Chap driving down a country lane late at night when the engine dies. So he gets out and does what everyone does - opens the bonnet and peers inside it.
Then he hears a voice say "it's your carburettor". He looks up but there's no-one about, so he returns to looking at the engine. Then the voice comes again "it's your carburettor".
He jerks up his head and looks about - there's no-one but a horse looking over the hedge. So he says "That's strange, I could have sworn I heard someone talking to me". The horse says "Yeah, it was me, it's your carburettor!"
Chap runs 3 miles down the road to the nearest pub and dives in and demands a triple brandy. Landlord says "You look very stressed sir, are you alright?"
Chap says "No - I'm going mad. I broke down back along the road and while I was trying to sort out what the problem was, this horse stuck its head over the hedge and said it was the carburettor. Gimme another triple".
Landlord says "What colour was the horse?"
Chap says "what does it matter? Whatever colour it was I'm still going mad".
Landlord says - "Well there are two horses in that field - a brown one and a white one."
Chap says - "Well it was dark, but so was the horse, so it must have been the brown one - why? what different does it make?"
Landlord says "Well in that case sir, it probably is your carburettor - the white horse knows bugger all about cars!"