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I would carefully reinspect that orifice again, maybe you did not get all the debris out.

Manifold Pressure: is the outlet side of the Gas Valve going to the burner.

Allowable Gas Max: is the maximum gas pressure applied to the input side (Supply side) of the Gas Valve.

Supply Pressure Min: Is the minimum pressure (input side or Supply side) for the Gas Valve to function correctly.

I think part what you are missing in this situation is the Gas Valve is also a pressure regulator. So with a supply pressure between 5.5 and 14 the manifold pressure should be 4.0.

With your other intermittent issue, it is quite common with Aquastats like that for the solder joints that connect the relay to the circuit board to crack and fail. You can't see them without removing the circuit board from the housing which usually requires the sensor bulb and its capillary tube carefully removed from the Well also.

So cleaning the heat exchanger is not a good idea?

And replacing a defective gas valve after verifying that was the cause of the pilot failing to stay lit is not a good idea?

And the Aquastat problem (if it exists) has not been determined as of yet. So no more parts than needed so far as I can see.

And I don't see anyone stepping up to help out with the cost of a new boiler for this guy.

I try to look for the lowest cost options for each situation. I don't just say "Replace the whole thing” when I discover several problems at the same time. I look at the condition of the overall system and then decide whether the option to offer a replacement boiler is in the consumer's best interest. When Asked if it is better to repair or replace by a customer, and I ballpark the replacement boiler job, done the right way, they usually make the informed decision to repair.  

The existing Valliant boiler is not leaking and from the photos I have seen so far, it looks like it is good condition, a replacement CI boiler will not reduce the fuel usage in any quantity to justify a replacement of a perfectly good piece of cast iron that is already there.  A new Weil McLain boiler connected to the same system may not solve the problem if the system has flaws. It will just move the system’s problems to the new boiler.   (hence my tag line below)

I say we find out what the real problems are before we jump to the conclusion that the only right thing to do is replace the boiler.  That’s my two cents (but we all know that is really 4 cents now)

What you just explained sounds normal.

Now that you have the heat exchanger cleaned out the burner will operate as it was designed.

@EdTheHeaterMan Said on March 3 @ 1:30 “Your vent may be blocked, or the heat exchanger may have carbon building up in between the sections.“ 

Now that the dust and carbon is all gone the boiler will vent the byproducts of combustion (Flue Gas) properly.  The water in the boiler will get hotter faster (the dirt acts like insulation) and then the thermostat will get satisfied sooner. 

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