’t think you can achieve that with ANY SU’s with standard needles and the fuel we have available today.

I would buy two sets if new needel, both a bit richer than standard and install, adjust per ear and then carefullt test on a dyno and/or meter the AFR / CO2 %.


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Brilliant, thank you. I’ll invest in the needles and be happy with a richer idle for the time being :grinning:
I hope that fixes it all. Maybe I can find a needle that works right away; some thinking and searching first, and I was content with my twin carbs. They are nice to set up and all made sense but that. We can get up to 102 octane here (Aral ultimate) but it’s really wasted on my 8:1… and I did not buy 9:1 pistons to have that peace.
Cheers, David

PS: I feel like I’m on the right track now when I search for “HD8 needles modern fuel” here on the forums!

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I have no experience with HD6 nor HIF6 and I know I should also do something about the H4’s on the MKV (or H6, I admit I don’t remember what they are) but just haven’t gotten that far, and it does not rev that high anyways.

Over here even Shell’s V-power 99+ still has 5% ethanol so it does not help.

As the MKV is less than 8:1 I have also run it with lawnmower/outboard small engine fuel €1,99 EUR/liter. :slight_smile:

I would still first like to hear from someone else who has changed needles to achieve better balance in AFR idle/high revs with modern fuel, like Joe Curto or people close to or inside Burlen UK?


Ps. I may have to check, but IIRC the originals were UE on the E-type HD8’s and I bought UM and UB and whichever was ”one step” richer worked for me. Gave ca 228HP (DIN) in the dyno and safe AFR at 750rpm all the way up to 5,500rpm. Or maybe the original was UM and I bought UE and UB…

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I’ll see what I have in mine and source one step richer. It’s just a small stumble when it is set right at idle so I will compare that to the experiences of others. I see that at least UM, UB, UE, UO, UN are the same at the first points and then they begin to get thinner. I’ll look at the needle now and see what I have, some more research, and then I’ll go shopping :slightly_smiling_face:

Ps.2. I checked ftom Burlen UK, E-type 4.2 standard needles were UM. So I bought UE and UB. And yes, the carbs on MKIV and MKV were SU H4’s. :slight_smile:

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Yes, now someone who knows more should chime in, but I think that would mean richer mixture at a higher opening, which would allow to adjust to a less rich idle without getting too lean at WOT. But better to first check with an expert or ask Burlen UK.