the breather are supposed to work? You know thw ones... they control the flap at the end of the air breather snorkel. I have terrible carb icing and feel the flap is openning too soon.

Here is the way I think it works... the vacuum is applied to a tempurature controled valve in the top lid of the ait breather. As long as the temp in the breather is below a certain temp, the vacumm passes through the valve to another valve in the side of the breather body ( I don't know what this one is or how it works). The vacuum hose then routes to the round valve at the end of the snorkel that operates the flap. This flap when closed (i.e. vacuum applied) directs warm air from the heat stove via the tube from the heat stove to the air snorkel and into the main breather body and to the carb. This warm air is supposed to prevent the carb from icing.

Well, in my case, the flap closes when I start the truck. But in weather with high humidity and temperatures near freezing (typical spring weather on the Canadian east coast) the carb quickly ices up and she idles poorly till it gets really warmed up. I check the flap and find it alomost completely open. Now if I apply the vacuum directly to the valve working the flapper it closes and stays closed. Carb icing is very minimal then... but the flap is always closed no matter how warm the engine gets.

So, how long should the flapper stay cloed? Do I replace the temp controled valve in the top of the breather lid? Everything else in the system is in perfect condition... the heat stove is new as is the tube from the heat stove to the air snorkle. Anyone have any tips to prevent carb icing????

 

Simply put the valve is opened when the engine is cold allowing heated air to come from the riser and shroud around the exhaust minfold to the carb. As the temp rises the vacuum motor closes the valve allowing cold air to flow into the carb from the snorkel. If it is not working correctly then check to see if it will move at all. Maybe it is just stuck. When the truck is cold you should see the valve is open (no vacuum on the motor) . As the motor heats you should get vacuum on the hose going to the motor (the little round thing on the snorkel). When the truck is hot you should have full vacuum.

Unfortunately the vacuum diagram on the truck is crap and in code so it is difficult to figure out what line leads where. It sounds like a PVS is switching too soon, or the bimetalic vacuum switch is switching too soon. You may be better off replacing the parts involved. On my truck the valve is spring loaded so it closes (cold air in) when no vacuum is applied. The manifold vacuum goes to the bimetallic thingamajig inside the air cleaner. I suppose that it gradually closes as the temp increases and allows the spring to close the valve.

Hope this is helpful in some small way.

 

Manifold vacuum should go to the bi-metal sensor, then to the TVS (thermal vacuum switch) then to a retard delay valve, then to the air cleaner diverter valve. Full vacuum cold closes the diverter allowing hot air to enter the engine, the delay valve keeps the diverter closed (for heat) on short accelerations but will bleed off slowly on full throttle even when cold due to loss of vacuum. The bi-metal sensor heats up and opens a small valve inside itself to bleed off all vacuum when hot and the TVS in the side of the air cleaner is the on off thermal switch for the whole air cleaner inlet system. Suggest bi-metal sensor first.