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Working on an old carrier gas boiler today. As soon as I turned it on I could smell that sooty smell when a boiler is dirty and sooted up. Took apart completely and cleaned, when I put back and started I put my analyzer on it and the co was over 2000! Gas pressure was at 3.6", when I lowered it to 2.5 the co dropped to 60, then it would go up to 400 and go back down. The gas valve was replaced, did these older boilers have different manifold pressures? Why is the co going up and down and so high? It reminded me of an expansion valve that was " hunting".