Protecting exhaust valves

[promotor]

Yes, I do have the connecting tube mounted in the lower position.

Dave.

Maybe there's a possibility that air is leaking through the balance pipe O-rings and causing a lean mixture?

Or maybe there is water getting into cylinders 2 and 3 and causing super heating? Do you have to top your water up occasionally?

Just seems odd that it's cylinders 2 & 3 struggling - I would have thought that if fueling was correct and no air leaks then all cylinders would run to the same conditions. It doesn't make sense to be down to incorrect jets as exhaust valves 1 & 4 would also struggle as they share the needles that feed 2 & 3.

[david.g.chapman]

Good news!

I decided to swap all of the spark plugs for another old set of NGK BRP6Es I had lying around, and also raised the needles a bit to richen the mixture (not easy with my fixed jet federal carbs).

I then did 10 mins at 4000prm up the A12, and then 4 miles of mixed driving back through Chelmsford to Writtle.

Plugs 1, 2 and 3 were a pretty similar grey/brown colour (plug 1 slightly darker) and plug 4 was dark, but beginning to self clean at the electrodes to that same grey/brown colour. That sort of makes sense to me with the crankcase fumes mostly going to number 4 from the feedback pipe, and also with the mild disruption of the brake servo feed on air pressure to number 4. I think that number 1 would not have the same disruption once the headlights are down (or up for early cars).

That's good enough for me. So if in doubt - change the plugs!

I will see what happens in 12000 miles.

PS
I just saw your post - BobP. Being too lean at motorway speeds has occurred to me as well. There is a spreadsheet showing needle profiles somewhere and I might use it if the valves give trouble again.

Dave.