Rough idling and dieseling

Good morning Jaguar enthusiasts!

I am a proud owner of a 1976 XJ6 Series II.

However, I have had the problem for a long time that the engine does not really run smoothly when idling. The engine starts well, runs quite well while driving. When parking, the engine continues to run for ~ 0.5 sec.

I have already checked the following things:

Intake bridge checked for leaks (with smoke leak detector)

Ignition cable new

Ignition coil new

Ignition distributor 123 new

Ignition distributor cap including distributor rotor new

new and checked spark plugs (not oily and not sooty)

AED completely overhauled and adjusted (distance from the upper edge of the square screw to the sealing edge of the housing approx. 8.5 mm)

both carburetors (HIF7) were cleaned and got a service kit CSK 82 (Gaskets and Seals, main jet and needle valve and seat float bowl), then carburettor adjusted according to manual, damper oil checked (15w40), idling 830 rpm

Compression checked:

Cylinder - bar
1 - 12
2 - 12
3 - 11.6
4 - 12.1
5 - 11.3
6 - 11.7

Ignition (measured on the 6th cylinder, without vacuum): 11 degrees before TDC

Valve clearance measured (the WHB says 0.012-0.0014 for intake and exhaust):

Cylinder - Intake - Exhaust


The values ​​for intake (1 and 6) and exhaust (2 and 5) differ slightly. Is that still within the tolerance range?

Does anyone have another idea? Meanwhile I am desperate :frowning:


P.S.: Kindly excuse my bad English:-)

Good evening André,

great to have the figures at hand. Compression looks great, clearances at least totally undisturbing.

What is the static timing of ignition though? It should be 8° BTDC. What is your CO concentration in the exhaust? Anything around 3 - 4 % at idle should be good. What do your spark plugs look like?

A dangerous cause of dieseling may be carbon residues burning off - but that is rather a horror story it seems. Most often dieseling comes from too high idle (750 RPM in N are o.k.), too hot spark splugs (specs are Champion N11Y), too lean mixture or too late ignition. No need to be desperate - just take one step at a time.

Good luck


75 XJ6L 4.2 auto (UK spec)

I think the idle is a bit high as well, that would be the first thing I would try, as Jochen suggests 750 rpm would be my target, (not that I can get mine down to that unless it is in D or R :confused: ) because mine idles high I always shut it down in D.

Hi Andre. Everything the guys say. For rough idle when warm check the carb air balance with a length of hose or a UniSyn. Not familiar with HIF7 carbs yet but I think the tuning process is similar to my HS8’s. As above check the spark plugs - should be brown on the insulators. It took me a long while to get my 4.2 engine to idle well - part of the problem was the faulty AED. I know you have repaired your AED but it might be worth isolating it when the car is warm and idling by isolating the fuel and blocking of the warm air intake. Paul.

Andre if timing is too advanced it can cause dieseling and running on.I find the best way to set timing with a warm engine,providing carbies are set well is remove dist vacuum pipe loosen dissy,bring engine up to 3000 rpm then rotate dissy for optimum revs.tighten dissy.Easy to do and nothing to loose JOHN