High idle revs on XJ6 Series 2 engine out on the bench

I have a Series 2 XJ6 4.2 litre engine from a former kit car on my bench at the moment, which starts OK but has high idle revs (about 3,500 RPM) that I don’t seem able to lower to a suitable tick over speed. The engine has twin HIF7 carbs and a manual choke converted AED (both now fully stripped and reconditioned). I’ve connected up a new electric fuel pump (which was recommended to me for this engine from a well known Jaguar parts supplier in the UK) but all the usual XJ6 engine “management” aspects stripped off and /or blanked off - only the vaccum advance to the distributor remains from the first carb. New air filter fitted and carbs seem to be balanced with each other on my vacuum guage.

The timing appears set-up and correct, as does the AED and Carbs to the SU instructions. The engine fires up ok on manual choke (up to about 4,000 RPM), and soon warms up to enable push in of the choke cable - but the revs don’t drop below 3,500 RPM. Usual carb linkages are disconnected and idle screws set back.

Lifting the carb barrels does make the engine fall and stutter, but then regains to the high revs. I’ve yet to try cutting off the AED fuel line (fuel tap on order due next week!!) and I can’t see an obvious leak (fuel or air) anywhere.

Could the over revs be due to wrong fuel pump (and I’m now using E10 unlead petrol, standard in the UK), AED, carbs not set up correctly, too much air intake (ie: a leak), or some other issue if the XK engine does not have the usual sensors, pipework, etc connected?

I’ve had other earlier year XK engines working on the bench OK in the past but I must be overlooking something as I’ve been trying to sort this over-reving engine for weeks now without success :thinking:

What fuel pump is this? You don’t need a lot of pressure with carbs, but you do with fuel injection. Maybe the supplier read the wrong page on his computer and thought you had a fuelie? Or never heard of a carb.
I can’t imagine how you would get 3500 rpm unless either your throttle plates are wide open or you have a huge air leak from open smog pump ports or something.

Fuel and air:

Rob’s point about pump pressure - only need 3 - 4 lbs - over this will push past the float needle valve and have fuel running out of the jet bridge into the inlet manifold. If this isn’t happening at ignition “On” - fuel pump running - I would give this a “pass” for the moment.

It’s getting air from somewhere else - misaligned throttle plates won’t get you to +3000rpm. I suspect the air valve in the AED is contributing. Blank the air intake off (big pipe connection underneath). Send us a photo so we can check it is an AED not an ASC. Sometimes terms used interchangeably.

Pinch the fuel hose to the AED with soft jaws and Vise Grips.

Let it warm a little so that it might start with the AED isolated per above. Paul.

Definately is the AED with manual conversion. Fuel pump (as recommended by UK supplier) is an SU AZX1308, which looking at the SU Web site is a High Pressure pump, 18 gallons per hour at 3.8psi. I think the AUF222 or similar is a Low Pressure pump, but can anyone recommend an external pump for the Series 2 with HIF7 carbs and AED?

There is no flooding of the carb or AED float outlets, but I do notice a little residue of fuel in the both carb Chambers - maybe suggesting pump pressure to blame. If it was in just one chamber I’d be more suspecting one of the internals of the carb to look at.

The carb butterfly plates seem to seat and close correctly, and new needles, seals, floats, etc fitted in them and the AED.

Blocking the AED air intake does not seem to alter the high revs (once the engine has warmed up and the manual choke pushed in).

Clamping the AED fuel feed once the choke is pushed in again seems to have no effect, so I’m thinking it must be on the carb side.

I might spray some Cold Start around the manifold to see if there is a hidden air leak somewhere (which I suspect will change the revs if there is), but my thought after the initial feedback and suggestions is it could be the pump - so any further information much appreciated.

I found gasket damage in the AED, so air leak issue. At last my replacement order of spare parts has arrived!!!

With the manual choke conversion, where is the fuel needle level set? Information I have for the factory auto AED says turn the brass needle flange down clockwise until it just raises the needle off the square headed seat, then 3/4 turn back anticlockwise. This sets the fuel shoulder 8.5mm from the bakalite housing (which seems to agree with a tuning diagram I have from SU). This would mean when the fuel needle is fully depressed some fuel will still enter the chamber.

However, the manual conversion information from Limora seems to be saying take the brass flat head screw of the fuel needle down clockwise to close fully the fuel needle “full dead centre” - no mention of going anticlockwise by 3/4 turn. This will totally cut off the fuel when the manual choke cams press the fuel and air pistons down (choke off).

I’ve set it to the normal auto AED adjustment, and the engine revs still high, unless I cut the fuel to the AED, but no were near the very fast revs before the gaskets changed - so it must now be the sensitive AED adjustment needed. Anyone with any tips on the correct starting position for the fuel needle in the manual AED set up would be greatly appreciated.

You have the manual choke conversion installed so you need to follow the instructions for that. Or give Limora a call or five and ask them about it if it’s their kit.

The manual convertion kit was with my engine, but not fully fitted and not set up - there were no gaskets and the diaphragms perished!!!

Yes, I have followed the conversion kit information to set up - but everything was taken apart for the AED cleaning and rebuild kit (including the fuels needles and brass adjustment screw removed). The needle adjustment screw is back in to the starting position that the auto SU AED suggests (all the way down just to start to lift the fuel needle off its seat, then 3/4 turn anticlockwise). This seems logical to me, as the manual kit is replacing the top part of the original AED.

