1974 XJ6 series 2 4.2 litre auto
Having a strange problem with my beast! This week, I’ve taken her out for 3 long - well more than an hour each runs. Initially the car runs silky smooth and then after an extended period of time, I start to get hesitation and misfiring when I pull away from standstill or very low speed. It then gets progressively worse in terms of the hesitation and lack of power. Trip one this only appeared after 2 hours of motoring, Trip 2 after an hour – today’s trip after @ 35 minutes.
Once I’m on the on open road and up to speed, the performance and response is perfect with the engine responding to the slightest of throttle changes.
Wouldn’t seem to be a temperature thing, as during the periods of perfect response, I’ve been in traffic and the temperature has climbed as one would expect in the stop/start but all seemingly within acceptable norms; running at the N of NORMAL and only moving up to the highest of M – and then quickly back down again once on the open road again.
My basic knowledge of the SU carburetted XK engine tells me that there can only be 2 culprits for this. Either fuel or the spark. Since it operates perfectly at speed on the open road after the onset of the hesitation, it seems to suggest this isn’t a fuel supply problem – in which case maybe the plugs?
New and as far as I’m aware, correct plugs/points/filters were fitted less than 3 500 km ago.
I’ve just got back from the today’s trip so I’m too tired to pull the plugs right now – will do so tomorrow but I was wondering if any one can perhaps give me a “light bulb moment” as to what might be causing this perplexing problem? … Seems so strange that it can run silky smooth for such a long period only the for the issue to arise after long periods into each of the three trips.
Many thanks for any insight into this – much appreciated.