OLD (1986) 36c84-209 crossover

need to set it up with a manometer and combustion analyzer, replacing a gas valve isn't a diy job.

In case the cross-reference was wrong, this would tell us what we're looking for.

I refer you to the question I would ask the technicians that worked for me. It happens to be my signature tag line just below this comment.

I had two customers with that gas valve / mercury flame sensor set up. I seem to remember that the three prong Mercury Flame Sensor (MFS) was something that I replaced often on both customers. It seems that the original lasted 30+ years but the replacement MFS were problematic and didn't last very long. I actually remember that one of the MFS failes within one year and the place I purchased it honored the 1 year warranty. they were affiliated with Johnstone Supply at the time so the main office had a policy for that warranty. Today that store is a mom and pop supply house because they didn't have a large enough property to provide the minimum amount of floor / warehouse space and they only had street parking. Bye Bye Johnstone. Hello American Parts co.

Having two valves with almost the same symptoms is curious. I don't see why the wiring would be different, they are the same Model number series.

If you truly believe there is a wiring difference, prove it, is there 24 VAC getting to the Main Valve solenoid coil when there should be. If not, why not.

During a call for heat I would monitor the inlet gas pressure and verify there is 24 VAC across both solenoid coils with the focus on the main valve coil, since the pilot works.

Troubleshooting is about following the evidence, not about what you want the defect to be.

I doubt the 40 year old valve is exactly the same as the new one. It would not surprise me if the solenoid is different. Engineers like to eat so they redesign things that otherwise appear to work fine. And they may have simply redesigned it to do exactly what you are seeing, old valve possibly partially opening, the new one does not partially open under the same conditions.

This electrical circuit looks easy to troubleshoot. You say the gas pressure is good so only the electrical remains.

Also it would not surprise me that a defect with the Mercury flame sensor switch contact behaved differently with valves that are 40 years different. The pull in current may be different.

And for all I know from here, the old main valve may simply be leaking and not really opening, hence the low pressure to the manifold.

To me there is two things to prove, gas pressure and if the is there 24 VAC getting to the Main Valve solenoid coil when there should be. If not, why not.

If that doesn't work out, you can upgrade the entire gas train and ignition system using this kit:

https://www.supplyhouse.com/Resideo-Y8610U6006-Intermittent-Pilot-Control-Conversion-Kit-for-270000-BTU-Units?utm_adgroupid=171237273553&utm_targetid=pla-2447861271497&utm_product_id=Y8610U6006&utm_matchtype=&utm_adtype=pla&utm_category=General&gad_source=1&gad_campaignid=21861371907

This kit uses standard components and will be easier to maintain in the future.

it is very unlikely that the mv solenoid is opening but not opening al the way because of a bad flame switch or voltage, since it is driven by gas pressure it is all or nothing. you can check both with a voltmeter, in fact i think you can check the mv and pv solenoids directly with a voltmeter.

it is possible a manufacturing defect like a bad diaphragm or something that got assembled wrong or is missing is causing it to not open fully.

someone cold have set the regulator to the pressure you are seeing or it has the wrong spring or something.

a regulator could be bad or a valve closed or some other restriction causing the pressure to drop on the supply when the mv opens.