Mysterious Engine Failure - Ideas, please

[Graham Barker]

My 1937 Ruby on a recent run: the engine coughed and misfired to a standstill after about 8 miles.  Checked carburettor jets at roadside and found them clear. Restarted motor which was still hot but it coughed and misfired to a standstill after about a minute.  So got tow truck home.
Drained petrol tank - a little rust powder came out. Blew back through fuel line and it was clear.  Dismantled fuel pump and interior was clean - I had overhauled it with new diaphragm 12 months ago. Incidentally there is a cartridge type fuel filter between the fuel pump and the carburettor. Checked this and found that it blows through quite freely. Manual prime with fuel pump worked well.  Engine started fine so went driving, but it coughed and misfired to a standstill after about 2 miles.  Tow truck home.
Replaced ignition contacts engine ran for a few seconds then coughed and misfired to standstill.
Replaced coil, condenser, distributor cap and spark plugs with successive tries at starting the engine.  By the end of this process the engine will not run at all - only a misfire or two.
Battery has been checked and found healthy and had it fully charged during these latest tests.
Any ideas on what to do next?

[Tiger]

Check ignition timing
The only other thing I can think of is a partially block exhaust
I am confident you will soon find the cure

By the way you don’t say whether you have a healthy spark at the plug.
Also are the plugs wet after stying to start ?

[Hugh Barnes]

Have the distributor drive gears stripped?

[Andy Bennett]

sorry if daft but you don't say if you reused the old fuel or replaced it with fresh?
Just that this is just the problem I had on our New Year's day run having not used the car for a few months and after new fuel off she went.

[David Stepney]

I had a parallel problem with my car shortly after I bought it. I would appear to run out of petrol after a few miles, especially if I had been climbing in third or second. I had the tank professionally cleaned, overhauled the carb and fitted a new fuel pump. All to no avail. In desperation, and because it was the only part I hadn't either overhauled or replaced, I fitted a new fuel line front to back even though I could blow through it quite happily. Problem solved!

The only thing that  that i could think was that, over the years, the lumen of the fuel line had become restricted either by corrosion or crud.

[Steve kay]

David is quite right about possible fuel line problems. Being able to blow through the line is not an accurate indication of its condition. It should be closer to gently breathing through it as the pump is working at only about 2 psi.

[Graham Barker]

Thanks for comments so far.
The fuel was fresh. 
I blew through the petrol line from the pump end using only my lungs - no air compressor involved.
I have not checked the spark at the plugs.
Graham

[David Stepney]

"I blew through the petrol line from the pump end using only my lungs - no air compressor involved."

I could with the old line too. Air is a lot more fluid than petrol, so it is no guarantee that sufficient fuel is getting through to the carb under load.

[John Cornforth]

Hi Graham

Piecing together all of your descriptions, I would be more inclined to suspect ignition/electrical rather than fuel.

I can only suggest you re-check the spark side of things, the possibilities below are ordered from most to least likely:

Condenser. Even new ones can be faulty, try another
Contact Breaker Points. Oil or tarnish on faces. Clean with thin card and petrol, avoid abrasive paper
LT supply to coil via ignition switch. Check with a test lamp or voltmeter.
Earth from distributor to engine block. Is the clamp firm ?
HT leads. Unscrew acorns and check, especially the king lead
Rotor Arm

When cranking, connect one HT lead to a spare sparkplug resting on the head. There should be an easily discernable spark even with a plug gap opened up to 40 thou, as the gap isn't under pressure.

If you have established a good spark and still no joy, time to re-visit the fuel supply.

---

Hi Graham again

Having now read your other post about distributor wear and points, I think they would be the first thing to check. The distance from the base of the cam to the tip is only about 40 thou, so it's not hard to see how a bit of spindle bush wear can have a damaging effect. The smaller the points gap, the worse this will be.

Maybe your gap closed up due to the initial bedding down of the heel on the cam ? As others have suggested, in the short term try opening up the gap to say 18 to 20 thou, and check it on all 4 lobes.

[A G Wood]

My two cents (and apologies if I'm giving egg sucking lessons):
You don't say whether you've checked the distributor rotor for HT leakage to earth (as suggested by John C)?
The usual side of the road method is to remove the centre HT lead (the one which goes to the coil; what some call the "king" lead) from the distributor cap, and with the cap removed, so you can see them, rotate the engine until the contact breaker is closed, switch the ignition on and holding this lead 1/4 inch from the rotor, snap the CB points open, either with your thumb (or a screwdriver if like me, you're not shock resistant...) Any sign of a spark to the rotor is a fail. Pro tip, insulated pliers are your friend for holding the lead.
Extra edit: Also try directly filling the float bowl of the carburettor with fresh fuel with the pump to carburettor pipe disconnected (you will have to arrange some method of avoiding fuel flow from the plumbing if you do this for safety). This will eliminate all fuel components except for the carburettor.
There are lots of real experts on here who have real world experience that will chime in I'm sure.