Help Engine Misfire runing rough

[DaveT]

1935 Ruby has suddenly developed an engine fault after running smoothly with no engine problems. (Compression check on all heads 94 to 98psi)
 
The symptoms are as follows.
 
Engine starts easily and runs smoothly for 5miles or so. I then stop at traffic lights and on starting to run again the engine develops a misfire and runs rough. I change down and rev hard to build up speed the misfire continues to get worse then better but generally sounding like a misfire.
 
So far;
 
I’ve change the plugs, same issue but then disappears for a few miles before the problem returns.
I’ve changed the Bosch dizzy condenser (no change).
I’ve checked the point’s gap (0.015) and checked the rotor arm (no change).
I’ve swapped and changed the fuel pump and in line filters (no change).
I’ve cleaned out the 26va carb, removed the jets and blown them through (no change).
 
So at the moment I’m stuck as to how to proceed.
 
Any suggestions, ideas or thoughts on how to move on will be greatly appreciated and tried out.

Dave 

[Hugh Barnes]

Coils can break down when they get hot... ?

[Chris KC]

You don't mention cleaning or replacing the ignition points?
I wouldn't give you tuppence for a Bosch distributor but on a DK4A it's nearly always the points. 
I do that without thinking and only start scratching my head if it doesn't work.

[DaveT]

Thanks Hugh,

Great, I've got a new coil so one more thing to try Fingers crossed.

Dave

[DaveT]

I've just fitted the spare coil and then went for a short run up Hartside Pass (1900 feet) then back to Gamblesby, Ruby was running well, then, as I drove into the garage she started to misfire and wouldn't rev and if I pressed down on the accelerator the car backfired. 

So could it be the 26va carb as well as a coil??? I've got another carb so I might fit it and see what the next step is.

Again any suggestions greatly appreciated.

Dave

[Alan]

Sticking valve

[andrew34ruby]

Distributor cap and rotor arm are other possibilities. Internal cracks and/or a little dampness are possible.

[Graham Barker]

I had a similar fault. I made all of the replacements that you have made without success. Eventually the fault was traced to my Lucas distributor. There was a poor electrical connection through the screw that holds down the contact to the distributor base plate. This screw has to make electrical contact with a brass earth connection within the base plate. Electrical resistance increases as temperature rises. Hence the motor was fine when cold. The solution was a new base plate for the distributor.

[John Cornforth]

Hi Dave

The symptoms, especially the sudden onset / intermittency, make me think its more likely weak ignition than a fuelling problem.  I agree that coils can fail when hot.  Condensers can also fail when hot, especially modern ones. HT breakdown due to dampness usually goes away when things warm up and dry out, but some modern (i.e. not bakelite) parts like dizzy caps and rotor arms can become conductive when hot if they are not made of good enough quality plastic.

Clean the points, then draw a switch cleaner soaked piece of business card through, to remove any particles

Check the earthing of the points via the distributor to the engine block. This can come and go with vibration if dodgy.

LT feed to coil from the ignition switch OK ?  Try a direct feed if suspect.

A voltmeter will be useful when checking the LT side of the ignition.  Expect no more than about 0.5 volt drop from the battery to the coil SW terminal, and no more than about 0.2 volts drop from the coil CB terminal to the engine block.  Both these checks carried out with the points closed and the engine not running. If you can use croc clips for the probes, it leaves hands free for wriggling connections, ignition key etc to test for intermittency.

[squeak]

I had the exact same symptoms with fuel vaporisation when waiting at the traffic lights, the car would barely make it across the intersection before finally clearing and running smoothly on.  Back in Australia I had the radiator cleaned out and no further issues.