I've had my '71 E-type (Old Scruffy) for about 20 years. A PO had removed the secondary throttle linkage and plugged the holes on the manifold. The car always ran okay but I had a stutter on high revs (~3500) just before a gearchange which I never was able to tune out. The carbs have been converted to adjustable jets.
After acquiring a set of HD8s I wished to convert the car to a twin HD8 set up as a friend of mine had done on his SII E-type. I rebuilt them (new needles, etc) and fabricated a linkage. Set them up and cannot get the car to run right. The car will stumble badly on everything over slight throttle. The issue appears to be fuel starvation but I can't adjust the jets down low enough to correct that. Carbs were rebuilt using an SU service manual and the official "E" manual so the float level and jet centering are correct. The fuel pump is relatively new and is putting out sufficient flow (it worked with the Strombergs) and I can't find any vacuum leaks. The ignition was upgraded to electronic years ago and initial setting is at 10 degrees BTDC. Centrifugal advance is working.
The kits came with the UM needle and I'm wondering if this is too lean for this set up.

Any advice would be appreciated; I'm annoyed enough at this point to put the Strommies back on.

 

I'm not experienced with conversions from US spec to UK; mine is an original UK. car. However, there may be a clue in your comment that you converted to electronic ignition years ok. Might be why you always had a stutter at approx 3500 or higher on the Stroms as well.
I had the same issues after fitting a new electronic unit. Tried new coil etc etc. The supplier eventually told me to change the leads from the copper based ones to silicone. Boom. No misfiring all the way to the red line. Trouble is copper leads set up a field which can interfere with the electronic ignition. Are you on silicone leads, and do you have suppressor plug set up?
Only a thought, and may be more to do with how you set up the carbs.

 

Other possibilities that could cause fuel starvation......low fuel pressure, needle valve stuck closed, piston sticking in dashpot or sticking on damper rod, jet not centered causing needle to bind, throttle plates not opening simultaneously, air filter restricted. UM needles were stock for the twin HD8 carbs on the 4.2L 420 Series sedan. Perhaps a needle swap would be useful for fine tuning the performance, but UM needles should not cause the bad stumbling. Cheers, Richard

 

UM needles were also stock on the UK triple SU set up. Stick with them. Unless you've made other changes to the air filter system, cams exhaust etc all you'll do by changing from UM will be to increase fuel consumption

 

Since the original problem was at 3500 RPM, I am thinking it was ignition. If you have the smog mechanical advance only distributor, you may have to go to aftermarket vacuum advance distributor to match the bigger SUs. I converted my 1968 XKE to Pertronix D177600 with matching coil, ignition wires and NGK 7327 BP5EY spark plugs.

To confirm it is ignition and not SU installation; move initial timing from 10 to 15 degrees to see if off idle response improves. Vacuum advance will respond more quickly than mechanical.

Air filters must be on SUs for tuning, otherwise you get too much air.

The original manifold has 1 3/4" throat, so with 2" carburetor, you can have a disruptive lip on manifold that may not effect low RPM, but at wide open throttle you will have undesirable turbulence. Place a 2" gasket on the manifold and mark lip, that can be smoothed with a rotary file.

The new linkage geometry, should have full travel of accelerator peddle (90-95%) as you get full open throttle discs.
Rgds
David
https://www.jaguarforums.com/forum/e...change-221747/
https://www.jaguarforums.com/forum/e...ast-4k-210395/
https://www.jaguarforums.com/forum/e...retion-214389/

 

Did some checking tonight; there appears to be no issue with fuel flow to the carburettors.

Interesting that you mention needing the air filters to be on the car for tuning - I've been told the opposite for years.

The friend I mentioned in the original post did mention that he had to recurve his distributor - you might be on to something there. I did snick a few more degrees advance on the last test drive and it didn't seem to make much of a difference but I haven't confirmed how much. It may not have been enough. I do have the copper core leads, so the interference theory is worth a look.

I'll take a look at everything tomorrow but I'll have to wait for the local weather to improve for a test drive.

 

Few thoughts on the SU HD8 set up: Manifold is limiting at 1 3/4" diameter vs. SUs at a 2" diameter. Approximately 30% more area for the SU than original carburetor. This means that the airflow velocity over the metering jet in the venturi will be less, hence less vacuum = leaner mixture. So a richer needle may be required.

