Help Engine Misfire runing rough

[andrew34ruby]

Hi Dave

 HT breakdown due to dampness usually goes away when things warm up and dry out,

Agreed John, usually, but you might be surprised how many cars in the 1980s I came across that started perfectly from cold, then two miles down the road they missfired and or stopped completely. The inside of the distributor cap had a fine mist of condensation on the inside.

[DaveT]

Thanks for all your replies relating to the misfire.
 
I have now checked the following,
 
Cold compression check all 95psi
 
Changed the points as I found a breakdown in the wire between the points and the condenser and then changed the distributor cap and rotor arm.
 
The engine started immediately and ran fault free for a couple few minutes before the misfire and loss of revs started. Further hot compression check, all still 95psi.
 
Engine refused to start again after 20 minutes so I decided to remove the head and start thinking about looking for a sticking valve.
 
Attached is a photo of the block, any ideas why a couple of valves are grey and the rest heavily coated in carbon?
 
The block is a sports block with larger valves and a HC compression head on a standard 2 bearing engine, which has run fine since the rebuild in 2015
 
 
 
  Condition of valves..png (Size: 413.3 KB / Downloads: 259)

[andrew34ruby]

Often a slight water leak into a cylinder results in a spotlessly clean valve.

[Charles Levien]

Hi Dave,

Perhaps look at your valve seatings and tappet  clearances — a light valve grind might be useful unless they are all good. The pale grey valves are both exhaust which run hotter anyway.
It’s really a case of checking the basics.

[Derek Simmons]

Double check the valve tappets are correct (and lock nuts tight) and that the Pot 1 spark plug is firing regularly as a lighter coloured valve head could suggest the misfiring is coming from there. That’s what I found with my 37 Ruby!

[John Mason]

I don’t know if this is of any use but thinking out of the box. The two exhaust valves that are pale looking are both to one side of the block being served by one branch of the inlet manifold. Could it be that that the mixture from the carb is not equal to both branches. Thus two cylinders being richer/weaker than the other two.

John Mason.

[DaveT]

Thanks for those suggestions, I'm going to carry out a decoke, but before I take the valves out I'll spend time checking the existing gaps before I back of the settings to remove the valves for lapping in.

All good fun.

Dave