Poor pick up on a series 3 XJ6 3.4 litre

[CliveRothwell]

Rebuilt head
New HIF 44 twin SU carbs
Set up on rolling road by respected motor engineering wizard to sort high fuel consumption.
123 electronic ignition, new leads and plugs.
Owned for 9 years.

From idle, the car seems reluctant to pull away smoothly, the engine appearing to fluff and almost mis-fire and indeed, given too much pedal, it will backfire slightly if you are not careful. The car has always done this, even before all the work noted in the intro.

Today, I raised the idle from the recommended 750 rpm up to 900rpm in Park. It has improved things slightly.
I assume that all automatics lower their revs when put into Drive by some 200 - 250 rpm. I suspect that pulling away from just 650 rpm is a lot to ask and the engine baulks and fluffs before overcoming the resistance and then pulling away properly. I don't want to raise the rpm any further. Other than noting that the autobox atf oil level is high, even when cold, I am struggling to figure out what the issue is. Any thoughts please?

[mike020150]

Raising the idle revs may mask the problem but shouldn't be necessary. Do you have the most recent MoT emission test results available? They might give you a clue about the mixture strength. If the car was set up in response to a complaint of high fuel consumption, it would only takes a tiny extra tweak towards the weaker side of an ideal mixture to make the car feel sluggish. Equally, could you have overlooked any of the obvious simple checks, like use of the correct viscosity oil and oil levels in the carb dash pots? Mike.

[CliveRothwell]

Thanks Mike. I am confident mixture levels are spot on. Engine oil is correct at 20/50. Genuine SU carb oil was used in the dash pots and was checked recently. I'm wondering if the ATF is causing too much drag due to it being at a rather high level.

[mike020150]

Hi Clive,
The ATF definitely won't have a bearing on the issue. Typically, these annoying problems have simple solutions so I'd suggest going back to square one and working your way through the basics looking for things like air leaks, cracked or leaking exhaust manifolds or anything which could cause a weak spark, like poor coil earthing. Don't take anything for granted and double check everything! Mike.

[piman]

Hello Clive,

it's a prejudice of mine but I don't like these 123 distributors, especially the one with just a handful of curves selected by dip switchs. There's not a chance it will match your engine properly.
That aside, my first check would be with the timing, if the engine does not pink then advance the timing until it does then back off a shade. That is a simple dynamic way of setting the timing, with that done do a timing check at high revs to ensure that it is not overadvanced by the 123, your manual should give the advance curve figures including what full mechanical advance is and at what rpm.

Alec

[CliveRothwell]

Thanks Mike and Alec. It's nearly winter, so I should have plenty of time to try your suggestions.