I’ve been emailing and phoning the conversion kit manufacturers for some 6 weeks - but never a reply or technical person to speak with!!! What is not clear to me in their instructions is the physical starting position of the fuel needle/adjustment screw - that is critical before you grind off the fuel tappet body to size.

Search the archives, I have seen a writeup of this job

Put up pictures of what you have, I think a couple of guys have done similar conversions

I have the full manual choke conversion on my HS8s, the AED is gone
Its excellent

The full kit is no longer available, but it is possible to make up your own,
using a mix of new Burlen, and used and/or fabricated parts

I’ve HIF7 twin carbs on the engine. Unfortunately can’t find an archive Link on the forum for manual AED conversion set up.

Here are some photos of the bottom end of the AED and the manual conversion kit in place.

It is critical in my mind to get the position of the fuel needle and square headed adjuster screw in the correct position BEFORE the fuel tappet body length is ground down to the recommended 0.030" (0.762mm) gap to the flange.

I’ve taken down the fuel needle adjuster screw to just lift the needle head off the top (fully closed), then back anticlockwise 3/4 turn as recommended for the standard and original auto AED. The means the fuel needle is partly open.

On this setting the start up engine runs very fast, with very poor control of reducing the manual choke position. Taking the needle adjuster down 1/8th turn (clockwise, and reducing the fuel flow) does make a slight improvement.

I just need to know the correct starting position of the fuel needle adjuster screw before shortening the fuel tappet to the correct length.

Any experience or advice would be greatly appreciated.

Hi David, I will have a look and see if I can find the instructions that came with the same conversion that I bought several years ago. I will send them to you if I find them. I have to admit that I ultimately removed it and went “fully manual” more recently. Paul.

OK Paul, but I suspect they will be similar to my Limora instructions:

And the SU information for the standard AED unit (which I’m using as the “starting point” for the fuel needle position)

Whilst I understand that HS8 carbs can be converted and have full manual choke over-ride, I’ve not seen this type of set up on the twin HIF7 carbs I have - only the Limora conversion kit.

If anyone has successful changed HIF7 to manual choke over-ride to get rid of the AED altogether, or for me to try the correct fuel needle position for the Limora kit, then your comments would be greatly appreciated.

I’m not vey familiar with HIF carbs, but for a full manual choke setup to work you need an external lever setup that can raise & lower the jet & hold it in place. (such as HS8 or E-type HD8)

I just had a quick Google of HIF7 and it seems they dont have this facility ( but I could be wrong ?)

The next question is the AED the same as the HS8 unit ?

If so, I have definitely seen a detailed writeup of how someone got his running well, apparently its quite fiddly

I can’t see how the original HIF7 carbs on my engine can be modified like some people have done for manual choke of the HS8’s, but maybe it is possible.

According to Limora’s information their kit works with Jaguars HS8 and HIF7 carbs, and I thought the Jaguar AED was the same for both types of carbs. Maybe the fuel needles or needle starting heights are different - there are no serial numbers I can see on the manual conversion kit AED on my engine, but assuming it was with the HIF7 carbs (which I do know are original to the engine).

As you say the AED is very very fiddly to set up, but the starting point of the fuel needle height (and if that was a different set up position in the factory calibration between HS8 and HIF7 carbs) remains my question, and in my view the most crital aspect before the Limora head mechanism goes on and the fuel tappet shortened to the 0.030" gap recommended.

They are indeed as my instructions are.

That’s correct Tony. The needles are fixed apart from the screwed mixture adjustment. Enrichment is done differently - I have these on a Rover V8, with manual “choke”. David’s might not have the same facility.

I’ll give contacting Limora one more try. If no response I’ll set the AED to the SU rebuild instruction for the standard auto unit. Then I’ll shorten the fuel tappet to length to get the 0.030" gap.

Well, after serveral weeks of taking the carbs on and off and the AED apart and numerious rebuilds I have finally found the problem!!!

The AED was set to the original and standard auto needle set-up height, and the Limora manual conversion kit fitted and set with the fuel tappet shortened to 0.030". However, the uncontrolable high revs was eventually found to be the culprit of the two butterfly discs within the twin HIF7 carbs. All the seals, fittings, and these discs were part of the Full Service Kit I purchased from SU, and these butterfly discs have spring loaded poppet type “valves” on them (as they were in my original engine).

The springs on them were either the wrong type or defective - neither had much “spring” in them, and one was clearly bent.

My solution was to cut off these poppet valves, and solder their seating plates directly onto the butterfly disc - making them fixed. OK I may run the over-run engine risk, but for the first time the engine now runs smoothly and the idle speed nicely set.

I supect that these weak springs simply let uncontrolable amounts of air mix into the engine, and had insufficient strength to fully close these vents shut when the engine should have been ticking over. I see SU sell these butterfly discs with or without the poppet vents, but the full service Kit for the Jaguar XJ6 came with these poppet vented discs, and that was what was in the original HIF7 carbs before I stripped them for restoration (unfortunately I didn’t keep the old ones, as it might have been interesting to compare the spring sprength between the old and new ones). I’ve noticed that other forums for British Classic Cars with SU Carbs such as MG, Rover, etc suggest using the solid plate butterfly discs and replacing the poppet type ones.

Here are a few pictures of the modification carried out:

CONCLUSION: Don’t take for granted that any new parts recieved from suppliers are indeed working correctly or to original Jaguar specifications. So no matter how carefully you fit your new part, if it is defective in the first place it just isn’t going to work - expect the unexpected .

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Good solution, those poppet valves are mainly an emissions thing although besides better engine braking you may get some popping on the overrun now which is not a problem or risk. Good warning too, new parts are rarely better.