From some forums comments:SUHD8 Needles: by christopher storey “It is quite common to find UO or UE needles on 4.2, particularly if the standard air filter setup has been altered. Heuer ( David Jones ) and I have discussed this on this forum many times . I am of the view that all the richer needles achieve is to consume more fuel - a lot more fuel! I have always used UM on both of mine, and get 22-23 mpg in normal use, but I am not a hard driver Usually, needles are changed to match a modification to the engine such as higher profile cams or a freer flowing air filter. If everything is standard on your car, I am not sure that having richer needles would have made that much of an improvement on their own.

Thus, for UM, UE and UO , ( 3 of the commonest E type sizes ) increase of 7% between UM and UE, and 5% increase between UE and UO. “

PISTON SPRING: “If the spring is too weak, the piston will be elevated to a level higher than its optimum position causing the engine to run too lean. If the spring is too strong, the piston will not rise to its optimum position causing the engine to run too rich.”
Why is this so? Surely, if the piston is higher than it should be, the engine will run richer, and if the piston is lower than it should be, the engine will run leaner? Wrong! Remember the volume of air the engine draws is proportional to its speed. So… If the piston rises too high the throat area will be larger than optimum, the vacuum between the piston and the bridge will be low. That is the air velocity across the jet will be low, drawing less fuel, thus causing the mixture to run lean.

The opposite occurs if the piston does not reach optimum height the throat area will be smaller than optimum… The air velocity will be high between the piston and bridge causing a larger volume of fuel to be drawn through the jet assembly causing the mixture to run rich. There are 4 of springs available for the SU type carburettor each with a different compression loading. Blue 2.5 ounce, Red 4.5 ounce, Yellow 8.0 ounce, Green 12 ounce

Joe Curto is a long-time rebuilder and parts supplier for British carburetors. He founded British Superior in 2002 when he purchased the patterns and molds from the long-established Salcorp, a company that produced high-quality rebuild kits. Our shop is open five days a week 9:00am-6:00pm EST.
"We are here to troubleshoot your problems and answer any questions about carburetor rebuilding and tuning."

I would ask Joe Curto about Needle selection and Spring selection. Confirm with him what dampener oil weight to use. He did a great job for me, in sorting out my 2 to 3 ZS carburetor project. He has all the bits and pieces you may need.
CONTACT 718-762-7878 JoeCurto@Aol.com

Once you get ignition and new carbs dialed in, you will enjoy the the smooth power again, all the way to redline.
Rgds
David

 

I've installed O2 sensors, and I've done a fair amount of experimenting with tuning. If you're using ethanol fuels, which is mostly what you find in the US, UM needles will be too lean. So be careful of comments you read on British forums. Your problem is even more involved, because you're using UM needles intended for a three carb setup on a two carb manifold. Think it through: you can't really expect the car to use 1/3 less fuel than a three carb car. You're going to have 1/3 more airflow through each carb, and you'll need that much more fuel flow. I think you're going to need much richer needles and maybe stronger springs. You might need something as extreme as a UT needle.

What oil are you using in the dampers? A quick way to diagnose a very lean condition is to switch to 90W gear oil. You can also weight the pistons with a stack of washers. If the situation improves, then you need to richen the mixture. You can't adjust inappropriate needles with the jet height adjustment: If you had enough adjustment to get it to run well at mid load, you'd end up overly rich at idle. But there won't be enough adjustment even for that.

 

I must say I didn't really notice you were going to 2 X SUs; most people who convert go to the triple SUs with a new manifold. Not really a massive advantage in what you are doing. The UK spec car produced about 40 bhp more than the US one, but that wasn't just down to the carbs. The UK cars had a higher compression ratio, no emission controls, different timing, 3 carbs rather than two etc. Strombergs in them selves are ok, so going from 2 stroms to 2 SUs won't really gain anything (other than the sort of issues you are experiencing now) Might it be worth selling the SUs and going back to the original set up?

 

I think he's invested in the SU's, there's no harm in following through. He really should consult Joe Curto or Burlen to get a better needle and spring combination.

