I thought that I should clear up the mystery before I disappear on holiday next week. I am afraid that no-one has got near the answer, but maybe I can come up with another (easier to solve) mystery and give the mugs away at my open day in July.
It seems that everybody followed the same line of thinking as I originally did, that the white smoke must be steam due to water getting into the combustion chambers. When I first introduced the mystery back at the end of March I said that 'John Roe had visited my workshop whilst I was puzzling over the problem and made some comments that were extremely pertinent' the most pertinent of his comments was that "white smoke can also be caused by over-fuelling'", "when the mixture is too rich you get more water (steam) and carbon monoxide, as well as unburnt fuel (moisture) in the exhaust gases". I initially dismissed that as the CO level was spot on at tickover - but then again I wasn't getting the smoke at tickover!
The other clue that I had missed was the dampness of the pistons when I took the head off (see my posting of April 11). The pistons were all wet with fuel. I had totally ignored that clue as I was looking for where water could be getting in
So following the idea that water must be getting in somewhere I decided to try some fresh clean fuel (see my posting of April 25). I decided to disconnect the fuel line from the tank just before the fuel filter and connect a pipe from a spare can of fresh fuel. First I warmed up the engine to see if the smoke was still there (it was). To my surprise, when I disconnected the fuel line, a spurt of fuel shot up from the pipe a good six inches above the end of the pipe. Now the end of the pipe must have been at least a foot above the level of the fuel in the tank, so the only explanation was that the tank has become pressurized. I quickly stuck the pipe back on (I don't like jets of fuel splashing around the engine bay!) and mopped up the fuel. I then went around to the back of the car and removed the fuel cap. We started the car again and this time no smoke. I put the fuel cap back on and within a couple of minutes it was smoking again.
So the cause of the smoke was the pressure in the tank. As the car warmed up the exhaust will have warmed up the fuel tank increasing the vapour pressure and evidently the tank was not adequately vented (hence my comment about Johno's post). In fact that came down to my installation of the MGF fuel cap, so I must take the blame for causing the problem as well as taking so long to find it.
The SU fuel pump is closely matched to the SU float and needle valve so that it supplies fuel at the correct pressure - if you have ever fitted something like a Facet competition electric pump then you will know that you need a fuel pressure regulator before the carb(s). The diaphragm of the SU pump will let any pressure from the tank through as it just supplies a pressure difference when it pumps the fuel. This extra pressure, must have been enough to just pressurize the float chamber without causing it to overflow. The vent pipe comes vertically out of the top of the float chamber, so it must be possible to raise the pressure by a couple of inches of head before it overflows. A couple of inches of head may not be much, but it was clearly enough to create a big difference between the mixture at tickover, when the needle is sat well down into the emulsion tube, and the mixture as soon as you opened the throttle. With the fuel cap off, I needed to richen up the tick over mixture a fair bit, so even then the pressure must have been pushing more fuel through.
The real cause of the problem was down to my fitting of the MGF fuel cap. It totally sealed the tank. The arrangement with the MGF fuel filler neck was that the pipe that provides the vent for the Mk2 Metro fuel tank becomes a fuel return pipe for any back splashes caused during filling. Owners of Mk4s, with Rover Metro tanks will know that they have a fuel return pipe and a vent pipe. The tank breathing was easily rectified, by teeing in a second pipe and fitting a little non-return valve at its top.
DSC_0025 by
Alistair Courtney, on Flickr
DSC_0026 by
Alistair Courtney, on Flickr
With those fitted I would like to be able to say that the car sailed through the MOT, but D612 PNT still had a trick or two to play.