Boiler Burner Issue?

[Plumber69]

Boiler sparks, pilot lights, main burner comes on. There's 6 burner tubes, the front 6-8 burner ports in all tubes loose flame instantly, shortly after pilot losses flame. Then recycles. Inlet gas pressure is good, manifold pressure is good, burner tubes are good, manifold orfices are clean, heat exchanger is clean. I can get it to run if the shutoff valve is in half off position. Any ideas

 

[Dana]

Is it the correct gas type?

 

[Sponsor]

 

[Plumber69]

Is it the correct gas type?

Yes, boiler is about 5-7 years old. It's a new problem.

 

[Fitter30]

Brand model serial number. Has something changed with the flue ? Does it have a barometric damper thats closed?