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Intermittent Misfire at motorway speed

Hi everyone

I have had an intermittent misfire which clears itself after 20 or 30 seconds when driving on motorways at near 70 mph.

At 1st I thought it might be dodgy Sparkrite electronic ignition so I replaced that & fitted a brand new distributor at the same time too. I also replaced the spark plugs and leads and then the fuel pump for an electronic one (as the tick tock one was sometimes reluctant to start to operate after standing for a month or two).

On my way to and back from the Brooklands Classic car day today (it was a great day & absolutely full of cars) the same fault reappeared when the only way I could maintain speed was to floor the throttle until the problem cleared but on one occasion this caused some 'popping' in the exhaust as presumably unburnt gases lit in the exhaust pipe.

So I am now unsure what to do next! I have checked the compressions and they were all around (a lowish figure) of 125psi & I have checked all the tappet clearances & made adjustments.

Any ideas please??

ChrisP

Dirty fuel in the carbs?

Bob Hubbard