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diagnostic from reading the plugs - what to do next. - Fuel System/Carbs - The Lotus Forums - Official Lotus Community Partner Jump to content


diagnostic from reading the plugs - what to do next.


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Hello all.


I did a 200 mile round trip on the M4, A40.  So I was tooting along at speed without lots of stopping/starting town traffic.  I used about £45 of fuel!  The engine progressively ran rougher as the day went on.  On pulling away the sound instead of being high, cheerful and spirited, sounds low, grumbly and throaty.  The acceleration was very slow. the engine would respond, but none of the usual whipping push from the back seat and feeling of power.  I confess I put foot to the floor on the open motorway and topped out at 100mph.  I kept the throttle there for a good minute and no rise in speed.  I have got higher in the past.  Any hill rise in incline even on the motorway required a drop to 4th.  Over Easter I had driven to Northern France and it was a different story, so I figured something up.

Once home, I took out plugs and the photo attached reads left to right cylinders 1, 2, 3, and 4 (with 1 closest the timing belt and cabin, 4 toward rear of car).

1 - sooty

2- sooty

3 - clean as a bell

4 - sooty and a bit oily

the oil COULD have been a oil build up at the foot of the plug as I dropped it when taking the plug out so i'm not sure if oil on the electrode came from my baffy fingers...

The car is Esprit S3, n/a.  super well cared for.  New valves went in earlier in the year at great cost so that bit is covered. I'm now a dab hand at balancing the carbs (thanks to the forum) so I THINK this is a question of new plugs, check mixture and re-balance carbs. BUT wanted to check for anything else it could be?

  • distributer timing/advance (can this work loose? - surely not).
  • HT leads?
  • over fuelling or under fuelling?
  • when is it good practise to change plugs?
  • when plugs 'die' is it this quick decline in performance measured over a few hundre miles?
  • I've got Valeo R76H which i've been assured are the same spec as NGK BPR6ES

thank you all.  I'll post up the resolution, as ever, when solved.








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