Its sorted! - but your post put me on the right track to the solution; (which was 'user error'!!). You said that black smoke was either too much fuel or not enough oxygen. So, this morning, I went straight to the truck and removed the air filter casing and withdrew the air filter. I did this since this was at least the easiest 'first step' to finding the reason (and to be honest NOT wanting to find it was a turbo-problem!). As I took the casing off the air filter, I saw a beech leaf (from the autumn) and this at least reminded me that stuff can get sucked into the filter. My eye was then caught by a flash of green inside the empty cannister. I put my hand in a and caught the corner of a green rag which was compressed inside the inlet tube to the filter housing. The was about half the size of a tea towel and was blocking the intake entrance into the housing by about 95%!!

I then recalled, that an hour or two before the symptoms had occured I had checked my oil level (something I do only every two months or so.

Obviously, I had wiped the dip-stick with the cloth and then instead of tucking it behind the battery I had tucked it forward of the battery position and right under the air intake conduit. When running, the rag had been sucked into the intake and then right along the air conduit until it lodged in the filter housing when it hit the filter element depriving the engine of 95% of the air it needed! There was enough air for tick-over and moderate throttle opening but not for high revs.

There was also one other symptom which at the time escaped me. During my first look at the problem, I removed the oil filler cap from the top of the rocker cover and was surpised to feel not a moderate back pressure but a strong SUCTION into the oil filler hole. I remember thinking; "thats funny"! but I didnt cotton on to the fact that this too was a symptom of air starvation. So, all's well and it didnt cost a penny - naturally, I will be more careful where I place loose bits of rag in the future!! Cheers, Terry