Hesitation/misfiring from standstill? …

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.

I had a similar problem, although not identical. It turned out to be the condenser in the distributor although it had been replaced less than 5000kms previously. I replaced it with the ‘old’ one and the engine was perfect again. I have since taken the points out and replaced them with electronic ignition and it was definitely the correct thing to do IMO, although I was reluctant to make this change initially.

Your problem may not be the same as mine but it’s an easy thing to check. It took me a week to find my issue as I went looking for fuel problems to start with and some wise heads on this forum pointed me in the right direction!

Just my thoughts,

Bob

Two other electric items do not like heat.

  1. Carbon core HT spark wires.

  2. Oil filled ignition coil.

On different critters solved by replacement materials.

Carl

My thoughts went to the condenser as well, grab a can of Freeze spray and next time it starts to play up spray your condenser with it. If that doesn’t have any effect spray the coil as well as that may be breaking down. But go for the condenser first.

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If you do have the mechanical points, Dave; check points for pitting, which may be caused by a faulty condenser - and dirty points affects current flow/spark energy. How long since the replaced points? When accelerating vacuum falls and cylinder pressures increase, requiring better sparking - and you may be on the marginal side. Points wear also influences dwell angles and ignition timing - both may influence sparking…

Frank
xj6 85 Sov Europe (UK/NZ)

Thanks for that Frank - new points and condenser fitted in March and I’ve done less than 3 000 km since then but they’re seemingly more and more likely as the culprits but what I’m battling to get my head around is how the car can run so nicely for such a long time and then only does the problem kick in - if it’s the condenser maybe it has some sort of time-related fault ie., it starts to break down in efficiency having been “used” for a period of time? …

Hi - thanks for your input - Yeah I read your past posts and it seems that the condenser may be the problem - I’ll try replacing over this weekend if I can find the right one - classic car parts suppliers are not exactly common place here in cape Town!

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Questions arising, Dave…

During acceleration from standstill or low speed the engine hesitates and misfires after driving for ‘some’ time; no such problems earlier in the drive - it suddenly appears, and persists? There is no indication of engine temp variations during the drive, relating to the ‘episodes’? Have you visually inspected the point’s surfaces - any sign of discoloration or pitting? Have you checked points gap? Have you observed the tacho behavior during ‘episodes’; as long as the ign functions properly the tacho should follow engine rpms - ie tacho should be steady? Does engine respond the same way when accelerating from higher speeds?

How long does the ‘episodes’ last; until speed/rpms rises or you ease off the gas pedal. Point here is that the box should downshift with enough pedal input. If downshift does not occur; the engine may be in the wrong gear and will struggle…

The condenser does not ‘tire’ while being used; it either works or does not. The ignition voltage is related to how quickly the coil current is cut; points arcing prolong the cut-off and lowers coil voltage - the condenser, working correctly, reduces arcing, raising ign voltage. A finer detail is that the current is flowing while the condenser ‘fills up’, also slightly delays cut-off - a reason while EFI ign voltage is higher; the cut-off is much cleaner…

That the parts have recently been replaced is not really a proof of anything; any part, new or old, may fail in its own good time…:slight_smile:

That said; other factor should no be disregarded. As you step on the gas; ignition timing changes due to vacuum inputs - failed vacuum inputs may then cause hesitation. While fueling is not really suspect; the standard procedure when engine is misfiring is to instantly change to the other tank - and the engine will recover, a clue. As you accelerate; fuel demands instantly jumps up, while cruising consumption is far less…

That all ‘episodes’ happen after some driving is just a complication - it does not provide any certainty. But the coil gets hot while driving and is a possible suspect. It may be marginal; failing to deliver in some circumstances, but not in others.

That the ‘episodes’ seems to appear after shorter periods of driving may be a clue, but to ‘what’ is not crystal clear…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)

Gosh Frank, I really appreciate your time and trouble taken in your analysis - gave me much to ponder

OK so the points definitely look pitted - but work will have to cease now as I try and find an offset 1 5/16 ring spanner (or 33 mm I believe from the forum) so I can turn the engine to set the points gap.

