New owner, 77 xj6 s2 4.2 Engine trouble

Hello everyone my names Chris, from Adelaide, South Aus.

Ive finally conceded to asking for assistance regarding my XJ6.

Ive been tinkering away for months now, reading all the bits and pieces i can scattered all over the internet etc, but ive finally come to needing to ask for help.

April last year i picked up a old 77 model XJ6 S2 that had been sitting at a farm for a while. Long story short it needs some love and work to get it back on the road for a reliable weekend cruiser through the Barossa.

Its got a semi colourful history to which i will discuss at a later date, but the short version is im the 4th owner since new and the cars only got 33xxx KMS on it.

Im having issues getting the engine to run correctly, it needs the BW65 auto rebuilt, interior tidy up, aircon system updated and pump reinstalled, bit of paint and panel.

The auto i will sort out as i have the complete rebuild kit waiting to go in, paint and panel ill do over time, interior im already onto cleaning and retrimming, aircon will be a next summer issue, but my main concern is this bloody engine!

In not new to engines, ive had many older vehicles and im not shy to working with the classics, SU carbies, etc etc.

So to date, the engine. Upon purchase i was told it was a “good engine” and “Runs smooth” to which were alarm bells that someones been fiddling. Fast forward to inspections, someone had redone the original dizzy with new points, coil and sort of new wiring, old plug leads, new plugs (not correct gap and range) and the head looks like it had been off at some stage in its life.

The dizzy i have exchanged and replaced with a s3 electric dizzy, new leads and new plugs.

I checked the engine timing today and it appears all to be within spec, since the head had been off. I did make adjustments to the upper timing chain tensioner, as i do believe it was over tight which as a result was rotating the intake cam minutely advanced. Wouldnt have said it was 90 degrees to the rocker flange.

The cams are what id call the s3 style, with the 4 bolt cam gears.
I checked valve clearances and it was on the higher side of tolerance, but were all equal.

Ive completely rebuilt hte SU carbs, theyre HD8, which i believe were S1 carbies. All good there, rear carby diaphragm was shot, but the rest of the carbs were actually in pretty decent order. Ive removed and disables the cold start mechanical valve as this was flogged. was more of a fuel leak then anything beneficial. I can always rebuild it and reinstall should i get the engine running right to start with.

So symptoms, hard starting, not smooth running, backfired through carbs when approaching 3k rpm, sluggish to rev ( i have no reference as what is deemed normal), overall its like as if the timing is way off and the engines struggling.

The timing has been set to 8 degrees, sometimes as advanced as 12 degrees, but again after a lengthy time inspecting and having even the old man check it out we determined that was all set (for now - to at least get it started and half running)

Ive done a comp test and all but one cylinder were equal, the odd one out did eventually come up to compression equal to the others. Suspect it was from sitting around too long.

Im now at a bit of a loss as to what to aim for next, its not an overly complicated engine and its whats becoming most frustrating is that i cant put my finger on whats holding it back. i was nearly confident it was a cam timing issue that someone who did the head job had reinstalled it wrong but after todays checks im now lost.

Has anyone got a whisper of any idea to trial, im nearly tempted now to remove the whole engine and pull the front end off and double check timing is all 100%.

Also has anyone got a decent timing guide, something i can print out and work through.

I cant find the workshop manual i once found online anymore, any help would be greatly appreciated!

First thing I would check is that you have the spark plug leads connected correctly? Lots of people are not aware that the XK engine has No1 cylinder next to the fire wall.


Oft forgotten, but there is a heap of info here:

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Hi Robin, Interestingly enough i came across that little gem of info fairly early on it my discovery.
Can confirm that i have the leads correct, thankyou.

Obviously, without choke?
The chain should be not overtight but tight; the cam timing must be adjusted so it is correct then, not the other way round; cam timing off by as much as the width of the notch can cause it to idle rougher or smoother but it will run…

Hi Nick, thanks for the link, i have also come across this site before and read quiet a bit of the info early on.


This kind of problem brings to mind a situation a friend was in 20 odd years ago here in Norway. A wealthy guy in Oslo had a S3 XJ12 that had been customised by a dealer in the UK, more power and such. It had been the rounds to every specialist in Southern Norway that claimed knowledge of engines, every single one tweaking and fiddling a bit before passing it on. The car still ran nowhere near it’s potential.

My friend, being a true engineer, got out the manual, started at page 1 and checked and adjusted everything in order. When finished, the car ran like a train. Job done.

Which is a testament to systematic thinking, not the tech savant random approach.


