help diagnose this

Brian,

I dont know if it can happen on a Ford block, but the "hardest to diagnose" mis-fire I ever had was on a little Anzani block on a 28 Frazer Nash I ran. It would run fine when cold then gradually start to mis-fire.

It turned out, after months of investigation and expensive replacement of parts, to be an internal crack in between the inlet tract and an adjacent exhaust port. As the motor heated up the crack opened up and burning exhaust gas was getting into the inlet tract causing a wierd mis-fire.

Dont know enough about the Ford block inner structure to know if it can happen but just thought I'd throw that one in...

Tom.
 
Hi Tom, Yeah, after chucking all the usual electrical stuff into this old girl, I wondered if the heat riser in the intake manifold had a crack or similar in it, replaced intake with another, no difference. I'll be trying a vacuum gauge on it next week, see what that shows.
 
Exhaust valves all properly clearanced when cold (or hot)?

This engine is running stock non adjustable lifters. Ran fine for several thousand miles, before misbehaving like it is.
 
Do you smell a little gas when it fails? May not be applicable in your case but, i've had a pin hole leak in the fuel pump diaphragm that would enlarge as it warmed up. The leaking gas would drain out of the pump onto the ground.
 
I have had to slip a plastic tube over the adapter plate contact spring on one unit to stop it intermittently earthing on the distributor body
 
I would look at the plugs and see if I could find the cylinder that the problem,at lease the side you think and remove the head.
The valve guide could be cracked,worn,carbon build up,inspect the valves.
 
I had a very simuluar problem in my wifes 302 Bronco. There was a fuel line too close to the rad hose (P/O removed fan shroud and installed extra rad hose to route away from fan) Once I realized and replaced problem solved and no more vapor lock. Do you have any lines close to heat?
 
exhaust valve without enough clearence. When it gets hot and expands, clearence goes away and it doesn't hold comp.
 
Jumping in here. Have you checked all the ignition primary circuit wiring and grounds(earths)? A loose connection will give those symptoms. My pickup showed the same type of issues a couple of years ago. Turned out it was a loose/dirty terminal on a ballast resistor.
If you are running a "divers helmet" distributor check the rotor. Those can short to the shaft. A piece of heat shrink tubing over the narrow portion of the shaft may help.
A quick vacuum leak check can be done with WD40 or Carb cleaner spray. While running spray either one at the manifold mounting surface all the way around, at the carb base to manifold and carb base to main body. If you have a vacuum leak you will hear the engine change.
Good luck finding the problem.
 
I haven't got back to this issue as of yet as the vehicle is some drive away. I will definitely give an update after my next session on it. Seth's recent post about bad condensor indicates much the same problem, we've tried three new condensors with no change, we will try yet another. Thank you for all your suggestions, much appreciated.
 
Brian, the first two things that come to my mind are 1. coil & 2.condenser

I had a similar problem once on my 39 pickup. I was trying to run an electronic unit on 12v negative ground. I would get down the road a few miles with engine up to normal operating temperature and all of a sudden the engine would shut off as if I turned the switch "off". After things would sit & cool off, the engine would fire back up and I could drive a few more miles until things got hot again. As it turns out I was getting too many volts to my coil. The coil would get so hot you could not touch it and it would just break down. It also caused the electronic unit to cut out. Drive the car and when you notice the engine wanting to backfire or cut out place your hand on the coil and see if it is hot. I solved this by doing two things. I installed a points system and placed a resistor ahead of the coil.

On the condenser, it would cause the engine to run extremely rough and hard as hell to start. I got a Vertex condenser from Bubba. You may indeed just have the incorrect condenser or bad condenser. I got lots of backfire from a bad condenser.

Let us know what happens here.
 
With the crappy unleaded fuel we have now, could have a valve tightening up. Rechecking the lash couldn't hurt.
 
Well, we,er all guessing, so here is my guess. Sticking valves. Pour the marvel mystery oil done the carb, in the oil, in the gas tank. Make sure you let us know when you find the problem. Walt
 
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