The heat risers are bimetallic spring operated flaps that direct exhaust up onto the bottom of the intake manifold when open. Should have a bimetallic spring visible, or under a sheetmetal cover on the exhaust under the carbs.

You should adjust the idle on the carbs to draw equal air with the link between them off, then adjust the link so that the idle and air flow don't change when it's re-installed.

Glad it's working better -- that twin carb setup is great when it works, and a huge PITA when it doesn't!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!