I’ve mentioned before, the popping sound on over run and had spent some time on the S2 Forum trying to figure out why it was doing it. The learned folks on the forum collectively came up with the following…
- Air entering the exhaust system on over run (a detailed topic in itself which I won’t bore you with)
- Poor valve gaps setting
- Poor timing
- Mixture too rich
- Combination of all of the above (which would mean I’d done a dreadful job on the build and I should be utterly ashamed of myself)
I could eliminate all of these… until last weekend when I noticed a build up of soot on the engine block. I could also hear a faint “ticking sound” (the best word to describe it) on acceleration. I assumed this was the coil which is right on the other side of the bulkhead from where I sit in the drivers seat.
But after finding soot on engine block and head at number 1 and 4 cylinders the dots connected. The “ticking sound” was exhaust gas bypassing the gasket. This in turn would lead lead air being entrained into the exhaust system on over run…. causing the popping. (See item 1 above)
I had fitted the composite gasket that came back with the engine from Turner Engineering. Given their excellent reputation it never occurred to me the gasket might have failed (which it had… almost immediately)
I just happened to have a replacement metal gasket kicking around, so let get on and change it 🙂
Then, the 9 fixings holding the manifold assembly to the head are removed. With a light tap with a soft mallet, the assembly can be pulled away… the inlet manifold is cast aluminium and the exhaust manifold is cast iron… and quite a weight.
There are two types of gasket depending on engine number… maybe I’d fitted the wrong one originally ? Hopefully the parts manual will help.
The parts manual states:
Part number 274171 (upper in the picture above): metal and fibre type joint washer up to engines numbered 25283642J Except Switzerland. 25282861J for Switzerland.
Part number 564307 (lower in the picture above): corrugated tin plate type joint washer up to engines numbered 25283643J onwards, except Switzerland. 25282862J onwards for Switzerland. (With corrugated tin plate type joint washer, the exhaust manifold and cylinder head are in metal to metal contact)
For security I’m not going to type the engine number here (although it’s probably in a picture somewhere)… but it ends in ………653H so we need part number 274171 (the upper gasket in the picture above)
The motor sounds completely different on tick over and no longer pops on over run… something I am particularly pleased about.