Different coloured exhaust valves

[Chris Edgar]

This one has me baffled (not unusual really)

I removed the engine from the convertible yesterday, as I am going to take it to a local works to regrind crankshaft, check cyl bores etc...

I took the head off & noticed that the exhaust valves on cyls 1 & 4 are as clean as a whistle. They look almost new.
The valves in 2 & 3 are black with carbon.
All the inlet valves are evenly black.

This tallies with the colour of the plugs in 1 & 4( good colour), compared with 2 & 3 ( blackish) last time I looked.

So, is the mixture different for 1 & 4 ? is that possible? The inlet manifold is standard for the 1275 engine but the carb is HS6
( 1& 3/4"), larger than standard.
Or, could this be due to oil getting into 2 & 3 ? via valve guides or piston rings?

The engine uses next to no oil & the compressions, tested about 500 miles ago, were 170 psi each cylinder tested dry.
The piston crowns all look the same...carbon coated.

hoping for some pearls of wisdom!
Chris

[bmcecosse]

They do share the same exhaust port - so there can be some differences compared to the end valves, due to the waste gases being slightly restricted leaving these cylinders compared to the end cylinders. It's the major limitation of the A series I'm afraid - along with the shared inlet ports 1&2 and 3&4 - which also tends to cause some mixture variation. If you consider the firing order 3421 - then #3 sucks hard and then when #4 valve opens - the column of mix is already moving in the inlet port from it's run into #3 and so #4 is slightly 'supercharged' and similarly with #1 getting the benefit of #2 having sucked in mix just before it's valve opens. This too upsets the mixture..... So all in all - don't worry about it too much. If it really bothers you - consider an 8 port head.......which solves all these problems........ but isn't cheap unfortunately.

[beero]

Had the same with mine, 1 and 4 were okay, 2 and 3 were burned. I think it must be a heat thing where they share the exhaust port.

[Chris Edgar]

Thanks for the replies, Roy & Beero
Roy, I suspected that you may know something about this!

Be interesting to see what the man at the engineers has to say. He has been running the business for nearly 40 years.
regards
Chris