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December 17th, 2006

Working on it

Minor update

This past week, I've:
  • Hacksawed Fenchurch's inlet/exhaust manifold in two. This is a common modification to slightly increase the maximum power output. Separating the exhaust manifold from the inlet manifold eliminates the hot-spot where the two halves meet. I think this was originally intended to do something useful in extremely cold weather, but its main effect is to heat the incoming fuel/air mixture and reduce its density. Unfortunately the two halves are only separated by a thin sliver of metal and it's in a very awkward position, so I wound up accidentally breaking through into the intake half by a fraction of a millimetre at the thinnest point. I've patched up the hole as a temporary fix and will soon be upgrading to a more efficient after-market aluminium inlet manifold I've bought on eBay. I dread to think what my insurance company is going to say when I tell them I've modified the engine, even if it is only a very tiny mod (it's not like I've fitted a supercharger)...
  • Rebuilt the old, rather worn rocker assembly. This involved dismantling it (being careful to keep all the bits in the right order), cleaning decades of black gunge off everything, turning a special tool and using it to press out all the old bushes and press in the new ones, reaming the new bushes, and putting it all back together on a new shaft.
  • Put the cylinder head, manifolds, etc. back on the car and set the tappets. After doing this the engine fired right up first time and ran as smooth as silk, but I had to shut it off after a few seconds due to the lack of radiator, water pump, coolant, etc.
  • Fitted a new water pump. The old one was a bit corroded and bunged up with all sorts of crud, plus since I was replacing it anyway I took the opportunity to change to the later Metro type with no bypass hose fitting on it and block up the bypass fitting on the cylinder head, thus eliminating the bypass hose altogether (something the later A series engines had as standard - it took them a long time to realise it wasn't really necessary and had a habit of failing and being awkward to replace). One slight complication was that there are two locator pins that are supposed to be fixed in holes in the block and fit into holes in the water pump casting to align it, but one of them was jammed solid into my old water pump instead of the block. I had to turn up a replacement pin to just the right diameter and whack it into the block, then I discovered that the distance between the locating holes in the block was about 1mm greater than the distance between the corresponding holes in the new water pump, so I had to use a small burr tool to elongate the holes in the pump casting until it fitted snugly in place.
  • Replaced the vacuum advance unit. These have a diaphragm in them that tends to fail eventually, allowing air to leak into the inlet manifold and preventing it from advancing the ignition when the throttle is closed like it's supposed to. AIUI the effect this has is to increase the fuel consumption slightly. I had to take the distributor off to do this and found that, contrary to what the workshop manual claimed, the timing is affected when you put it back, even if you take it off by undoing the two mounting bolts without slackening the pinch bolt. In fact I started the engine after putting it back together and it sounded rather rough, so I checked the timing and found that it was firing at about 30 degrees after TDC, which is way out (it's supposed to be about 4 degrees before TDC). It just goes to show that these old engines will often still run, albeit poorly, even if they're extremely badly adjusted indeed.