I think the title may be a bit misleading but wasn't sure how to shorten it up. Here's what's happening:

I have a robertshaw 780-715U ignition control unit on a gas furnace. When heat is called for the first time (usually in the morning), the igniter sparks, the pilot lights, 5 seconds later, the main valve opens and all is good. The house warms up and the call for heat turns off. The problem then starts with the second cycle. The second call for heat has the ignitor spark and the pilot light but no main valve opening. After the pilot lights, the spark stops, but then resumes about 5 seconds later. This unit is configured with a combined sensor/ignitor.

I've tried replacing the sensor ignitor but the symptom is the same (purchased robertshaw 1751-729). I also tried using A white rodgers 50d50-843 but that doesn't spark at all though the pilot valve opens on that unit.

In trying to diagnose the 780-715U, I check voltage between the main valve lead and the "white MV/PV" lead and I do not get any DC voltage when I would expect the main valve to be open. I have to say that I'm not 100% sure about that voltage reading because the ignitor is sparking and I'm a little nervous about getting close to that ignitor lead.

While I'm okay with buying another 780-715U, I had assumed the white-rodgers would be okay.

Do you all have any suggestions for further troubleshooting?

 

Welcome to the forums.

It's best to start out with the make and model of your furnace and then what control board it uses.

That's because it doesn't know it's lit. It sounds like you have a pilot issue. More than likely a dirty pilot orifice.The flame must be big and firm in order to keep the sensor satisfied. The main gas valve will never open until the pilot flame is 100% proven.

 

Nope, you will never get a DC voltage because the system is AC. Set your meter accordingly.

 

Gotcha. The furnace is a GE model BLU080F942B0. I'm not sure what the make and model of the gas valve is.

That makes sense. Duh. A transformer without a rectifier will still produce AC. I've changed my meter to detect AC voltage. The first time, I get 26 volts on both pilot and main but on the second time around, I do get 26 volts on the pilot but zero on the main. I think that has to do with the post below...

If it's truly a dirty pilot orifice, then wouldn't the main valve not open on the first run as well? If this matters, the ignitor/sensor glow orange after being in the path of the pilot burn for a few seconds on both the first and second calls for heat. Here's a picture of the pilot burn showing the ignitor/sensor on the second round:

https://goo.gl/photos/MXerVYVbNVXQmYQ48

How does one clean a pilot orifice? Emery cloth?

 

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The ignition control turn the spark on when it's not detecting that the pilot is lit.

So it's initially detecting the pilot and shutting the spark off, then failing to detect the pilot and turning the spark back on.

Your picture of the pilot is very poor, but the pilot flame appears to be an incandescent white flame. That strongly suggests that the pilot orifice is dirty, since the pilot ought to be a sharp blue flame like a small blowtorch.

A dirty pilot flame would be a good source of the problem and symptoms you describe.

Carefully observe the pilot flame and see if it is no longer striking the flame sensor when the spark turn on again.

 

The white portion of the picture is actually the ignitor/sensor and it's white (more of a bright red in reality) because of being in the path of the flame. The surrounding blue is the pilot flame.

I'll go ahead and clean the pilot orifice as I don't think that's been done for at least 5 years.

 

I'm showing that furnace is over 35 years old.... is that right ?
Have you ever had it professionally cleaned..... you should have.

A gas appliance should be routinely cleaned and inspected for integrity.

 

Apologies for not posting my results on this thread sooner...

PJMax was right, cleaning the pilot orifice resolved the problem. For those who may find this thread via google, it's important to note that my initial cleaning didn't solve the problem but that's because I didn't take the pilot thingy completely off. Once I took it off, I could clean all parts of it which did the trick. We're now all warm and toasty.

As for having a 35 year old furnace, yes, it most likely is. The HVAC person who replaced the valve about 5 years ago claimed to do "maintenance" on it but I wasn't here when he did that maintenance so I'm not really sure what he did. If I wanted to do that myself, is that something a DIY'er could do?