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Just replaced my old intake and exhaust manifolds with new manifolds in part to get the heat riser assembly installed back into my M38. Old exhaust manifold didn't have the metal flap inside and was completely rusted solid.

Got everything installed but the heat riser assembly seems to be quite stiff. You can manually move the weight (which does turn the spring/rod/metal flap) easy enough by hand but it will stay in place when you let go. I was expecting the weight to always want to drop towards the ground (don't know if that is open or closed) and slowly open when the motor is warmed up but I doubt the spring has enough strength to move the weight as is.

With the new manifold sitting on a bench, the heat riser assembly was lose, the weight/spring/metal flap would open and close just by turning the manifold side to side. It only bound up when I bolted the intake manifold to the exhaust manifold and then bolted that to the motor.

Is it normal to have a small bit of resistance in the heat riser assembly? Since everything on here is new, maybe it has a small break in period?

From my experience it should be pretty light. The weight should always want to drop towards the ground - it's the spring contracting that opens it. You can test it by applying some heat with a torch and watch it open up. Make sure that the bolt attaching the exhaust pipe to the manifold is not too long or it can prevent the flap from opening fully. It's a scary moment when that manifold starts to turn cherry red!