It’s been decades since I last dealt with points but my recollections from my days with Minis and MGBs etc is that the 2 sides of the points were pretty much the same size/ diameter - whereas on this set, the one contact is definitely smaller in diameter than its opposite number - is this a new development in points design and/or manufacture?

  • or maybe rather my memory is faulty!

Just as an aside to demonstrate how not only the sourcing of parts is a challenge in itself in the Southern Hemisphere - here’s what greeted me when I went in to my garage to start work on my XJ ! …

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Old time mechanics had a tool to activate the starter and as such bump the engine over. O have a pair. But, i am a log way from you.

Make one. Just apply power at the rw wire on the starter relay.

I began one on my car. Semi permanent…

Hi Dave,

Just remove spark plugs and what I do is temporarily tighten the alternator belt (very easy to do) and then use the nut on that to turn the engine. This is a lot easier in my view than trying to turn the nut on the crankshaft pulley and less scraped knuckles.

Don’t forget to re-set the alternator belt after though - don’t ask :crazy_face::crazy_face: !

Bob

What I did in a pinch was jam a screw driver into the fan/pump pulley where the fan clutch is bolted to. That was just enough to turn the engine, no further tensioning. Alternator sounds better if accessibility is good, no tensioners in the way.

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Difference is of little consequence, David - contacts don’t carry extreme currents. But theoretically one smaller is more likely to make ‘better’ contact. Bought as a set it may be a deliberate manufacturer set-up - for honest reasons. A platinum file is designed to smooth the points…

If you have a working spare coil around; you may also try that?

Frank
xj6 85 Sov Europe (UK/NZ)

For those who responded to this thread or simply followed it – here’s the latest!

My twin 17 year old sons both tested positive for Covid last week, so we’ve all been stuck in isolation, which didn’t help matters. A good mate of mine tracked down and bought for me a 34 mm ring spanner – what an enormous spanner that thing is! – which, with a bit of fiddling, I managed to get on the crank bolt to turn the engine (couldn’t for the life of me turn the engine using the water pump pulley or the alternator as suggested – try as I might!). I set the points. Cleaned up the rotor and the internal distributor cap connections; changed the condenser (realised it hadn’t been changed when I did the first plugs/points change just after I acquired her back in March – stupid me!); cleaned up the plugs (weren’t bad) and she started third turn of the key after 10 days off the road. Up to operating temperature and she revved freely – I’d like to think better/free-er than prior – took her out for a road test and she purred.

I haven’t yet done a long journey to see if I get the same problem of hesitation on low speed acceleration after a long period of running but that will come. I bought a new coil which I’ll fit if the problem occurs again as the next possible culprit if it re-occurs but I wanted to do pretty much one thing at a time, so I can be reasonably certain of what caused the issue if it happens again.

Many thanks to one and all for your input/support – greatly appreciated.

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Good job Dave,

if your 17-year-old sons are the same as in your profile pic your profile pic must have been uploaded around the same time as mine:-) All the best for their recovery.

Congratulations to your success!I had similar issues on my car over around 18 months and changed the coil to no avail. In my case a good carb cleaning did the trick. So I guess persistency and consequence is what matters. In most cases it won’t help much to just throw new parts at such a problem.

All the best

Jochen

75 XJ6L 4.2 auto (UK spec)

Hi Jochen,

Always nice to hear from you.

Yeah that pic was taken 5 years ago when they were 12 - I really should update! Now the one is very nearly my height (1.89 m) and the other is a gym fanatic and is built like the proverbial brick sh*t house!

Did a wedding in the XJ6 yesterday and she performed faultlessly - bursting into life each time with the merest hint of a turn of the key - very satisfying - I haven’t replaced and checked points in a car for over 45 years when I was on my 20th Mini or so, so it was very satisfying to seemingly get it right first time around - my guess is that they guy who put the last set of points in and gapped them wasn’t as meticulous as I was in making sure the gap was as accurate as it could be - still don’t know whether this problem described above was points or condenser and I need to take her out for a 2 hour drive to see if I gotten rid of the problem but so far - so good!

I just have to keep reminding myself that the XJ6 is really just an oversized Mini with the same basic mechanics and in my day, there was no job I wouldn’t attempt myself, other than perhaps a gearbox re-build!

Cheers.

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