 

Some budgetary numbers for getting your XKE sorted ( IMO, the ignition system update is required for either carburetor setup, ZSs or SUs)

New After-market distributor for Jaguar XK engine Amazon
Pertronix D177600 Plug and Play advanced Distributor $ 248,99
Pertronix 40511 Flame-Thrower 40,000 Volt 3.0-ohm Coil , Black $42.99

Rebuild your original Lucas Distributor
British vacuum Unit
Canterbury, NH 03224
Feel free to call with any questions 603-731-1788 bvu@britishvacuumunit.com

IMO, one of the top specialty distributors overhaul shops. They recommend “jaguar XK engines distributors be calibrated and curved for maximum performance, cooling and mileage, to the XKE distributor #41060spec, with a mechanical advance of 18-20 degrees at the crank", with initial timing set at 10 degrees BTDC, for a totalmax- advance of 28 to 30 degrees at crank. Using the OEM vacuum unit #54415894, (7-14-8)for optimum smoothness and economy.” “We often modify the curve for modern fuels and from emissions vacuum retard to performance vacuum advance, and re-curved on our Sun distributor machine. Distributor re-building range from $169.95 to $225.00, Usually 2 to 10 days turn around.”

E-BaySpark Plug Wires Brand:
Lucas Type: Ignition Wire Manufacturer Part Number: HP900 Price: $28.95 Each Direct Fitment: Fits 1984 XJ6 Vanden Plas
NGK Ignition Wire Set For 1982-1987 JAGUAR XJ6 L6-4.2L Engine. Price: US $33.74 Manufacturer Part Number: 54393 Fitment Type: Direct Replacement

Spark plugs Amazon/E-bay/O’Reilly/NAPA NGK 1233 BPR5EY V-Power Spark Plug ITEM#NGK1233 Price: $3.74 ea. (BP5EY has been discontinued)

SU HD8 needles and springs estimate ~ $70 -$90
Rgds
David

 

Nice numbers. Totally irrelevant to the OP's problem. He can change around ignition components all day, but none of that fixes the fueling problem. He needs MUCH richer needles for this setup.

 

I wanted to give you all an update.

I removed and went through the carbs again out of my own paranoia and I had put them together correctly. I talked to my friend again - he performed the twin SU set-up on his E when he restored it some years ago. He stated that he needed new needles but couldn't remember which size he had to use. He recurved the distributor advance since he'd put hotter cams in the engine. He didn't think I needed to putz with the distributor since my engine is otherwise stock. I shot an email to Mr. Curto yesterday and I'll see what he has to say.

I went with the SUs because the car always ran okay but I thought it would benefit from being able to breathe a little better. Plus, I had the SUs on the shelf - didn't have the three carb manifold and I am not paying what people want for one. I just want a solid runner; it doesn't have to be concours correct. Don't have any money in the SUs other than the rebuild kits; if I can't get them to work to my satisfaction I'll put the Strombergs back on.

 

Putting the Stroms back might be the easiest option. See how it goes, but as I said earlier, two SUs in themselves won’t improve the breathing without all the other things I mentioned

 

The last "Stock XKE (sports car) Engine" in US was 1967... 3 SU HD8s, high compression pistons and a 40617A pre smog distributor... in UK with 98 -102 RON gas, this configuration continued on. The 1968 and beyond US engines were an "Emissions Tricks" to keep selling cars in California. (Twin ZS lean carbs, secondary throttles and a special distributor with no Vacuum or with Vacuum Retard) . ZS came with no needle or jet adjustment, and a lead seal to make sure you didn't tamper with carb, in order to make it run smoothly. Removing the secondary throttles cuts out the air fuel mixture warming path, necessary for a decent idle and early acceleration. So with no secondary warming path, adjustable needles were added by owners to ZSs in order to achieve acceptable low RPM performance. Later, aftermarket adjustable jets became available.
Going back to the older Twin or Triple SU carburetors (run richer), requires the correct Distributor Curves for smooth performance, without overheating. The 41060A distributor dealt with the lower compression engines and lower octane numbers.

