Hi there.  Been away a couple of days.  Sorry for the late reply.  I reckon I have although I haven't been able to take the car for a test drive - the wet weather has just set in.  I'm 99% confident.  If not, I'll report back.  

So have You solved the Problem by fixing both Issues?

  

Just thought I'd provide the result of the original post.  After refitting the intake manifold with new gasket, the intake manifold improved slightly.  My first run indicated that I may have solved the problem of poor contribution from cylinder #8.  Things definitely improved as the manifold vacuum reading increased into the low side of a normal reading.  I then rechecked all other vacuum fittings - including the brake booster.  I discovered that the booster diaphragm did have a hole in it which was a big part of the problem.  Interestingly, the first time I plugged the booster pipe, the problem didn't disappear, since the intake also had a leak.  Given that there were two issues - an intake manifold gasket fault and a booster diaphragm leak - it makes it harder to diagnose the problem.  One naturally assumes there is one fault, not two or many!  I have previously worked in maintenance engineering, so I'm annoyed I got fooled by this issue of multiple failures.

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One would a$$-u-me the secondary IGN operation has been verified. And it seems all the mechanical's have been verified.

If it has, it sounds to me as if #8 is running too lean and will not fire the mixture or the vacuum leak is blowing out the flame.

But GOD knows, I have been wrong before. Just ask the wife...

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The vacuum source is within the carb. It pulls filtered outside air through the tube in the intake manifold where the air is heated to operate the choke. The carb is calibrated for the extra vacuum draw and any leakage will upset the factory carb fuel trim.

If yours is 56 H4000, simply plug with a rubber cap the air intake side (@ the carb air-horn) and remove the inlet fitting to the choke housing and check for outside air leakage with a MITY-VAC to see if it will hold vacuum. Service the system until it will hold vacuum and re-attach.

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I didn't see any response to observing the quality of the spark as suggested in this post. I think it would be a helpfull clue to know this.

OK so where is the Vacuum Source? Through the Choke? Where do I seal it to check if its a vacuum leak Source? Cap the threaded fitting on the Choke Housing. I did try the Propane Test and didn't find any change in RPM.

  

OK.  That makes sense.   So I shouldn't have blocked off the mushroom fitting.   I will remove the brass fitting on the inlet manifold that connects to the booster, etc. and find and make sure it seals properly with a thread sealer or similar.  I have directed a propane spray (with wand) around it when the engine is running but it doesn't appear to make a difference.

Yes, it is a fresh/heated air source and operated by manifold vacuum. It has to be air tight.

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