Hi David, the cam timing was pretty damn spot on, the notch on the exhaust cam was 90 degrees and the intake was nearly on 90 degrees i would say. for an idea the edge of the notch would have been no more then 1mm off 90 degrees. loosening the adjuster and allowing the chain to relax i could rotate the intake back to 90 degrees (mere mm movement) and then redone the adjuster.

my only question though, which ive read from other forum discussions, the slack on the upper chain is meant to be under the intake cam? that when the engine is in rotation there is a certain amount of chain slack under the intake cam. the tensioner torques the chain between the cams, but not enough to pull the chain completely tight around the three chain sprockets?

It does; it pulls the cams both inwards. Once the engine rotates the slack will collect on the length of chain furthest from the driving wheel. If that’s under the intake cam, it’s not meant to be there, it’s just how it is.
Slackening, turning a cam, tightening again does nothing. 1mm is no Problem anyways.

For serious adjustments you‘d have to unbolt the cams, remove the circlip, adjust the serrated plate, then bolt it all up, tension, turn the engine and recheck.

essentially this is the situation im in. Im a bit of a engineering systematic kind of person, that if i had access to a suitable manual i would start from the beginning. it has to be something so simple.

have you an exploded view of the cam gears and a setting of adjustment for it?

Its nearly as if the cam gears could be a tooth or three out. such fine adjustments, but without truly knowing whats been done to this engine prior to my ownership, i would be very tempted to tear that all down and reset the static cam timing.

Are you certain it’s not fuel starvation or ignition problems? Hard starting sounds like the enrichment isn’t working, the backfiring sounds like the ignition is way off.

Just a checkpoint: You know the direction of ignition wires on the dizzy is counter clockwise?

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I fail to understand this part of your post. A 1977 XJ6 will have either SU HS8, Strombergs or some may have the HIF7… The SU type has no diaphragm - perhaps you have the strombergs. The type of carb will determine the cold start mechanism.
But putting all this aside, I think the most likely cause is spark timing. Very easy to get it mucked up in these cars. Some drawings in service manuals are wrong when it comes to the distributor lead connections. Work on this area first.

When i first started this venture i thought it was a spark and fuel related issue.

Got my old man involved, being a mechanic from back in the day having played more extensively with all this gear, we both determined the timing and dizzy was true and correct. Even when i sourced the s3 dizzy and installed we double checked it all.

Fuelling was the next issue i went after, i tore down the HD8 SU’s to realise the rear SU was a little flogged out and compared to the front wasnt fuelling correctly. the diaphragm was stuffed and the float levels all wrong.

Went down the path of buying the rebuild/service kits and followed the manual to the letter on setting them all back up.

I wont lie, i was excited to think my issues lay all in the carbs, but nevertheless the issues persisted.

Regarding a hard start i can accept that as i removed the “choke” mechanism from the HD8 setup as this was leaking fuel and appeared to be not assisting whatsoever. I latched up a bushing to delete it from the base of the front SU.

The engine when i can get it running, warmed up and sort of idling will shake about. Like as if its missing and or terribly out of balance. Again, putting the mind to a timing related issue.


this is a google image, but a copy of the carbs i have installed on my jag.

im fully expecting them to have been swapped from the original SU’s

further to information this is the details ive used for rebuilding/resetting them.

diaphragm is attached to the base of the jet assembly.

If your chain is reasonably tight and the notches aren’t out by more than a few mm leave it for later.

HD8 are by far the best carbs you can have. You need to get the ASC working, do the mixture checks (does it smooth out when you spray a little brake cleaner?), and balance them. Maybe it’s just running on just one carb via the balance drilling, or very lean.

Make sure the cam timing lines up to physical TDC (the damper isn’t reliable).

For the cam gears, the sprocket, that has teeth inside. The sprocket is sandwiched between the camshaft and the plate with the pin that you see from the front. That plate slides into the teeth of the sprocket; remove the plate and you can turn the camshaft while the sprocket stays in place. Then you put it together, install a screw, turn the engine and install the other screws.

To me it sounds like it may be a vacuum leak or mixture issue where you should start. You are lucky with the HD8 if I didn’t say so before.

Get your hands on a haynes manual (ideal since you have a s1/2 mix) or a workshop manual. Yes, there are free scans on the internet but the haynes is really worth it.

when i pulled the plugs and had a look at the burn, the rear two cylinders look a tad lean compared to the rest of the pots. i put new plugs in it today and tried to get it running. ill pull them again tomorrow and see what the burn looks like. if it still looks the same ill fiddle with the mixtures further and see what i can make of it.

id eventually like to have the ASC working. with the leaking i figured until i can get it running smooth when warm and its happy to rev up, ill look at reinstalling it and fixing its issues.

as for TDC, physically checked it again today, trusty phillips head down the plug hole. fortunately the damper markings aligned spot on.

you can start to see why its all a bit strange whats happening. surely its something super simple, or a combination of small things stacked. either way its frustrating haha