It is important to get closer to the original 40617A curve, if you run rich side carburetors: Triple SUs or Triple Webbers as well as Twin SUs or modified Twin ZSs.
If you have low compression and an automatic Transmission, the 41060A (with modern vacuum advance) is a conservative choice.
Twin SUs will not flow more air than the Twin ZSs on a XK engine at stock redline, as the manifold is the limiting factor; (the highest HP & Torque for XK engine comes from the 6 individual inlet runners of the Triple Webber configuration)
When you get the both Ignition and Carburation dialed in, you will notice the difference; a 50 year old sports car that performs better than new.
Rgds
David

 

I need to make a correction to Post #7 . I pulled this from an old forum discussion, and failed notice this was for 1 1/2" and lower Dia. SU carburetors.
("There are 4 of springs available for the SU type carburetor each with a different compression loading. Blue 2.5 ounce, Red 4.5 ounce, Yellow 8.0 ounce, Green 12 ounce")

The correct springs for the 2" SU carburetors are:

Red & Green (1) 11.25 oz. 3.875 in AUC 4826
Light Blue & Black(1) 04.5 oz. 3.875 in AUC 2107 (2)
Light Blue & Red (1) 18.0 oz. 3.875 in AUC 4818 (3)

Notes: 1. 2' diameter throttle carburetors only 2. Used in place of AUC 5014 red & black. 3. Used in place of AUC 2091 blue & green.

It appears that the spring weights are too far apart for fine tuning??

Rgds.
David

 

You won't find the answer in the standard references, and forum discussions about other applications are useless. The only "correct springs" are the correct springs and needles for this specific application, and won't match anything in your references. I modeled this setup on SUNeedle, and it's coming up with UT needles with the yellow (8oz) springs, which sounds about right. The reason for this is that he's using two carbs that were originally intended for a three carb manifold. So each carb has to flow 1/3 more air at any given RPM and load. That will cause the pistons to rise too high and too fast unless the springs are corrected. Tuning by adjusting the jets may get it to run well in a very narrow band, and that assumes there's even enough adjustment. Once the springs and needles are dialed in, the car should run well. There's no need for ignition mods.

As I said above, he can check this by weighting the piston.The stock springs for a 3 carb E-Type were blue/black (4.5 oz). Adding 3 or 4 oz to each piston should more or less correct for excess rise. This can be accomplished by dropping a weighed set of washers around the damper tube. It won't interfere with the operation of the carb, it will just add to the force needed to raise the piston. Or a large washer can be placed inside each piston rim. Weighting will confirm is the additional force needed to delay full lift until WOT. Once the revised spring strength is confirmed, the correct spring can be selected. If an appropriate spring isn't available, it can be cobbled up by taking the next stronger spring and removing coils until the proper force is measured at 3.875" height. In the very worst case, the weights can be used with the stock spring. Then the needle should be selected. This can be done by seat of the pants trial and error with a selection of different needles, along with checking the plug condition after each test run. The better method would be to use a lambda gauge.

 

Update: I'm going back to the Strombergs. I wanted to see if the SUs would work without a lot of parts thrown at them. I have five projects going on at once (not counting a friend's MGB) and I'm not ready to go down a rabbit hole this deep - not yet.

I do appreciate all of the input and it gives me a clearer picture of what needs to be done to make everything work.

The distributor is a vacuum retard type so I may upgrade that regardless.

 

Understand that the "project is getting to be too much trial and error". Since you have you already have the SU HD8s, you may pick this project up again when time permits... Saw UK EBAY listing for XJ6 Twin SU intake manifold, if you can pick up a good one cheap, you will be able to get the extra air flow you were seeking, a quick look at parts manual for UK XJ6 with Twin SU HD8s, shows UM needle with a a Red/Green Dash pot Spring.... so that isn't too bad for a starting point.
Jaguar XJ6 Inlet Manifold Twin SU Carburetors £69.00Buy It Now, £6.99 Shipping, 30-Day Returns, eBay Money Back Guarantee ( UK shipping only for this one)

Rgds
David

 

A new manifold will leave him with the same problem...carbs set up for a triple manifold on a double manifold. The engine will draw the same volume of air with the SU manifold, only volumetric efficiency would improve.Changing the springs and needles would have fixed this, but he's discouraged now.